PALAEOENVIRONMENTS OF THE GULF OF CARPENTARIA FROM THE LAST GLACIAL MAXIMUM TO THE PRESENT, AS DETERMINED BY FORAMINIFERAL ASSEMBLAGES

Transcription

1 PALAEOENVIRONMENTS OF THE GULF OF CARPENTARIA FROM THE LAST GLACIAL MAXIMUM TO THE PRESENT, AS DETERMINED BY FORAMINIFERAL ASSEMBLAGES A thesis submitted in fulfilment of the requirements for the award of the degree of Doctor of Philosophy (Science) from the University of Wollongong by Sabine Holt (BSc. Hons) School of Earth and Environmental Sciences, 2005

2 CERTIFICATION I, Sabine Holt, declare that this thesis, submitted in fulfilment of the requirements for the award of Doctor of Philosophy, in the School of Earth and Environmental Sciences, University of Wollongong, is wholly my own work unless otherwise referenced or acknowledged. The document has not been submitted for qualifications at any other academic institution. Sabine Holt 22 nd February 2005

3 ABSTRACT This thesis presents a palaeoenvironmental study of the Gulf of Carpentaria, northern Australia, from around the Last Glacial Maximum (LGM) to the present. Foraminifers, microscopic unicellular aquatic organisms, occur throughout the sediment in the time frame studied. Data on the species composition and preservation of the microfossils found in the Gulf of Carpentaria cores are utilised to reconstruct past environments by comparison to the known assemblages of living foraminifers in various modern environments. The Gulf of Carpentaria is a shallow epicontinental sea, situated between Australia and Papua New Guinea, and is a maximum of 70m deep. It is separated from the Pacific Ocean to the east by Torres Strait, which is 12m deep at its shallowest, and from the Indian Ocean and Arafura Sea to the west by the Arafura Sill, which is 53m below sea-level (bpsl) at its shallowest. For at least ten thousand years in the lead up to the LGM (which reached its peak about twenty thousand years ago), and for about ten thousand years after, sea levels were lower than the 53m-deep Arafura Sill. The continental shelf in the Gulf of Carpentaria area between Australia and Papua New Guinea was exposed, creating a land bridge between the two islands, and a lake developed in the Carpentaria Basin. This palaeolake is termed Lake Carpentaria (named by Torgersen et al., 1983). Documentation of the timing in fluctuations in the extent and salinity of Lake Carpentaria provides information on local and regional climatic systems, such as the Australian summer monsoon. Constraining the nature and timing of the postglacial rise in sea-level which flooded the lake provides evidence for global eustatic sea-level reconstructions. Analysis of sediment cores from the Gulf of Carpentaria, beginning around 40ka cal BP (forty thousand calendar years before present), shows the existence of Lake Carpentaria (a large, non-marine water body of fluctuating extent) until sealevel rose over the Arafura Sill and inundated the palaeolake around 10.5ka cal BP. i

4 The earliest studied phase dates to around 40ka cal BP which is a marineinfluenced brackish water lacustrine facies where Lake Carpentaria is briefly at its maximum extent: 12m deep in its deepest section. The existence of such a large body of water (around 150,000km 2 ) supports the existence of a strong Walker Circulation in the region enhancing precipitation. Between 40ka and 18.8ka cal BP the non-marine, increasingly saline, Lake Carpentaria decreased to 7m maximum water depth, adding to the evidence of aridity around the LGM in northern Australia. At 18.8ka cal BP the lake freshened and monospecific bivalve, foraminiferal and ostracod populations dominated the still shallow (around 8m deep) lake. The lake was expanding, and from around 15±2ka cal BP, fluctuations are noted in the general trend of increasing precipitation. The recorded variations in precipitation intensity may result from stronger seasonality (i.e. monsoons) and/or interdecadal variability (e.g. El Niño Southern Oscillation). At 12.7ka cal BP Lake Carpentaria was at around 12m maximum water depth the maximum documented extent in the studied period. At this stage there was some exchange of waters with the Arafura Sea via tidal outlet channels in the Arafura Sill (indicating sea-level around 60m below present), seen as a marine influence beginning in the western margins at 12.7ka cal BP. At 12.4ka cal BP the sealevel had risen to the same height as water levels within the lake (58m bpsl). By 12.2.ka cal BP sea-level was up to 2m higher than the previous lake level, and flowed into the lagoonal Lake Carpentaria via channels in the Arafura Sill. By 10.5ka cal BP the sea-level had overtopped the highest surface of the 53m-deep Arafura Sill and the transition to marine conditions began in the Gulf of Carpentaria, confirming the accepted models of sea-level rise. ii

5 ACKNOWLEDMENTS The entire staff and associates of the School of Earth and Environmental Sciences at the University of Wollongong deserve many thanks for providing a stimulating and wonderfully friendly environment of learning. I am grateful to my supervisor Prof. Allan Chivas for providing a place for me in such an exciting project, and for his dedication to see me complete my goal. I wouldn t have chosen anyone else but the GoC crew to be locked up with in a cold-room for two years Dr Adriana Garcia: mentor and friend; Dr Jessica Reeves: twin soul; and soon-to-be- Dr Martine Couapel (with new addition Korigan): no-nonsense support crew! I owe much to the kindness and inspiration of Prof. Brian Jones, who always looked out for ways to help. I thank Prof. Paul Carr for his caring guidance, even though I only play in the mud! David Carrie also made great efforts on my behalf. And the enthusiasm and concern of Dr Dioni Cendon was much appreciated. The technical staff always helped above and beyond their call of duty: Aivars Depers, John Marthick, Penny Williamson, and John Reid (who not only had to cope with computers, had to deal with people like me stressed about computers, and never seemed to hold it against me). Nick Mackie, of the Engineering Department, guided me through hours of SEM, and Dr Winston Ponder, of the Australian Museum kindly took the time to identify mollusc shells used in radiocarbon dating. Thanks to the special people in my life, Mum who gave me all the love in the Universe, and Sunirmalya who gave me immense spiritual support. I would like to dedicate this thesis to the memory of John Head, not only a very helpful colleague, but a good friend. iii

6 TABLE OF CONTENTS page Abstract i Acknowledgements iii List of Figures viii List of Tables xiii CHAPTER 1 Introduction Introduction to the topic Introduction to the Gulf of Carpentaria Tectonic stability Water depth Location and climate The continuous presence of micro-organisms Previous related investigations Comparing investigations Previous results Synthesis of previous investigations The current project Thesis aims CHAPTER 2 Palaeoclimate and Sea-level change Introduction Global overview Changes in sea-level Changes in oceanic circulation and salinity Changes in temperature Atmospheric circulation (including rainfall) Walker Circulation, ENSO and WPWP Australian summer monsoon, ITCZ and Hadley Cell Combined effects Summary iv

7 CHAPTER 3 Foraminifers Introduction General systematics and biology Systematic nomenclature Features of foraminiferal tests The habitat of living foraminifera Introduction to living assemblages and indicator species Isolated water bodies Isolated brackish waters Isolated water bodies of marine salinity and higher Transitional environments Tidally influenced environments Lagoonal environments Marine environments Continental shelf sea environments Open marine environments Thanatocoenoses Summary. 95 CHAPTER 4 Methods Introduction Material selection and collection Seismic data Piston Core collection Core preparation and sub-sampling Foraminiferal analysis Sedimentary analysis Radiocarbon dating Selection of material Dating method Calibration of dates Summary. 104 v

8 CHAPTER 5 Results Introduction Sedimentology Carbonate Ooids Echinoid fragments Glaucony Quartz grains Pyritised fragments Calcareous nodules Iron Oxides Micropalaeontology Dating Facies Brackish Lake facies Saline Lake facies Exposed sediments Transitional facies Marine facies Palaeoenvironmental data from each core Core MD Core MD Core MD Core MD Core MD Core MD Summary. 176 vi

9 CHAPTER 6 Discussion Introduction Palaeoenvironments in relation to core location Northern lake edge Tidal channel influence Lake centre Palaeoenvironments through time Brackish lake: ~35-23ka cal BP Saline lake: 23-18ka cal BP Brackish Lake: ka cal BP Transitional: ka cal BP Marine: ka cal BP Implications for climate ~35-23ka cal BP ka cal BP ka cal BP ka cal BP Implications for oceans ~35ka cal BP ka cal BP ka cal BP Summary REFERENCES APPENDICES Appendix A New and incompletely identified species of Recent foraminifera Appendix B Species list of Foraminifera found in the Gulf of Carpentaria cores Appendix C. 251 Abundance of foraminiferal species at each 5cm sample depth in all cores vii

10 LIST OF FIGURES page Figure Location of the Gulf of Carpentaria. Figure Location of studied sections within the Gulf of Carpentaria. Figure Cross-section A-B of the northern Gulf of Carpentaria from the Arafura Sill in the west to Torres Strait in the east. Figure Location of sites, mentioned in the text, used to examine tectonic and glacio-hydro-isostatic deformation around Australia. Figure Predicted hydroisostatic uplift for the Gulf of Carpentaria region. Figure Interpretative line-drawing of a seismic section (line 17 of USGS survey of 1993/94) across the Gulf of Carpentaria from Gove to Weipa. Figure Location of the Western Pacific Warm Pool (WPWP). Figure Schematic diagram of atmospheric circulation over the South Pacific Ocean. viii

11 Figure Location of sites providing a palaeoclimatic record of sea-level, oceanic circulation and temperature in the region since the LGM. Figure Location of sites providing a palaeoclimatic record of atmospheric circulation in the region since the LGM Figure Schematic representation of planktonic and benthic foraminiferal habitats. Figure Summary foraminiferal and environmental ternary diagram. Figure Location of brackish and more saline isolated water bodies within the region where foraminifers have been found. Figure Location of tidally influenced environments within the region with foraminiferal studies mentioned in the text. Figure Location of lagoons within the region with foraminiferal studies mentioned in the text. Figure Location of sites within the region mentioned in the text where foraminiferal studies have been undertaken on the continental shelf. Figure Location of sites within the region mentioned in the text where foraminiferal studies have been undertaken on open marine areas. ix

12 Figure Schematic sectioning of sample slices showing sub-sampling. Figure Schematic cross-section of the Gulf of Carpentaria, showing the position and length of cores MD Figure Scanning electron microscope images of sedimentary components in the Gulf of Carpentaria cores. Figure Scanning electron microscope images of foraminifers from the Gulf of Carpentaria cores. Figure Radiocarbon ages (calibrated) within cores. Figure Sedimentological characteristics of core MD 29. Figure Relative abundance of foraminiferal species (>2%) as a function of depth in core MD 29. Figure Sedimentological characteristics of core MD 31. Figure Relative abundance of foraminiferal species (>2%) as a function of depth in core MD 31. Figure Sedimentological characteristics of core MD 30. x

13 Figure Relative abundance of foraminiferal species (>2%) as a function of depth in core MD 30. Figure Sedimentological characteristics of core MD 28. Figure Relative abundance of foraminiferal species (>2%) as a function of depth in core MD 28. Figure Sedimentological characteristics of core MD 32. Figure Relative abundance of foraminiferal species (>2%) as a function of depth in core MD 32. Figure Sedimentological characteristics of core MD 33. Figure Relative abundance of foraminiferal species (>2%) as a function of depth in core MD 33. Figure Sketch of facies diagrams showing position of cores in the Gulf of Carpentaria. Figure Brackish Lake Carpentaria ~35-23ka cal BP xi

14 Figure Saline Lake Carpentaria ka cal BP. Figure Brackish Lake: abundant shell population ka cal BP. Figure Brackish Lake Carpentaria ka cal BP. Figure Transitional environment of the Gulf of Carpentaria ka cal BP. Figure Marine environment of the Gulf of Carpentaria ka cal BP. Figure U-Th dated RSL estimates based on corals and other evidence for sites distal from isostatic movements. Figure Estimated eustatic sea-level. Figure Estimation of the varying level of Lake Carpentaria throughout its existence in the studied period. Figure Estimation of relative sea-level from the Gulf of Carpentaria cores. xii

15 LIST OF TABLES page Table Current and previous classification of foraminifers based on wall structure. Table Percentage composition of representative samples of glaucony. Table Conventional radiocarbon age, calibrated age, ANSTO number, species dated, and total weight of samples from cores MD xiii

16 Chapter 1 - Introduction CHAPTER 1 Introduction 1.1 Introduction to the topic For approximately the past two million years, the interval of the Quaternary Period*, the Earth has experienced major environmental fluctuations, alternating between ice ages and warm interglacials. The most recent ice age, termed the Last Glacial Maximum (LGM), reached its maximum around 20,000 calendar years ago (20ka cal BP), with an associated lowered eustatic sea-level as the accumulation of water within massive ice sheets lowered sea-levels world-wide. Estimates of the length of the LGM range from a few thousand years to over 10,000 years. One indication of the length of the LGM is maximum ice volume, variously reported as from 22-19ka cal BP (Yokoyama et al., 2000 and 2001a), to beginning around 30ka cal BP (Lambeck and Chappell, 2001). Most authors place the termination of the LGM at 19ka cal BP (e.g. Yokoyama et al., 2000; Mix et al., 2001), when temperature and sea-level began to rise to present-day levels in the post-glacial period. This thesis is part of a wider palaeoenvironmental tradition which seeks to understand the way in which various interactive systems of the planet have functioned in the past, which enables an explanation of the present, and may assist predictions of the future. The Gulf of Carpentaria, northern Australia (Fig. 1.1), is examined in this thesis, presenting a palaeoenvironmental study from the onset of the LGM to the present day. Around the LGM, when sea levels were lower, the continental shelf in the Gulf of Carpentaria area between Australia and Papua New Guinea was exposed, and a lake developed termed Lake Carpentaria (named by Torgersen et al., 1983). Information on local and regional climatic systems can be obtained from documentation of changes in the extent and salinity of Lake Carpentaria. Global eustatic sea-level reconstructions can be enhanced by information from the Gulf of Carpentaria on the post-glacial marine transgression which occurred with the rise in sea-level after the LGM. *Although the term Quaternary is not a formal chronostratigraphic unit (Gradstein et al., 2004), it is traditionally considered to be the interval of oscillating climatic extremes (encompassing the Holocene and Pleistocene Epochs) and is utilised in this thesis in this form. 1

17 Chapter 1 - Introduction Figure 1.1 Location of the Gulf of Carpentaria (modified from Chivas et al., 2001). tidal stations at Gove, Proudfoot Shoal and Weipa 2

18 Chapter 1 - Introduction An overview of the Gulf of Carpentaria and the characteristics of the gulf which make it useful for palaeoenvironmental studies is presented in Section 1.2. Previous investigations of the Gulf of Carpentaria are summarised in Section 1.3, and the current project is outlined in Section 1.4, including the aims of this thesis. 1.2 Introduction to the Gulf of Carpentaria The Gulf of Carpentaria is an epicontinental sea, located between Australia and Papua New Guinea (Fig. 1.1), and is positioned climatically within the zone of monsoonal influence. At present the gulf has a maximum water depth of 70m, an area of around 500,000km 2, and is bordered to the west by the Arafura Sill which is 53m below present sea-level (bpsl), and to the east by the 12m-deep Torres Strait. The shallow, low-gradient topography of the Gulf of Carpentaria reaches its deepest nearer the eastern side as shown in Figures 1.2 and 1.3. Note that the transect in Figure 1.3 passes through the northern section of the Gulf of Carpentaria, between the Arafura Sill and Torres Strait, and does not portray the deepest area of the gulf. A predominantly clockwise pattern of water circulation occurs around the margins of the Gulf of Carpentaria (Church and Forbes, 1981). The Gulf of Carpentaria experiences a complex system of tides, varying from semi-diurnal (two high tides and two low tides per day) to fully diurnal (a single set of a high and low tide per day). The Australian continental borders of the Gulf of Carpentaria are under a mostly microtidal (up to 2m tidal range) to mesotidal (2-4m) regime. A mesotidal regime also prevails in Torres Strait and around the Arafura Sill. 3

19 Chapter 1 - Introduction A B Line 17 Figure 1.2 Location of studied sections within the Gulf of Carpentaria (modified from Chivas et al., 2001). cores used in this study (MD 29-33), and others referred to in the text (GC 2 and 10A); the transect A-B detailed in Figure 1.3; and seismic line 17 from Edgar et al. (2003). 4

20 Chapter 1 - Introduction A 53m below present sea-level B 12m below present sea-level Torres Strait water depth (m) Arafura Sill Lake Carpentaria to Indian Ocean 50km VE = 5005 to Pacific Ocean Figure 1.3 Cross-section A-B of the northern Gulf of Carpentaria from the Arafura Sill in the west to Torres Strait in the east. Water characteristics of the Gulf of Carpentaria vary seasonally; Forbes (1984) considered that local seasonal processes influenced the hydrology of the gulf more than the characteristics of the adjacent seas. Lower atmospheric winter temperature causes cooler sea surface temperatures (SST), while the increase in monsoonal rainfall leads to a decrease in salinity during summer (Forbes, 1984). Proximity to the shore is also significant in determining water characteristics of the Gulf of Carpentaria. Strong vertical temperature stratification is present in the deeper central waters of the gulf during summer, while close to shore the waters are relatively well-mixed (Somers and Long, 1994). The relatively uniform salinities (35-36 ) of the central gulf during summer are contrasted with salinities in the coastal zone influenced by freshwater runoff during summer (Somers and Long, 1994). Although Somers and Long (1994) found turbidity to be generally low throughout the gulf, Harris et al. (2004) recorded a higher level of 6.5mg/L below 50m water depth in the central gulf. Both Somers and Long (1994) and Harris et al. (2004) documented lower turbidity values above coarser sediments (e.g. 2.9±1.9 mg/l above a substrate of coral reef and/or coarse carbonate sediment; Harris et al., 2004). 5

21 Chapter 1 - Introduction Although the distribution of surface sediments of the Gulf of Carpentaria is related to bathymetry, with coarser terrigenous material in shallower waters and a muddy central region (e.g. Jones, 1987; Somers and Long, 1994), the prevailing clockwise water circulation pattern in the Gulf of Carpentaria has also been shown to have an effect (Preda and Cox, 2005). The prime source area of both terrigenous and carbonate material is located on the eastern side of the gulf, and transportation leads to finer grained sediments and more stable species of carbonates (i.e. low-mg calcite) distal from the source area (Preda and Cox, 2005). Jones (1987) highlighted the importance of relict sediments in the Gulf of Carpentaria (such as stained quartz grains, ferruginous and calcareous pisoliths, ooids and reworked foraminifera). Three main depositional environments were identified by Jones (1987) in the western gulf, in water depths less than 50m bpsl, and were considered by the author to reflect broad sedimentation trends of the Gulf of Carpentaria as a whole. Jones (1987) found most modern sedimentation takes place in a narrow zone along the coastline, characterised by moderate to high sedimentation rates. Sandy deltaic deposits, and prodelta mud, occurs near river mouths (the Roper and McArthur Rivers in the western gulf), where alongshore transport ensures their widespread distribution along the coast. The lowest sedimentation rates occur in the nearshore zone (mostly in 10-20m water depth) where relict deposits from the LGM dominate. Relict sediments are also found in the deeper Carpentaria shelf area, which is more distal from fluvial input. However, a slightly higher sedimentation rate (derived from in situ fauna and from settling of transported terrigenous mud) results in a mix of relict and modern sediments. Sediment type is known to have an influence on the distribution of aquatic marine species such as prawns in the western gulf of Carpentaria (Somers, 1987). The following characteristics make the Gulf of Carpentaria an important area for palaeoclimatic studies of the Quaternary Period: Tectonic stability The Gulf of Carpentaria is situated on an area of the Australian continental plate that has been tectonically stable during the Quaternary Period (e.g. Marshall and Thom, 1976; Stirling et al., 1995; Haworth et al., 2002). Several studies 6

22 Chapter 1 - Introduction attest to this stability, such as the documentation by Stirling et al. (1998) of undeformed coral reef terraces of Last Interglacial age (around 125ka BP) on the Western Australian coast (Fig. 1.4). In sites around former ice sheets, post-glacial unloading has distorted the continental and oceanic crusts. The Gulf of Carpentaria is far from such LGM ice margins, therefore such glacio-hydro-isostatic effects are assumed to be relatively small to non-existent in the region. Although the isostatic effect may have been relatively small, many authors have cited evidence of its effect over northern Australia during the Holocene. Studies in the Gulf of Carpentaria region have demonstrated mid-holocene (7-5ka cal BP) high-stand sea-levels ranging from less than a metre above present to 2.4m above present sea-level. Woodroffe et al. (1987) found little evidence for a Holocene highstand, with the estuarine sediments studied from the South Alligator River (Fig. 1.4) showing at most a 1m fall in sea-level from the mid- Holocene. Microatolls from Torres Strait (Woodroffe et al., 2000; Fig. 1.4) also indicated a maximum fall in sea-level of around 1m for the mid to late Holocene. Woodroffe et al. (2000) consider this fall to be primarily eustatic, but locally modified by hydro-isostatic loading of the shelf. Based on the height of chenier plains around Karumba (Fig. 1.4), Rhodes (1982) distinguished a purely hydroisostatic fall in sea-level of 1.5m since the mid-holocene. Chappell et al. (1982), utilising chenier ridge data from Rhodes (1980) identify a 2.4m difference in relative sea-level since the mid-holocene around Karumba. The variations in the time and height of the Holocene highstand across the region have been recognised as consistent with hydro-isostatic deformations caused by the post-glacial rise in sea-level (Chappell et al., 1982; Lambeck and Nakada, 1990). Yokoyama et al. (2000) consider the major cause of crustal deformation to be post-glacial meltwater loading of the ocean floor, causing subsidence of the sea floor and coastal zone. 7

23 Chapter 1 - Introduction Figure 1.4 Location of sites, mentioned in the text, used to examine tectonic and glaciohydro-isostatic deformation around Australia. 1: West Australian coast from Cape Range in the north to Rottnest Island in the south (Stirling et al., 1998), 2: South Alligator River (Woodroffe et al., 1987), 3: Torres Strait (Woodroffe et al., 2000), 4: Karumba (Rhodes, 1982; Chappell et al., 1982), 5: Sir Edward Pellew Group of Islands (Forbes and Church, 1983), 6: Magnetic Island (Haworth et al., 2002), 7: King Island, Bass Strait (Haworth et al., 2002), 8: The Great Barrier Reef from King Island in the north to Stone Island in the south (Chappell et al., 1983). 8

24 Chapter 1 - Introduction However, Woodroffe (2000) notes that differences in palaeosea-level of 1-2m are difficult to discriminate from the large tidal range along the estuaries of northern and northwestern Australia. Although the Gulf of Carpentaria mainly experiences a microtidal to mesotidal regime, a simple classification based on maximum range does not reveal the average water levels. Centre Island in the Sir Edward Pellew Group (Fig. 1.4) illustrates the large range experienced the mean tidal range is only 80cm, but the tidal range can reach up to 3m (Forbes and Church, 1983). During the monsoon season, seiches also affect the relative sea-level around the Gulf of Carpentaria. Wind pushes water southward, creating fluctuations of 0.5m (salt flats in the southern gulf, Wolanski, 1993) to over 1m (Karumba, Rhodes, 1982). The evidence of hydro-isostasy of the Australian continental margin is contested by Haworth et al. (2002). Radiocarbon dating of calcareous material from intertidal organisms from Magnetic Island in North Queensland to King Island in Tasmania (Fig. 1.4) provided a north-south transect that showed a constant sea-level for the time slice studied. However, as Chappell et al. (1983) pointed out in a study of the Great Barrier Reef (Fig. 1.4), hydro-isostatic warping varies due to position relative to the ocean and continent, not necessarily latitudinally. Sites further within the continental plate are expected to display hydro-isostatic uplift, in contrast to shelf margins which have subsided since their flooding post- LGM. The Gulf of Carpentaria cores studied in this thesis fall within the zones of Chappell et al. (1982, 1983) with 0-1m predicted uplift post-lgm (Fig. 1.5), although the gulf may have experienced up to 2.4m uplift on the south eastern edge (Chappell, 1982, 1983; Rhodes, 1982). The Gulf of Carpentaria is located far from former ice sheets and therefore sealevel records are minimally affected by isostatic factors. Most of the record of sea-level change in the Gulf of Carpentaria region can be attributed to eustatic change, but it must be noted that there may also be some local hydro-isostatic adjustment of the shelf. The relative stability of the gulf ensures information on global climatic-induced sea-level fluctuations may be acquired without the necessity of performing significant corrections for tectonic effects. 9

25 Chapter 1 - Introduction Fig. 1.5 Predicted hydroisostatic uplift for the Gulf of Carpentaria region, modified from Chappell et al. (1983). The red contours depict height (relative to the present) in the mid- Holocene in metres Water depth The shallow nature of the Gulf of Carpentaria makes it a sensitive register of sea-level change (Chapter 2). At a sea-level 12m lower than present, the gulf is separated from the waters of the Pacific Ocean (via the Coral Sea) by a land bridge along Torres Strait between North Queensland and Papua New Guinea. At 53m below present sea-level, the gulf is also isolated from the Indian Ocean (via the Arafura Sea), by the emerged Arafura Sill. This area of land between Australia and New Guinea is a low-gradient basin, as previously noted in Figures 1.2 and 1.3. When sea-level recedes beyond the Arafura Sill, a nonmarine body of water (Lake Carpentaria) is able to form within the land-bridge between Australia and New Guinea. The formation of Lake Carpentaria as a result of a lowered sea-level is illustrated in Figure 1.3, showing a transect between the Arafura Sill and Torres Strait. 10

26 Chapter 1 - Introduction The Gulf of Carpentaria has been isolated a number of times throughout the Quaternary, as sea-level fell below the level of the Arafura Sill. Figure 1.6 depicts up to seventeen exposure surfaces (implying transgressive/regressive events) revealed in a seismic survey of the area (Chivas et al., 2001; Edgar et al., 2003). At the LGM, sea-level was at least 125m lower than present (e.g. Yokoyama et al., 2001a), isolating the Carpentaria Basin from the oceans, and permitting the formation of Lake Carpentaria. With the post-glacial rise in sealevel, waters from the Indian Ocean flooded into the basin over the Arafura Sill, and eventually a through-flow to the Pacific Ocean was established when sealevel rose further to breach the Torres Strait land bridge. The effects of this rise in sea-level are recorded in the transition from non-marine to marine sediments in the Gulf of Carpentaria (first noted by Phipps, 1966). Figure 1.6 Interpretative line-drawing of a seismic section (line 17 of USGS survey of 1993/94) across the Gulf of Carpentaria from Gove to Weipa (from Chivas et al., 2001 and Edgar et al., 2003), showing numbered basin-wide reflectors and incised channels (width not to scale) at exposure surfaces. 11

27 Chapter 1 - Introduction Evidence of palaeochannels in the Arafura Sill must be taken into account when reconstructing past sea-levels. Tidal exchange would have allowed marine water and organisms to infiltrate Lake Carpentaria when sea-level was lower than the -53m planar surface of the Sill. Large incisions 10-15m deep have been found in the Arafura Sill (Jones and Torgersen, 1988). Some of the infilled channels on the gulf floor have been cut and filled more than once in a transgressive/regressive cycle, indicating fluctuating water levels (Edgar et al., 2003) Location and climate The Gulf of Carpentaria is well placed to record the signatures of a number of significant climatic systems (Chapter 2). The land masses and surrounding oceans of the tropical region play important roles in the circulation of the world s atmosphere. To the north of Australia is the Western Pacific Warm Pool (WPWP; Fig. 1.7) which is the centre of enhanced tropical convection in the region. Nearly two-thirds of global precipitation occurs in systems located within 30 of the equator (Sturman and Tapper, 1996), and the processes involved are important forcing mechanisms for the global climatic system. With a drainage area of around 1,200,000km 2 (Torgersen et al., 1983), the Gulf of Carpentaria and its associated drainage basins cover at least three climatic zones (utilising the system proposed in Stern et al., 2000 and based on the Köppen Climate Classification System). The northeastern portion is classified as equatorial savanna, while most of the south and west are considered tropical savanna, with a small section of hot climate (winter drought) grassland in the southern area of the catchment. The similarity between the above zones, which are based on temperature and precipitation as well as ground cover, is that they reflect the influence of a classical monsoon climate. A monsoonal climate dominates most of northern Australia, including the Gulf of Carpentaria the majority of the annual rainfall for the region occurs in the four summer months December to March, although the monsoon season can last 12

28 Chapter 1 - Introduction WPWP Figure 1.7 Location of the Western Pacific Warm Pool (WPWP), where average SST is >28 C as defined by Yan et al. (1992). The solid line represents a merging of information from Martinez et al. (1997) and Gagan et al. (2004). Where these accounts differ around the Gulf of Carpentaria, the dashed line represents Martinez et al. (1997) and the dotted line Gagan et al. (2004). 13

29 Chapter 1 - Introduction from a period of two weeks to four months. The Australian summer monsoon (also called the north-west monsoon) is linked to the position of the Intertropical Convergence Zone (ITCZ). The ITCZ, or equatorial trough, is a broad convergence zone of easterly air-flow from the subtropical high pressure cells in each hemisphere. In the southern hemisphere summer the ITCZ migrates south, due to a summer increase in heat radiation off continental Australia (causing a low pressure cell to form in the Pilbara region). There it lies over the southern reaches of the Gulf of Carpentaria, where it is also termed the Australian monsoon trough. North-westerly near-surface flows bring moist air and rainfall, constituting the Australian summer monsoon. Cooling atmospheric temperatures eventually cause a high-pressure cell to form over Australia, pushing the ITCZ back north, and ending the monsoon season over the Gulf of Carpentaria. In association with the ITCZ, the Hadley and Walker circulations provide much of the variability in weather for the Gulf of Carpentaria. The Hadley cell, or trade-wind flow, is primarily due to the pressure gradient between the subtropical high and the low pressure area of the ITCZ and WPWP (Fig. 1.8). Movement of the ITCZ south during the Australian summer monsoon weakens the trade wind flow in the Australian region. The Walker Circulation is driven by temperature differences in the east and west tropical Pacific oceans (Fig. 1.8) and flows along the ITCZ. Fluctuations in the intensity of the Walker and Hadley circulations are termed El Niño-Southern Oscillation (ENSO) events. The interactions between all these climatic systems are responsible for the climatic regime experienced by the Gulf of Carpentaria at present. However, oceanic and atmospheric conditions were considerably different during the last glacial cycle. An absence or weakening of the Australian monsoon is suggested for the LGM, with the establishment of present-day conditions contentious both in the chronology and in the methodology of establishment. 14

30 Chapter 1 - Introduction Walker Circulation South Pacific Ocean Hadley Cell Figure 1.8 Schematic diagram of atmospheric circulation over the South Pacific Ocean, showing the relationship between the Walker Circulation and Hadley Cell over the maritime continent of Indonesia. Such dramatic changes in precipitation should be reflected in the expansion and contraction of the palaeo-lake Carpentaria. The six cores analysed in this study (Figure 1.2) are well-placed to record fluctuating lake levels. Their transect includes a large portion of what would have been sub-aerially exposed and lake-margin environments, as well as the deeper constantly inundated basin centre The continuous presence of micro-organisms The major proxy used to infer past environments of the Gulf of Carpentaria in this study is the foraminifers which were living in Lake Carpentaria and surrounding oceans. Foraminifers are unicellular aquatic organisms, the majority of the taxa are marine, but some species can inhabit fresher waters. The comparison of the total assemblages of species found in the fossil record with modern examples provides an analogue to past environments (Chapter 3). 15

31 Chapter 1 - Introduction Some species of foraminifers may live in strictly defined environments, but most are adapted to tolerate some range in environmental parameters. Therefore, for reliable palaeoclimatic interpretations, it is crucial to include the whole assemblage in any analyses, noting species composition, diversity and abundance. Only a few species are adapted to live in non-marine waters, while the greatest diversity of species occurs in waters of marine salinity. From the LGM Lake Carpentaria to the present epicontinental sea, the deeper areas of the gulf were constantly under water (Jones and Torgersen, 1988; De Deckker et al., 1988; Torgersen et al., 1988). Consequently, a continuous presence of aquatic organisms (e.g. Foraminiferea) is found preserved in the sediments of the deeper cores. The continuous presence of the foraminiferal proxy variable provides an excellent opportunity for palaeoclimatic reconstructions. 1.3 Previous related investigations Comparing investigations An important consideration when comparing previous investigations is the difference in dating methods and their corrections. Conventional radiocarbon ages are reported in years before present (BP), where present is 1950 AD, and are calculated on the assumption that the production of 14 C in the past has been constant. To facilitate comparisons between other dating methods, a correction is made to conventional radiocarbon ages, and they are reported in calendar years before present (cal BP). Material from this thesis is reported in cal BP. In the following paragraphs the uncalibrated carbon dates are presented as published by their respective authors, with a figure following in parentheses which approximates to the calibrated age, in order that they may all be compared. Note that the calibrated ages provided are only an approximation as not all of the original authors data necessary for proper calibration were available. The calibration to calendar ages (only possible on dates younger than 24ka cal BP) was performed with the program Calib v4.4 (Section 4.4 and Section 5.5 of this thesis). 16

32 Chapter 1 - Introduction Previous results Phipps (1966) was the first to note the transition from non-marine to marine conditions recorded in cores taken in the Gulf of Carpentaria. Although attributing the transition to the rise in sea-level at the end of the Pleistocene, the radiocarbon dates obtained and facies interpretation were "anomalous" in relation to the then known sea-level curve (Phipps, 1970). Bulk sediment was used for dating, but recent studies on gulf sediments by Head et al. (1999) have shown that anomalously young organic carbon is present to depths of over one metre, suggesting that dating should only be performed on individual calcareous fossils. Also confusing the facies interpretation is the fact that monospecific populations of the foraminifer Ammonia beccarii, interpreted by Phipps (1970) as indicating brackish shallow marine conditions, may also occur in non-marine environments. Based in part on the work of Phipps (1970), and relying only on bathymetric contours, Nix and Kalma (1972) postulated the existence of a lake within the enclosed -53m contour of the Gulf of Carpentaria (bathymetric contours shown in Fig. 1.1). They deduced the presence of a shallow, fluctuating, brackish lake, possibly contained at 20ka BP (24ka cal BP), drying between 17-14ka BP (20-17ka cal BP), and later drowned with the post-glacial rise in sea-level. The occurrence of this lake was verified by Smart (1977), who matched a sealevel record (Bloom et al., 1974) to 14 C-dated sediments from the Gulf of Carpentaria. It was suggested that from 47-26ka BP the basin was isolated from marine influence and consequently a lacustrine facies developed. A possible sea-level rise and marine connection was proposed between 26-23ka BP, and a subsequent fall in sea-level leading again to an enclosed basin between 23-11ka BP was postulated. The post-glacial marine transgression, when sealevel rose above the -53m Arafura Sill, was dated to 11ka or 12ka BP (13-14ka cal BP) by Smart (1977). Seismic evidence of the existence of Lake Carpentaria was presented by Torgersen et al. (1983). Directly below about 2m of Holocene sediments, up to one metre thick sedimentary layers were mapped "pinching out" at -53m and 17

33 Chapter 1 - Introduction -65m water depth, indicating the boundary of semi-permanent water bodies. It was suggested the -65m horizon could delineate the most recent maximum extent of Lake Carpentaria, while the -53m horizon could have been formed when sea-level rose above the -53m Arafura Sill and created an embayment within the gulf. However, Edgar et al. (2003) could not seismically distinguish such features in the upper few metres of sediment. Torgersen et al. (1983) also observed a submarine channel west of the Arafura Sill that had not been completely infilled at present, suggesting that outflow channels from Lake Carpentaria cut through the Arafura Sill and may have joined up with the channel to the west. This major channel, stretching across the floor of the Arafura Sea to the edge of the Australian continental shelf and to the Indian Ocean, was termed the Arafura Channel by Grim and Edgar (1998). Edgar et al. (2003) state that the Arafura Channel was probably the main drainage conduit from Lake Carpentaria during the last sea-level lowstand, channelling most sediment and water into the Indian Ocean. Palaeochannels within the Gulf of Carpentaria basin were mapped by Edgar et al. (2003) over at least 17 transgressive/regressive cycles. Note that seismic line 17, which is utilised prominently by the authors to illustrate channels, overbank deposits, and other features, runs very close to the cores MD 30, MD 32 and MD 33 of this thesis (Fig. 1.2). Formed under low-gradient, sub-aerial conditions, these channels are infilled from depths of a few metres to 80m. Their infill by acoustically transparent material indicates possible estuarine origin, and point-bar deposits recorded within many of the channels indicate fluvial and/or tidal deposits. There is also evidence of cut and fill of channels within a transgressive/regressive cycle, indicating variable water flow through the channels. Meandering channel systems are noted, with line 17 apparently cutting the same channel three times, with acoustically transparent overbank deposits between each channel. The preliminary results from a 1982 coring cruise (Torgersen et al., 1985) identified the brackish lacustrine deposits of the fluctuating Lake Carpentaria, dating to at least 35ka BP. Cores GC 2 and GC 10A from this cruise are important to later studies and are shown in Figure 1.2. Jones and Torgersen 18

34 Chapter 1 - Introduction (1988) utilised core material (including cores GC 2 and GC 10A) and seismic information from the 1982 cruise to provide further detail on the lake's hydrological balance. The presence and nature of the channels connecting Lake Carpentaria to the sea are of major importance to the viability of the lake. The shallowest margin of the basin is the Arafura Sill, a possible wave- and wind-built dune structure (postulated by Torgersen et al., 1983; confirmed by Edgar et al., 2003). The Arafura Sill was found by Jones and Torgersen (1988) to have a number of incised channels the limiting factor for Lake Carpentaria's formation around the LGM being channels eroded to about -62m depth (with channels up to 75m deep formed in the previous cycle of transgression and regression). The regression in sea-level below the deepest channels produced an enclosed basin and, within it, Lake Carpentaria. Jones and Torgersen (1988) found the lake was maintained by fluvial waters as a continuous presence since the LGM, with a water level perhaps reaching -57m (i.e. with water levels in the -62m channel up to 5m deep). A marine influence may have been present as early as 11ka BP (13ka cal BP) when the sea-level rose high enough to infiltrate the lake through the -62m channels. The authors suggest a sea-level of -45m was necessary for the establishment of fully marine conditions in the gulf by 9.5ka BP (10.7ka cal BP). Deposition of sediments during the post-glacial marine transgression increased the height of the lowest relief within the Arafura Sill to its present -53m. Edgar et al. (2003) corroborated the findings of Jones and Torgersen (1988) that over successive transgressive/regressive cycles, the Arafura sill has been excavated then filled in by streams draining and filling the gulf. Further sedimentary, geochemical, palaeontological, and palynological data from the 1982 cores (mainly from core GC 2) were reported by Torgersen et al. (1988). Note that MD 33 from the present study is situated close to GC 2 (Fig. 1.2). From the analysis of GC 2, Torgersen et al. (1988) suggested Lake Carpentaria was a large (>29,000km 2 ) shallow (<10m) fresh to brackish body of water from about 35-12ka BP. Consideration of the available data indicates Lake Carpentaria may have actually been closer to 180,000km 2 in size 19

35 Chapter 1 - Introduction (Chivas et. al., 2001). At around 35ka BP the presence of either a low-salinity estuary or lake is suggested by Torgersen et al. (1988), with microfossil assemblages recording no connection to the open sea. The following broad conclusions are drawn from their work: A moderately wetter climate is indicated from palynological data for the interval up to 26ka BP. Around 26ka BP, Lake Carpentaria was permanent to the -63m contour and periodically extended to -57m. Geochemical analysis shows a change in water chemistry and increase in salinity at about 23ka BP. The saline lake was strongly evaporative and became more seasonal in nature, as the precipitation of calcite around this time attests, before establishment of a permanent connection to the sea after 11.8ka BP (13.5ka cal BP) (although the estuarine/transitional sequence indicates still significant freshwater input at that period). The return to fully marine conditions was dated at around 8ka BP (8.8ka cal BP). Geochemical studies (Sr/Ca, Mg/Ca) of ostracod carapaces (De Deckker et al., 1988) from the 1982 cores (including cores GC 2 and GC 10A) investigated the fluctuating salinities of Lake Carpentaria. Sr/Ca values tend to reflect the host body of water, with higher values (consistent with the precipitation of lowstrontium and low-magnesium calcite) occurring in a lacustrine system (Chivas et al., 1985). The Mg content of the shell is a function of both temperature and Mg content (related to salinity) of the water (Chivas et al., 1986). Values indicative of lacustrine conditions were found from around 40ka BP to around 13ka BP (15.5ka cal BP) by De Deckker et al. (1988). Between around 26-13ka BP the authors state unusually high Sr/Ca and Mg/Ca values indicate the effects of a differing water chemistry, with less calcium available during the biological calcification of the ostracod carapaces. The analysis of 87 Sr/ 86 Sr on individual ostracod valves from the Gulf of Carpentaria cores GC 2 and GC 10A (McCulloch et al., 1989) was used to make palaeoenvironmental inferences, based on the fact that Sr isotope values reflect the composition of the water mass in which they have formed. Holocene marine water has a low 87 Sr/ 86 Sr ratio, whilst the highest values are obtained from continental waters, especially those from an "older", more radiogenic, landmass (e.g. Australia). Prior to about 35ka BP, the authors suggest a brief 20

36 Chapter 1 - Introduction incursion of marine waters over the Arafura Sill, which reduced the Sr isotope values but did not significantly alter the lacustrine environment of Lake Carpentaria. For the period up to around 13ka BP (15.5ka cal BP) (when the Arafura Sill was breached) Sr isotope values are indicative of lacustrine waters. A marked increase in 87 Sr/ 86 Sr values just before 13ka BP suggests the lake received an increase in the proportion of runoff from northern Australia relative to New Guinea. McCulloch et al. (1989) suggest that at about 12ka BP (14ka cal BP), the connection to the open ocean over the Arafura Sill was completed, although the limited circulation within the gulf created transitional estuarinemarine conditions. These existed until about 7-8ka BP ( ka cal BP) when Torres Strait was transgressed and a fully marine water body was established. However, colonisation of the Gulf of Carpentaria by aquatic marine organisms (shrimp larvae) was shown by Rothlisberg et al. (1983) to be an ongoing process still occurring today. Penecontemporaneous change of sediments, water mass and biota from lacustrine to marine is unlikely. The major influence on sediment trace-metal composition, in core GC 2 examined by Norman and De Deckker (1990), is physical mixing of the two chemical end-members (marine and continental) visible in the core. Terrigenous clay overlain by a more biogenic carbonate facies marks the transition from lacustrine to oceanic environments. Surficial sediments of the Gulf of Carpentaria were analysed by Cox and Preda (2003), with Zn and Cr the dominant trace metals found. Cox and Preda (2003) state that sediment grain size and composition has a major effect on trace element distribution (e.g. most trace metals are found in the central and north-western areas of the gulf where fine-grained Fe-rich sediments occur). Deposition of eolian dust in GC 2 was studied by De Deckker et al. (1991) and De Deckker (2001). Between 35-10ka BP (i.e. during the existence of Lake Carpentaria) the authors found the deposition of this dust to be on a 2.6ka periodicity, with maximum aridity at the LGM (21.5ka cal BP). Other episodes of peak dust deposition did not appear to correspond to cooler events in the northern hemisphere. The authors propose the cyclic drier periods could be evidence of the intensification of the eastern trade winds over northern 21

37 Chapter 1 - Introduction Australia, causing a change in atmospheric circulation, which would result in a temporary failure of the monsoon over the Gulf of Carpentaria. At least three submerged coral reefs, found via sonar by Harris et al. (2004), occur in the south eastern Gulf of Carpentaria. The three reefs are situated on the sea floor on around 50m water depth, and are built up to a maximum 28m bpsl. Although mostly relict surfaces, they have a thin veneer of living coral, but the reefs have never caught up to present sea-level due to insufficient Holocene coral framework deposition and/or delayed recolonisation. Two extensive surfaces on the reef flats (of 27.3±1.4m and 30.6±0.58m) are considered by Harris et al. (2004) to be palaeosea-level indicators, occurring on more than one reef. The morphology and depth of the reef surfaces indicate that they formed mainly when sea-level was 25-27m below its present position. A stabilisation of sea-level at this height during the most recent marine transgression and/or in previous highstands (50-120ka cal BP) is implied Synthesis of previous investigations All studies of the palaeoenvironment of the Gulf of Carpentaria have revealed a general trend of lacustrine conditions transgressed by the post-glacial rise in sea-level, culminating in the present open shallow marine environment. Lake Carpentaria existed from at least 40ka BP, until the breach of the Arafura Sill around 12 ka BP (14ka cal BP). Prior to 35ka BP a brief rise in sea-level may have influenced the waters of Lake Carpentaria (McCulloch et al., 1989). From 35ka BP to 11ka or 12ka BP (13ka cal BP or 14ka cal BP) Lake Carpentaria was a low-salinity lake, isolated from ocean waters, and permanent at least to the -63m contour (Jones and Torgersen, 1988; Torgersen et al., 1988). From 26ka BP until the breach of the Arafura Sill, the lake waters periodically extended to the -57m contour (Torgersen et al., 1988). A change in water chemistry lead to a depletion of calcium carbonate available for shell formation through calcite precipitation (dated at around 26ka BP by De Deckker et al., 1988). This process may have also been associated with a rise in salinity (dated at 23ka BP by Torgersen et al., 1988). The first indication of the most recent marine transgression occurred 22

38 Chapter 1 - Introduction when ocean waters infiltrated through to the lake through the channels of the Arafura Sill, recorded as a transitional estuarine facies. This has been dated at 11ka BP (13ka cal BP) by Jones and Torgersen (1988), but McCulloch et al. (1989) propose an earlier date of 13ka BP (15.5ka cal BP), which seems to agree more closely with other authors findings that the connection to the open ocean was permanently established by about 12ka BP (14ka cal BP) (Smart, 1977; Torgersen et al., 1988; McCulloch et al., 1989). Sedimentary facies indicating the establishment of marine conditions prior to the breaching of Torres Strait are recorded by Jones and Torgersen (1988) at 9.5ka BP (10.7ka cal BP), when sea-level was -45m. As sea-level rose over the Arafura Sill and the gulf became an embayment, it may have slowed down at around 25-27m bpsl, creating the reef flat surfaces noted by Harris et al. (2004). The return to fully marine conditions with the transgression of Torres Strait has been dated to around 7-8ka BP ( ka cal BP) (Torgersen et al., 1988; McCulloch et al., 1989). However, it must also be noted that aquatic marine species may not have fully colonised the gulf when marine conditions were first established (Rothlisberg et al., 1983). Although previous studies have provided some basic information on the existence on Lake Carpentaria from the LGM until its inundation by the postglacial marine transgression, much refinement is needed in the extent and timing of the fluctuation in lake levels and of the transgression. In this study, the use of individual microfossils for AMS radiocarbon dating revises and constrains some of the earlier findings (Chapters 5 and 6). 1.4 The current project Previous studies had demonstrated the viability and relevance of sediment cores from the Gulf of Carpentaria to palaeoenvironmental reconstructions. A large, multidisciplinary project was conceived to investigate longer records, in more detail and with a variety of proxy records. This thesis is part of the larger multidisciplinary project on the Gulf of Carpentaria, partly funded by two grants from the Australian Research Council, and headed by Allan Chivas (University of Wollongong). A number of other organisations, institutions and individuals are also involved, including Patrick De Deckker (Australian National University) and 23

39 Chapter 1 - Introduction Sander van der Kaars (Monash University). The majority of their initial work is contained in Chivas et al. (2001). In 1993/94 a high-resolution ship-borne seismic survey of the Gulf of Carpentaria was conducted by the United States Geological Survey, covering 4444km. About 17 reflectors (indicating transgressive/regressive events and/or erosional unconformities) occur (Fig. 1.5), associated with large (up to 80m deep) palaeochannels (Chivas et al., 2001; Edgar et al., 2003). Consideration of these data indicated where sediment cores of the Gulf of Carpentaria might be best taken to capture such palaeoenvironmental information. Six cores were collected in 1997 by the research vessel Marion Dufresne in a joint Australian/French/USA cruise through the gulf, and as part of IMAGES III (International MArine Global change Study). The cores, MD through to MD , range from 4.2 to 14.8m in length, and were collected in water depths ranging from about -59m to -68m in order to provide a representative transect across the palaeolake from depocentre to lake edge. The positions of cores MD to MD , referred to as cores MD 28 to MD 33 throughout the thesis, are shown in Figure 1.2. Material from these cores forms the base of this study, and is stored at the University of Wollongong. Chapter 4 details the methods employed in the analysis of these cores. In this project, the longest core, MD 32, has been most extensively studied, providing a palaeoenvironmental record of the past 130ka (Chivas et al., 2001; Reeves, 2004). It demonstrates the existence of the non-marine Lake Carpentaria during the previous glacial maximum and associated sea-level lowstand around 130ka BP. The rise in sea-level to the Last Interglacial, and the subsequent lowering of sea-level to the LGM, are recorded as a fluctuating marine and non-marine sequence, especially significant due to numerous oscillations of sea-level around the height of the Arafura Sill. A micropalaeontological record of the top 150cm of all the six cores is presented in this thesis, spanning a maximum age of about 40ka. Foraminiferea are the main focus, but other organisms such as molluscs, echinoids, ostracods 24

40 Chapter 1 - Introduction and charophytes are noted for their palaeoenvironmental indications, and sedimentological data are considered (Chapter 5). The detailed analysis of the top 150cm of all cores is designed to provide the maximum amount of information on fluctuations in the extent of the palaeolake around the LGM until its inundation by the post-glacial marine transgression (Chapter 6) Thesis aims The broad aim of this thesis is to delineate the palaeoenvironmental history of the Gulf of Carpentaria from around the Last Glacial Maximum to the present. Micropalaeontological data (particularly foraminiferal species and assemblage data enabling the allocation of facies) from all six cores are presented, to a core depth of 150cm. Two major areas of investigation were pursued: sea-level change, utilising sedimentary and foraminiferal/ micropalaeontological facies changes, and climatic events recorded in the rainfall/evaporation budget of Lake Carpentaria, utilising sedimentary and foraminiferal/ micropalaeontological facies changes. The thesis is divided into six chapters, the first giving an introduction to the setting of the Gulf of Carpentaria and previous geologic and palaeoclimatic investigations. The second chapter gives a background of global and regional climatic and oceanic changes during the late Quaternary. The third chapter introduces foraminifers the organisms upon which the present palaeoclimatic study is based. Chapter 4 details the methods used in the present study. Chapter 5 presents the results for each of the six cores studied, from the initial micropalaeontological, sedimentological and 14 C data to the facies determined. The sixth chapter discusses the results in relation to the palaeoenvironment of the whole Gulf of Carpentaria, and examines implications of this study for the global and regional records of climatic and oceanic change. 25

41 Chapter 2 Palaeoclimate and Sea-level CHAPTER 2 Palaeoclimate and Sea-level change 2.1 Introduction The environment of the Gulf of Carpentaria is affected by local phenomena as well as global changes in climate. This chapter provides information on local and global climate and sea-level change for the studied period. Mechanisms for climate change are outlined in Section 2.2, and the components are considered separately in Sections Changes in sea-level (Section 2.3), oceanic circulation and composition (Section 2.4), land and sea temperature (Section 2.5) and atmospheric circulation (Section 2.6) are examined. Areas needing further research are mentioned in Section 2.7, and the rationale of this thesis is explained considering these points. 2.2 Global overview Global climate change is a result of a number of influences, including the sun, greenhouse gases, and the internal variability of the climate system. The system is in a state of continuous flux: from the extremes of massive continental ice-sheets to almost ice-free poles. Interaction between variations in atmospheric CO 2 and solar radiation has been proposed as the driver of long-term (Phanerozoic) climate change (e.g. Berner, 1994). Variations in the Earth's eccentricity of orbit, axial tilt, and orbital precession lead to differences in the seasonality of solar radiation reaching the Earth's surface (termed the Milankovitch Cycles). The response of the climate system to solar forcing depends not only on the amount of radiation, but also on its seasonal distribution over the globe, and feedback mechanisms connected with clouds, water vapour, ice cover, atmospheric and oceanic transport and other terrestrial processes (Beer et al., 2000). Ocean-atmosphere interactions in the tropics are considered by many authors to hold the key to explaining the last glacial cycle (e.g. Cane, 1998). Cane (1998) postulates that orbital forcing and internal forces may cause unstable ocean-atmospheric interactions in the tropical Pacific, altering sea surface temperatures (SST), influencing 26

42 Chapter 2 Palaeoclimate and Sea-level atmospheric convection in the area, and ultimately altering the global climate through teleconnections such as the present El Niño Southern Oscillation (ENSO) circulation. Tropical sea surface temperatures may also influence global climate by the direct association between lower temperatures and reduced evaporation, and therefore a decrease in greenhouse gasses leading to global cooling (Lee and Slowey, 1999). However, Kukla and Gavin (2004) suggest that warming tropical oceans were the principal cause of Quaternary glaciations, as the increase in the equator-to-pole temperature gradient intensified the hydrological cycle and led to increased ice sheets in high latitudes. The process of oceanic circulation in a variable climatic system may generate further change; due to the different response times of the surface mixed layer and deep-ocean, high-frequency random fluctuations in climatic forcing may generate responses at lower frequencies (Gaffin et al., 1986). The Quaternary Period has been characterized by massive shifts in climatic conditions, with global temperature changes and the cyclic growth and decay of ice sheets causing sea-level changes and reorganizations of ocean/atmosphere circulation. The most recent glaciation is, at its height, termed the Last Glacial Maximum (LGM). Problems occur in the definition of the LGM, as transitions into and out of the LGM climate state may have occurred at different times in various places. Environmental Processes of the Ice age: Land, Oceans, Glaciers (EPILOG) have defined the LGM as the most recent interval when global ice sheets reached their maximum integrated volume during the last glaciation (Mix et al., 2001). This is centred around 21-20ka cal BP and is widely believed to have terminated around 19ka cal BP (e.g. LGM from 22-19ka cal BP, Yokoyama et al., 2000). Mix et al. (2001) state that this period of maximum ice sheet volume extends from 23ka to at least 19ka cal BP, but Lambeck and Chappell (2001) and Lambeck et al. (2002) place the initiation of maximum ice sheet volume earlier, at around 30ka cal BP. Expanded ice sheets during the LGM lowered sea-level by at least 120m (CLIMAP, 1981), with modern estimates around 125±4m (e.g. Yokoyama et al., 2001a). The expansions and contractions of ice sheets influence global climate by affecting the planetary albedo, atmospheric and ocean circulation, and the 27

43 Chapter 2 Palaeoclimate and Sea-level hydrological cycle (Clark and Mix, 2002). In the Gulf of Carpentaria region, lowered sea-level during the last glacial period reduced marginal seas and led to the exposure of continental shelves. The altered land/sea distribution in turn affected oceanic circulation and heat and moisture exchange in the entire region. The warm and shallow marginal seas in the tropical west Pacific transport a large amount of moisture into the atmosphere, and the lack of these areas during the LGM reduced precipitation in the area. Earlier studies (CLIMAP, 1976) assumed that, while high-latitude temperature depression was 10 C or more during the LGM, tropical areas had only cooled by about 1 C. However, many further studies on a variety of proxies have refuted the earlier hypothesis, and many authors propose that tropical sea surface temperature decreased by about 2 to 3 degrees Celsius (e.g. using oxygen isotopes on foraminifers, Curry and Oppo, 1997). The decrease in tropical SST may have been as much as 6 C at the LGM (e.g. Sr/Ca ratios of corals, Gagan et al., 1998). Temperature estimates vary between proxies studied, models used, regionally, and between land and sea. After the LGM, the deglaciation was a time of climatic fluctuations. There was obviously an increase in temperature as the ice-sheets melted, although this temperature increase was not of a uniform pace, and certainly not of a similar level world-wide. Similarly, the increase in atmospheric moisture was not experienced as an increase in precipitation in all areas of the globe. The Australian monsoon is generally thought to have weakened during the LGM, and strengthened considerably only during the Holocene. However, the date of onset varies according to the region or the proxy examined. Most estimates of onset are centred around 16-11ka cal BP, and a possible onset or strengthening event is also recorded around 6.5-5ka cal BP. The following overview of the literature examines the climatic fluctuations of the Quaternary Period from the LGM to the present in the region of the Gulf of Carpentaria. 28

44 Chapter 2 Palaeoclimate and Sea-level 2.3 Changes in sea-level Expansion and contraction of ice sheets is the major influence on global sealevel, and the component of sea-level change due to ice volume alone is termed eustatic. However, many sites record variations in sea-level due to local crustal movement, collectively named glacio-hydro-isostatic (i.e. movement due to ice and water). The local sea-level is termed relative sea-level (RSL). Sea-level data from sites far from former ice sheets are considered to broadly indicate eustatic levels. Eustatic sea-level changes are yet to be exactly determined for the LGM, although a review of data by Peltier (2002) confirms the accepted view of a LGM low stand of at least 120m below present sea-level (bpsl). Studies have shown a net decrease in global sea-level at the LGM ranging from 120m to greater than 150m (e.g. 120m CLIMAP, 1981; m Jongsma, 1970), although most authors place the LGM sea-level between these two extremes. During Marine Isotope Stage 3 (MIS 3, around 60-30ka cal BP), sea-level was m lower than today (Chappell et al., 1996; Waelbroeck et al., 2002). Uplifted coral terraces from the Huon Peninsula (Fig. 2.1) indicate sea-level approached its maximum low-stand, marking the beginning of the LGM, about 30ka cal BP (Lambeck and Chappell, 2001). This is supported by micopalaeontological data from the Bonaparte Gulf (Fig. 2.1), investigated by Lambeck et al. (2002). Lambeck et al. (2002) state that ice volumes increased only slowly during the next 10,000 years after 30ka cal BP. The same Bonaparte Gulf cores provide more detailed micropalaeontological data from the centre of the LGM (22-19ka cal BP), allowing Yokoyama et al. (2000, 2001a) to reconstruct relative sea-levels. At the LGM, RSL is considered to have been between -125m and -121m (Yokoyama et al., 2000), or -125±4m (Yokoyama et al., 2001a). The authors state that the stability of the Australian tectonic plate ensures little glacio-hydro-isostatic adjustments need be performed on the data to reflect global eustatic changes. However, in comparison with other studies, and correcting for isostatic effects, the authors find the LGM glacio-eustatic sea-level to have been between -135m and -130m (Yokoyama et al., 2000), or -135±4m (Yokoyama et al., 2001a). 29

45 Chapter 2 Palaeoclimate and Sea-level South China Sea Sulu Sea Pacific Ocean 20 Indian Ocean Coral Sea 5 7 Figure 2.1 Location of sites, mentioned in the text, providing a palaeoclimatic record of sea-level, oceanic circulation and temperature in the region since the LGM. 1: Huon Peninsula (Lambeck and Chappell, 2001; Chappell and Polach, 1991; Edwards et al., 1993), 2: Bonaparte Gulf (Lambeck et al., 2002; Yokoyama et al., 2000, 2001a); 3: Sunda Shelf (Hanebuth et al., 2000), 4: Gulf of Carpentaria (Torgersen et al., 1988; Chivas et al., 2001; De Deckker et al., 1991), 5: Vanuatu (Cabioch et al., 2003; Gagan et al., 2000), 6: Great Barrier Reef (Chappell, 1983; Larcombe et al., 1995), 7: Port Hacking (Baker and Haworth, 2000), 8: Coral Sea (Corrège and De Deckker, 1997), 9: off Philppines (Stott et al., 2002), 10: off Sumatra (Gingele et al., 2002; De Deckker and Gingele, 2002), 11: North West Cape (Wells and Wells, 1994; van der Kaars and De Deckker, 2002), 12: Leeuwin Current cores (Gingele et al., 2001), 13: North West Shelf (James et al., 2004), 14: Okinawa Trough (Shieh et al., 1997), 15: Wanda Swamp (Hope, 2001), 16: Bandung (van der Kaars and Dam, 1995), 17: South China and Sulu seas (Thunell and Miao, 1996), 18: South China Sea (Steinke et al., 2001; Waelbroeck and Steinke, 2002), 19: off Sepik River (McGregor et al., 2002) 20: Ontong Java Plateau (Lea et al., 2000). Note: although specific sites are marked (either as points, representing cores, or lines, representing transects), some may have two numbers indicating different sites within the area of resolution. Repetition of numbers indicates more than one site for that study. 30

46 Chapter 2 Palaeoclimate and Sea-level Waelbroeck et al. (2002) agree with Yokoyama et al. (2000, 2001a) that in sites distal from former ice sheets RSL may be equated with eustatic sea-level, and utilise this approximation in their study. The composite RSL curve constructed by Waelbroeck et al. (2002) from global studies utilises data from Yokoyama et al. (2001a). Waelbroeck et al. (2002) subtract 10m from the curve of Yokoyama et al. (2001a) in order to convert the data to their version of RSL (i.e. approximately proportional to eustatic sea-level), giving a LGM low-stand of around 135m bpsl. It must be noted that this figure of -135m corresponds with the glacio-eustatic sea-level of -135±4m of Yokoyama et al. (2001a). This thesis compares results to the sea-level curve of Yokoyama et al. (2001a), as it best reflects local conditions in the Gulf of Carpentaria, as well as providing most detail for the time period since the LGM. At around 19ka cal BP (Yokoyama et al., 2000) sea-level rose at a rapid rate. Data from coral reef terraces on the Huon Peninsula (Chappell and Polach, 1991; Fig. 2.1) and micropalaeontological records from Bonaparte Gulf (Yokoyama et al., 2001a) constrain this rise as up to 30mm yr -1, for around 500 years (Yokoyama et al., 2000, 2001a). Yokoyama et al. (2000) note that after the 19ka cal BP rapid rise, sea-level rose at a much slower rate for the next years, before the onset of the main phase of post-glacial melting after 17ka cal BP. In a comparison of sea-level data from seven different regions (including Bonaparte Gulf), Lambeck et al. (2002) estimate that sealevel rose at about 3.3mm yr -1 between 19-16ka cal BP, and that the main phase of post-glacial melting lasted from ka cal BP with sea-level rising about 16.7mm yr -1. A period of rapid sea-level rise (termed meltwater pulse 1A or MWP 1A) has previously been recognised around 14ka cal BP (e.g. from U/Th of corals in Barbados, Fairbanks, 1989 and Bard et al., 1990). Cabioch et al. (2003) note that MWP 1A is apparently best seen in corals from tectonically active island arcs. In the sedimentary record of the Sunda Shelf (Fig. 2.1), MWP 1A is identified as lasting from ka cal BP (Hanebuth et al., 2000), although difficulties in calibrating 14 C mean the evidence is not certain. In the Bonaparte Gulf cores (e.g. Yokoyama et al., 2000) no evidence of MWP 1A is noted. 31

47 Chapter 2 Palaeoclimate and Sea-level Lambeck et al. (2002) also do not find enough evidence in their review of data to confirm the occurrence of MWP 1A. During the lower sea-levels of the LGM and post-glacial period, the Gulf of Carpentaria (Fig. 2.1) was separated from the open ocean by land bridges between Australia and New Guinea presently 53m bpsl (Arafura Sill) and 12m bpsl (Torres Strait). The rise in sea-level breaching the Arafura Sill has been dated to around 12ka BP (uncalibrated 14 C, Torgersen et al., 1988), while the latest research on the Gulf of Carpentaria indicates an age of 9.7ka BP (uncalibrated 14 C, Chivas et al., 2001) for the most recent marine transgression over the Arafura sill. Calibrated into calendar years, these are respectively around 13.5ka cal BP, and ka cal BP. A stabilisation, or lower rate of rise, of sea-level around the Younger Dryas has been noted in various studies (e.g. Edwards et al., 1993) based on corals from the Huon Peninsula; Fig. 2.1). Lambeck et al. (2002) also note a possible short plateau in the rate of sea-level rise from ka cal BP, corresponding to the Younger Dryas cold spell. A return to rapidly rising sea-level after the Younger Dryas is suggested by many authors (e.g. meltwater pulse 1B of Fairbanks, 1989). Cabioch et al. (2003) note that between ka cal BP, coral growth on the Vanuatu reef studied (Fig. 2.1) was unable to keep up with sea-level rise, supporting the existence of MWP 1B, with sea-level rising at a rate of 23mm yr -1. However, Lambeck et al. (2002) place the sea-level rise after the Younger Dryas at a fairly uniform rate of about 15.2mm yr -1 until about 8.5ka cal BP. Nakada and Lambeck (1989) point out that although northern hemisphere icesheets had mostly melted by 7ka cal BP, and eustatic sea-level had reached near-modern levels, it was still a few meters below the present level, suggesting melting of the Antarctic ice sheet as a source of the additional ocean volume. A general trend in the Australasian region is of a RSL high-stand of a few metres above present in the mid-holocene (i.e. around 6ka cal BP), falling to present levels in the late-holocene. However, Chappell et al. (1982) found mid- 32

48 Chapter 2 Palaeoclimate and Sea-level Holocene high-stand levels ranging from near zero to 4m above present around Australia. As Chappell et al. (1982, 1983) noted, tilting of continental margins due to increased water load from the post-glacial rise in sea-level ensures different sites record widely different RSL values. In addition, Nakada and Lambeck (1989) note the variable times of the RSL high-stand and subsequent fall to present levels throughout Australia and the South Pacific region. Controversy exists as to whether the rise in sea-level since the LGM has been oscillating or smooth. A review of data from the Australian region by Hopley (1987) indicates that within a one metre envelope, sea-level has risen smoothly since around 12ka BP (uncalibrated 14 C, ~ ka cal BP) to a mid-holocene still-stand, reaching modern levels around 6ka BP (uncalibrated 14 C, ~6.5ka cal BP). Although, Hopley (1987) notes that relative levels may have varied by up to 5m. Coral evidence from the northern Great Barrier Reef (GBR; Fig. 2.1) indicates smoothly falling sea-levels since a mid-holocene high-stand (Chappell, 1983). However, Larcombe et al. (1995) found that intertidal indicators (such as in-situ oyster beds) gave the most consistent sea-level curve for the GBR (Fig. 2.1) during the Holocene, and postulate fluctuating RSL from this data. Larcombe et al. (1995) found a stillstand or minor fall in rising sealevels at 10.5ka BP (~11.4ka cal BP), further falls at 8.5ka BP (~8.9ka cal BP) and 8.2ka BP (~8.6ka cal BP), and stillstands or minor falls in rising sea-level at 7.8ka BP (~8.2ka cal BP), 6ka BP (~6.3ka cal BP) and 5.5ka BP (~5.8ka cal BP). Also arguing the case for an oscillating Holocene sea-level, Baker and Haworth (2000) present biostratigraphical evidence from Port Hacking (Fig. 2.1) suggesting that RSL was around 1.7m above present from about ka cal BP, before a fall from around 3.5ka cal BP, and a possible 0.4m fluctuation around 1.9ka cal BP. 2.4 Changes in oceanic circulation and salinity The Coral Sea lies to the east of the Gulf of Carpentaria, connected via Torres Strait, while further east is the Pacific Ocean. Corrège and De Deckker (1997) show that salinity in the Coral Sea (Fig. 2.1) remained between about in the lead up to and during the LGM (around 50-20ka cal BP), dropping during the Holocene. 33

49 Chapter 2 Palaeoclimate and Sea-level The Western Pacific Warm Pool (WPWP, also called the Indo-Pacific Warm Pool) is the largest mass of warm ocean water on earth, and is defined as the region where SSTs exceed 28 C (Yan et al., 1992). The Gulf of Carpentaria is on the border of this zone. It is also a region of decreased sea surface salinity (SSS), as precipitation is higher than evaporation. Seasonal variation of salinity in the WPWP is about 1.5 at present. During the LGM, the lowered sea-level (with associated increased land surface in the Indonesian region) resulted in reduced monsoonal rainfall within the Indonesian Archipelago, causing the waters of the Pacific Ocean to increase in salinity. On the basis of foraminiferal oxygen isotope studies throughout the tropical western Pacific, Martinez et al. (1997) suggest that the WPWP could have had salinities around 1 higher than modern values, indicating lower effective precipitation in the region. The equatorial portion of the WPWP was found to experience less dramatic salinity change, with the highest salinities found furthest from the equator (Martinez et al., 1997). Seasonality may have also increased at the LGM, as Stott et al. (2002) note summer SSS up to 2 higher than today (from oxygen isotopes of the dominant summer foraminifera Globigerinoides ruber), with winter salinities near modern values (from G. sacculifer) near the Philippines (Fig. 2.1). Martinez et al. (1997) also noted longer-term fluctuations in SST and SSS in sites marginal to the WPWP, indicating its expansion and contraction throughout the Holocene at the scale of thousands of years. One such fluctuation (a change in isotopic values indicative of a decrease in SST and/or SSS) is noted by Linsley and Thunell (1990) in the Sulu Sea during the Younger Dryas, and is thought by Thunell and Miao (1996) to purely reflect salinity changes. Presently, north of the Gulf of Carpentaria, the Pacific and Indian oceans come together through the Indonesian Archipelago, and there is a net southward movement of water, transporting the warmer and less saline WPWP waters into the Indian Ocean. During the last glacial period, the throughflow from the Pacific to the Indian Ocean is thought to have been greatly restricted due to the sealevel low-stand (Martinez et al., 1997). The reduction of the Indo-Pacific throughflow during the LGM implies that lower-salinity waters from the Pacific Ocean did not contribute significantly to the Indian Ocean, while the throughflow was also more saline than present as the WPWP was more saline. Sedimentary 34

50 Chapter 2 Palaeoclimate and Sea-level analysis of a core south of Sumatra (Fig. 2.1) by Gingele et al. (2002) show a reduced amount of smectite from 35-20ka cal BP (indicating reduced throughflow transporting the material), with a smectite peak from 20-18ka cal BP. The peak in smectite is assumed by the authors to indicate increased velocity of the throughflow, although not necessarily increased volume. Analysis of the same core off Sumatra (Fig. 2.1) indicates a substantial increase in salinity near the surface of Indonesia s oceans during the glacial period and a possible stratification of the water column (De Deckker and Gingele, 2002). De Deckker and Gingele (2002) reveal that the onset of deglaciation is marked by the absence of the diatom Ethmodiscus rex and an increase in carbonate content from 19-12ka cal BP, possibly indicating better mixing of the water column. The demise of E. rex, and the presence of kaolinite in the core, demonstrate that by 12ka cal BP the present circulation pattern was established (Gingele et al., 2002). A decrease in carbonate content and increase in terrigenous material around 12ka cal BP is considered by De Deckker and Gingele (2002) to herald a decrease in salinity and the re-establishment of monsoonal conditions in the area. The Leeuwin Current, a warm water flow along the west coast of Australia, is directly linked to the Indo-Pacific throughflow. There is evidence (planktonic foraminifers off the North West Cape, Wells and Wells, 1994; clay composition in 3 cores along the Leeuwin Current path, Gingele et al. 2001; Fig. 2.1), for the decrease or absence of the Leeuwin Current during the LGM. The cessation of ooid formation (due to decreased salinity) on the Australian northwest shelf (Fig. 2.1) around 12ka cal BP is cited as evidence of the onset of the Leeuwin current after the LGM (James et al., 2004). Increased activity of the current since 5ka cal BP is indicated by the presence of Indonesian Pteridophyta pollen in a core off the North West Cape (van der Kaars and De Deckker, 2002; Fig. 2.1). Studies of oxygen isotopes of foraminifers from the Okinawa Trough (Fig. 2.1), led Shieh et al. (1997) to suggest that the modern global oceanic circulation pattern has been established since 8ka BP (uncalibrated 14 C, or ~8.5ka cal BP), as seen in the presence of a continuous influence of a warm water mass since 35

51 Chapter 2 Palaeoclimate and Sea-level that period. Referring to the area of the WPWP, De Deckker et al. (2003) state that the present oceanographic and climatic system may been active since 10ka cal BP. 2.5 Changes in temperature In the lead up to the LGM, Corrège and De Deckker (1997) found ostracod assemblage and geochemical data indicated bottom water temperature in the Coral Sea (Fig. 2.1) during MIS 3 to be similar to today s temperatures, and from this infer that SST was also similar to present. A cooling trend is noted beginning 39ka cal BP (Corrège and De Deckker, 1997). Pollen analysis of Wanda swamp (Fig. 2.1) by Hope (2001) reveals an increase in cooler species 35-17ka cal BP, which may also indicate lower precipitation or more seasonal climates than present. Temperature estimates for the cooling of the tropics during the last glaciation range from little or no change at sea-level (e.g. CLIMAP, 1976; CLIMAP, 1981) to a decrease of up to 10 C at higher altitudes (Bowler et al., 1976). Most studies indicate a decrease in temperature of 2-6 C. In the northern Coral Sea, Anderson et al. (1989) found planktonic foraminifer assemblages of tropical-subtropical species displayed little change since the LGM, indicating SSTs remained fairly constant. U-series dating combined with Sr/Ca ratio analysis of coral reefs in Western Australia indicate LGM temperatures very similar, or at most only a degree cooler than present (McCulloch and Esat, 2000). In a review of various palaeoclimatic data and models, Farrera et al. (1999) estimate that the southern tropical Pacific and lowlands regions bordering the Indian Ocean cooled by 2 C or less. Last Glacial Maximum temperatures C lower than present in the tropical Indian Ocean are indicated by alkenones (Bard et al., 1997). Ratios of Mg/Ca in foraminifers from the tropical Pacific Ocean show a decrease in temperature of 2.8±0.7 C (Lea et al., 2000). The coldest SSTs in the oceans around Australia have been dated at around 20.5ka cal BP (Barrows and Juggins, in press; Barrows et al., 2000). Utilising planktonic foraminiferal assemblages, Barrows and Juggins (in press) found a decrease of up to 4 C (compared to present 36

52 Chapter 2 Palaeoclimate and Sea-level values) in the eastern Indian Ocean at 20.5±1.4ka cal BP. This temperature difference was found to be mainly between 0 and 3 C elsewhere along the equator. A similar study, reanalysing the foraminiferal assemblage data used in the initial CLIMAP reconstructions of the equatorial Pacific found a temperature difference of 3-4 C at the LGM compared to the present (Mix et al., 2001). Farther afield, even greater temperature differences at the LGM have been found: utilising oxygen isotope analysis of corals, Guilderson et al. (2001) document a LGM temperature around 4.5 C lower than present in Barbados (tropical Atlantic). Analysis of noble gasses in groundwater from the Brazilian lowlands led Stute et al. (1994) to predict a LGM temperature 5.4±0.6 C lower than present. Pollen data from tropical lowlands around the world also has suggested a large change in LGM SST; 5-6 C (Rind and Peteet, 1985). Other studies indicate LGM SST lowering around 6 C in the area of the Gulf of Carpentaria (post-lgm Sr/Ca ratios of corals in the GBR, eastern Indian Ocean and Indonesian seaway, Gagan et al., 1998) and tropical west Pacific (global general circulation model with insolation, CO 2 levels, ice sheet topography and albedo, and sea-level specified, Bush and Philander, 1998). There are conflicting views on the extent of SST cooling during the LGM in contrast with land temperature. High-altitude sites are indicators of different, although interacting, climatic systems representing atmospheric as opposed to sea temperatures, and many high altitude sites display greater average LGM cooling than nearby SSTs, even accounting for the difference in elevation. Lowered LGM snow-lines throughout Australia and New Guinea led Bowler et al. (1976) to predict a depression of up to 10 C, compared to present temperatures, at these high altitude sites. Snow-line and palynological data indicate similar LGM temperatures on Mt. Jaya in New Guinea (5-9 C colder, Hope and Peterson, 1976). In analogous studies of New Guinean snowlines and tree line depression, other authors found a decrease of 6 C (e.g. Webster and Streten, 1978; Hope, 1987), to 6-8 C (Hope, 1996). Elsewhere, oxygen isotope analysis of a tropical glacier in Peru indicates a LGM temperature around 8 C colder than present (Thompson et al., 1995). 37

53 Chapter 2 Palaeoclimate and Sea-level Some authors propose large temperature differences between the sea and land during the LGM. Rind and Peteet (1985) state that consistency between the snow-line data and SST at the LGM would require large increases in the value of the vertical lapse rate (temperature change due to elevation). In a review of snowline depression in the tropics, Porter (2000) noted the disparity between LGM temperatures at high altitude (5-6.4 C decrease) and low altitude (SST decrease of 1-3 C). Mix et al. (2001) also looked at studies from throughout the tropics, including high-altitude sites in New Guinea and Africa, and found a difference between high altitude (temperature decrease of around 6 C) and low altitude sites (SST decrease of C). Other studies do not find a large discrepancy between SST and land temperatures at the LGM. Thomas (2000) documented a similar broad-scale cooling at high and low altitudes of about 5-7 C throughout the tropics (various studies, mainly in South America and Africa). In the Bandung plain and surrounding highlands (Fig. 2.1), van der Kaars and Dam (1995) suggest an LGM decrease of 4-7 C based on pollen assemblages. With a range of results for both SST (1-6 C, commonly quoted figure of around 3 C) and highland temperatures (5-10 C), it is clear that temperatures in the tropics did not experience uniform change throughout the last glacial cycle. The Australasian region is subject to variable SSTs, as short-term modern climatic events such as ENSO can vary SST by up to 2 C (McCulloch et al., 1996), which is within the range of the postulated glacial/interglacial temperature changes. In addition, increased seasonality of SSTs in marginal seas may partly explain the discrepancy between the tropical palaeotemperature estimations in this region (Wang, 1999). Wang (1999) noted LGM seasonality was 4 C higher in marginal seas compared to the open ocean, possibly due to an intensification of the East Asian winter monsoon. This enhanced seasonality was documented by Thunell and Miao (1996), who found glacial winter SSTs to be 7 C lower than present while summer SSTs were only about 1.5 C lower in the South China and Sulu seas (Fig. 2.1). A further explanation for the larger-than-expected difference between land and SST may be found in the drier atmosphere and lower level of cloud cover, which would have reduced nocturnal temperatures at elevation in the region (De Deckker 38

54 Chapter 2 Palaeoclimate and Sea-level et al., 2003). A common explanation is significantly steeper vertical lapse rates during the LGM compared to the present (e.g. Rind and Peteet, 1985), possibly accompanied by a weaker hydrologic cycle (e.g. Farrera et al., 1999). The post-glacial rise in temperature was not regular. Alkenone analysis from several tropical Indian Ocean cores shows abrupt warming at 15.1ka cal BP of 1.5 C, and a pause in the gradual SST warming at 12.2ka cal BP (Bard et al., 1997). The Younger Dryas is evident in these cores by a small cooling from ka cal, with a gradual warming of 1 C between 11-7ka cal (Bard et al., 1997). However, a more dramatic change is noted in the corals of Vanuatu (Fig. 2.1). There, the Younger Dryas is seen as an event that decreased SST to 4-6 C below present levels, from ka cal BP, with temperatures rising abruptly in the next 1500 years (Beck et al., 1997; Gagan et al., 2000). Utilising oxygen isotopes of foraminifers from the South China Sea (modified from Steinke et al., 2001; Fig. 2.1), Waelbroeck and Steinke (2002) state that at the late glacial-holocene transition winter SSTs also abruptly increased (within 500 years) by 2.1±0.7 C. However, other studies have found no evidence of the Younger Dryas temperature depression in the region. Thunell and Miao (1996) modified the earlier assumption of Linsley and Thunell (1990) that changes in foraminiferal oxygen isotopes and assemblages in the Sulu Sea reflected cooling at the Younger Dryas. Changes in water composition due to discharge of isotopically light meltwater into the ocean were instead proposed. Gagan et al. (2004) note the evidence of the Younger Dryas in the northern WPWP (i.e. the isotopic data from the Sulu and South China Seas), suggesting that this area may have been under the influence of the Northern Hemisphere-forced Asian Monsoon, whereas other areas of the WPWP show no indication of the Younger Dryas cold spell. In the overall rise in SST during the Holocene, Gagan et al. (2000) show there was a small decrease in SST at 8ka cal BP, and a temperature 1.2 C higher than present at the Holocene climatic optimum (6ka cal BP). Analysis of coral Sr/Ca off the Sepik River, Papua New Guinea (Fig. 2.1) by McGregor et al. 39

55 Chapter 2 Palaeoclimate and Sea-level (2002) indicate SST at 2.5 C below present at the beginning of the studied period (7.6ka BP U/Th age), warming to a SST peak similar to today at 6.1ka BP. Utilising Sr/Ca and oxygen isotopes of corals, Gagan et al. (1998) note that evaporation rates also increased at the climatic optimum. Since near modern sea levels were established around the mid-holocene, it is likely that SSTs also reached near modern values by that time. By 4ka cal BP, SSTs had stabilised (Beck et al., 1997). Various data indicate the tropics may have been the driver of glacial and interglacial cycles, although they are not in agreement as to which region initiated the transitions. Studies of Mg/Ca ratios of foraminifers by Lea et al. (2000) indicate throughout the Quaternary changing tropical SSTs precede both the build up and melting of ice by about 3000 years (from the Ontong Java Plateau, west Pacific, and Cocos Ridge, east Pacific; Fig. 2.1). Oxygen isotope values from a sub-tropical ice-core on the Tibetan Plateau show an increase in warming at 18.8ka cal BP (Sant and Rangarajan, 2002), which the authors point out is earlier than the Antarctic Vostok ice core (at 16.2ka cal BP) and other icecore records (15.7ka cal BP, tropical Bolivia; ka cal BP, Greenland). Sediment layers in tropical Andean lakes, analysed by Seltzer et al. (2002) suggest the post-glacial temperature and precipitation increase began as early as 22.5ka cal BP in the tropics (from a LGM at about ka cal BP), preceding the Northern Hemisphere deglaciation by thousands of years. 2.6 Atmospheric circulation (including rainfall) It is well known that many areas around the globe experienced a cooler and drier LGM. Studies of pollen records have shown that during the LGM precipitation was reduced over Indonesia (e.g. by around 30% in Bandung, van der Kaars and Dam, 1995; Fig. 2.2) and northern Australia (e.g. Lynch s Crater, Kershaw, 1978; Gulf of Carpentaria, Torgersen et al., 1988; Fig. 2.2). However, estimates of the amount of reduction in precipitation over the tropics range from 12% (the humid tropics in COHMAP CCM1 model, Kutzbach et al., 1988) to 65% (Southern Asia in UGAMP model, Dong et al., 1996). Kershaw s (1978, 1992) record of vegetation at Lynch s Crater may reveal a LGM reduction in 40

56 Chapter 2 Palaeoclimate and Sea-level Indian Ocean 10 9 Pacific Ocean Coral Sea 4 Figure 2.2 Location of sites, mentioned in the text, providing a palaeoclimatic record of atmospheric circulation in the region since the LGM. 1: Bandung (van der Kaars and Dam, 1995), 2: Lynch s Crater (Kershaw, 1978, 1992, 1986), 3: Gulf of Carpentaria (Torgersen et al., 1988; De Deckker et al., 1991; De Deckker, 2001), 4: Lake Eyre (DeVogel et al., 2004; Gillespie et al., 1991), 5: North West Cape (van der Kaars and De Deckker, 2002), 6: Lake Lewis (Hesse et al., 2004; English et al., 2001), 7: Kakadu (Nott and Price, 1994, 1999), 8: Rawa Danau caldera (van der Kaars et al., 2001), 9: Wanda Swamp (Hope, 2001), 10: Lake Tondano (Dam et al., 2001), 11: Gilbert River (Nanson et al., 1991), 12: Papua New Guinea (Tudhope et al., 2001). Note: Repetition of numbers indicates more than one site for that study. 41

57 Chapter 2 Palaeoclimate and Sea-level precipitation of 64% (Thomas, 2000). Local atmospheric patterns are inextricably linked to changes in the ocean - as precipitation is influenced by the amount of land exposed as well as sea-surface temperatures. The coupled ocean-atmosphere interactions are also linked to temperature, with lower glacial SST possibly weakening such interactions since cooler waters would not evaporate as much. The decreased moisture and heat transported to the atmosphere during the LGM due to the contracted low-latitude oceans also contributed to the palaeoclimatic variability of the area (e.g. fluctuations in the Western Pacific Warm Pool, An, 2000). A number of atmospheric circulations influence the Gulf of Carpentaria, including ENSO and the Australian summer monsoon Walker Circulation, ENSO and WPWP The maritime continent of Indonesia, including the Gulf of Carpentaria, provides a similar amount of heat radiation as do the continents of Africa and South America (Janowaik et al., 1985), and is therefore an integral part of the global system. The Walker circulation is driven by the difference in sea surface temperatures between the east and west Pacific. Air rises over the WPWP, and subsides over the cold tongue of water off the west coast of South America. The El Niño Southern Oscillation (ENSO) refers to the variability in strength of the Walker circulation. It is an anomalous large-scale ocean-atmosphere system associated with strong fluctuations in ocean currents and surface temperatures on under-decadal time scales. El Niño events occur when warm waters flow across the Pacific to the west coast of South America, reducing trade winds and causing drought in the Australian region, while the term "La Niña" describes the opposite phenomenon of cooler than normal waters off the South American west coast. This atmospheric circulation is obviously affected by changes in oceanic circulation and sea surface temperatures that accompany glacial and interglacial cycles, as well as the exposure of large areas of the maritime continent by lowered sea-levels during the glacial periods, and has corresponding effects on the climate of the region. It is known that the WPWP has experienced expansions and contractions on a scale of thousands of years, as foraminiferal oxygen isotopes from marginal sites indicate fluctuations in temperature and/or salinity (Martinez et al., 1997). During the glacial period, the 42

58 Chapter 2 Palaeoclimate and Sea-level temperature gradient between the east and west Pacific was larger by about 1 C (Lea et al., 2000), possibly increasing trade winds and weakening ENSO effects (Tudhope et al., 2001). Whereas, in the early Holocene SSTs were at near-modern values in Indonesia, while the waters off the South American coast were warmer than today (being driven by lower wind speeds which are linked to mid-high latitude sea-ice and the associated oceanic-temperature gradient, Sarnthein et al., 1987). The comparatively lower contrast between the east and west Pacific during the early Holocene means the Walker Circulation would have been weaker than at present, possibly generating more rain over northern Australia, although further interactions with other systems, such as the Hadley Cell, must be considered Australian summer monsoon, ITCZ and Hadley Cell In northern Australia the dominant system influencing rainfall has been the Australian summer monsoon. The Australian monsoon forms part of the Southeast Asian monsoon regime, and is linked to both regional and globalscale considerations in both hemispheres. A prime component of the Australian monsoon is the Intertropical Convergence Zone (ITCZ). The ITCZ is a zone of convergence of the subtropical high-pressure cells on each side of the equator, and at present is situated N of the equator in the Australian winter, shifting south to lie over northern Australia during the summer (Hastenrath, 1988). However, the ITCZ also shifts position over glacial and interglacial cycles. A weakening or complete failure of the Australian monsoon may be caused by the ITCZ shifting further north as is suggested for the LGM (Sturman and Tapper, 1996). The annual precipitation pattern of the Indo-Australian region is greatly influenced by the migration of the ITCZ. The ITCZ is connected to the Hadley and Walker circulations. The Hadley cell is composed of the flow of trade winds from the eastern edges of subsiding tropical highs over the continent of Australia, towards the low-pressure zone of the maritime continent to the north of Australia (Sturman and Tapper, 1996). An example is during El Niño the reduced temperature gradient between the east and west Pacific is accompanied by a southward shift in the ITCZ and an intensification of the northeast trade winds with a weakening of the southeast trade winds, and a 43

59 Chapter 2 Palaeoclimate and Sea-level weakening of the Hadley cell (Koutavas et al., 2002), leading to less monsoonal rainfall for Australia Combined effects Higher precipitation is indicated in the pollen record from Lynch s Crater (Fig. 2.2) for MIS 3 (Kershaw, 1992). Further inland, around 40ka cal BP, Lake Eyre (Fig. 2.2) was a perennial lake, indicating a period of high rainfall penetrating into Australia s arid interior (DeVogel et al., 2004). However, van der Kaars and De Deckker (2002) record a reduction in summer precipitation in the pollen record off the North West Cape (Fig. 2.2) from 46ka cal BP, with even drier conditions initiated around 35ka cal BP. In contrast, Harrison (1993) states lake level data from sites across Australia and Papua New Guinea between 30-24ka BP (uncalibrated 14 C) indicate generally wetter conditions (similar to modern), inferred to be from a strong Walker Circulation. Moss and Kershaw (2000) note a disturbance in the pollen record (with increased charcoal concentration) between 45-35ka cal BP, perhaps due to human-induced burning, but also possibly a reflection of climatic fluctuations. In a review of data from Australia s arid zones, Hesse et al. (2004) state that most indicators point to dry or drying conditions from around 30ka cal BP. During the LGM, the decrease in precipitation on the Australian continent was evidenced in the expansion of the desert regions and increased mobility of dunes (Bowler and Wasson, 1984). Vegetation was also restricted to species that could survive arid conditions (e.g. Kershaw, 1978, 1986). De Deckker (2001), revisiting the data of De Deckker et al. (1991), note the largest peak of dust particles in a succession of peaks (with an average cyclicity of 2.6ka) occurred at 21.5ka cal BP in the Gulf of Carpentaria (Fig. 2.2), indicating maximum aridity at that time. Oxygen isotopes analysed from various sites around the western Pacific show increased salinity of the WPWP during the LGM, and therefore evaporation rates greater than precipitation for the region (Martinez et al., 1997). Between 20-10ka cal BP charcoal records from Indonesia and Papua New Guinea show evidence of increased human-induced burning, exacerbated by climate variability (Haberle and Ledru, 2001). 44

60 Chapter 2 Palaeoclimate and Sea-level In the Australasian region there exists some disagreement between studies as to the timing of the post-glacial increase in precipitation. Reinvigoration of the Australian summer monsoon and ENSO is widely believed to have occurred in the Holocene. However, the onset of activity in the Holocene is contentious. A late onset of the monsoon, around 6ka BP, has been suggested (e.g. Markgraf et al., 1992). In contrast, Hesse et al. (2004) cite evidence of a much earlier onset of the monsoon, quoting maximum flood dates of 14ka BP on the Fitzroy River and Lake Gregory and 19ka BP at Lake Lewis (Fig. 2.2). Sedimentary signatures such as levees and ridges associated with plunge pools in Kakadu (Fig. 2.2) record a continued regime of heavy precipitation throughout the LGM (Nott and Price, 1994, 1999). Extreme flooding events from 30-18ka BP (TL dates) lead Nott and Price (1999) to suggest the Australian monsoon was active (although latitudinally restricted) during the LGM, with activity declining after 18ka cal BP until reinvigoration in the early Holocene. English et al. (2001) verify that even from around 20ka BP (OSL dates), monsoonal conditions influenced central Australia, with a marked increase in fluvial activity present soon after the period of maximum aridity, affecting the Lake Lewis basin (Fig. 2.2) since 19ka BP. Hope et al. (2004) propose that in northern Australia the post-glacial increase in precipitation was accompanied by increased evaporation, resulting in lower effective precipitation (EP) and a drier climate until the early Holocene. Dust peaks in a Gulf of Carpentaria core (Fig. 2.2), indicating arid episodes, are found at 19.3ka cal BP, 15.8ka cal BP and 13.3ka cal BP (De Deckker, 2001), indicating a pattern of wet and dry climate. The synchronous change in charcoal levels from various lacustrine sites in Indonesia and Papua New Guinea, and Central and Southern America, lead Haberle and Ledru (2001) to postulate that the onset of the Walker Circulation established broad atmospheric connections by around 16ka cal BP. The expansion of forest cover in the Rawa Danau caldera, Indonesia (Fig. 2.2), from 15ka cal BP indicates a change to wetter climatic conditions (van der Kaars et al., 2001). Wyrwoll and Miller (2001) and Hesse et al. (2004) propose that the Australian summer monsoon was initiated around 14ka cal BP, based mainly on flood deposits from northern Australian rivers and central Australian lakes. Heavier than present summer rain over north-western Australia from 14 to 3ka 45

61 Chapter 2 Palaeoclimate and Sea-level cal BP is also chronicled by palynological analysis of a marine core by van der Kaars and De Deckker (2002). Hesse et al. (2004) note that local differences in the timing of the hydrological change mean that at present the initiation of the monsoon can only be constrained to within 15-13ka cal BP. A later increase in precipitation, around 13-12ka cal BP, is indicated in other studies of Indonesian lakes and also in the record of Kershaw (1986) from Lynch s Crater (Fig. 2.2). The pollen record from Wanda Swamp, Indonesia (Fig. 1.2), supports the notion of a drier climate 30-13ka cal BP, with a change to wetter conditions around 13ka cal BP (Hope, 2001). Lake Tondano, Indonesia (Fig. 2.2), studied by Dam et al. (2001), shows an initial small increase in the lake-level around 12.7ka cal BP (when peaty soil formation begins after a hiatus), with a rapid increase in the lake-level around 12ka cal BP (although the authors note the rise in lake-level may be due to tectonic effects). There is little evidence of an arid spell specifically linked to the Younger Dryas in the Australasian region. De Deckker (2001), re-examining data reported in De Deckker et al. (1991), notes that a peak in dust deposition in the Gulf of Carpentaria previously assumed to coincide with the Younger Dryas actually occurs earlier (13.3ka cal BP). Rising sea-level 12ka cal BP inundated the Sunda Shelf, leading Gingele et al. (2002) to propose initiation of the monsoon around that date. The Pleistocene-Holocene transition (11.5ka cal BP) is traditionally synonymous with the change from glacial conditions to a climate closer to the present environment. Nanson et al. (1991) present apparently contradictory evidence of a northern Australian climate that was both wetter and drier from the termination of the LGM into the early Holocene. Both calcrete and ferricrete were found to form virtually coeval by Nanson et al. (1991) in the alluvium of the Gilbert River (Fig. 2.2). The coexistence of these dry versus humid indicators may only be due to a previously unrecognised sensitivity to minor climatic or groundwater changes, yet it could also indicate a pronounced seasonal or interannual difference in precipitation. Increased moisture is noted in Lake Eyre (Fig. 2.2) from 11ka cal BP, with the formation of a permanent lake (Gillespie et al., 1991), and Wyrwoll and Miller (2001) suggest regional establishment of full monsoonal conditions by this date. However, Wyrwoll et al. (1992) note that swamp and alluvial records from northern Australia indicate the monsoon may 46

62 Chapter 2 Palaeoclimate and Sea-level have been weaker until 6.5ka cal BP. Based on a series of pollen records throughout Australia, New Zealand and South America, Markgraf et al. (1992) note a later onset of the Australian monsoon. The breaching of the Torres Strait is proposed by Markgraf et al. (1992) as a cause for the onset of the monsoon in the mid-holocene (around 6ka cal BP). The Holocene climatic amelioration (increase in EP) is notable in the plunge pool records of Nott and Price (1999) from northern Australia (Fig. 2.2), with the most extreme floods noted in the period 8-4ka BP (TL dates). As Gagan et al. (1998) note, even a small increase in temperature, such as the gradual post-glacial rise, and particularly the peak experienced at the Holocene climatic optimum (around 6ka cal BP) would greatly increase the amount of water transported to the tropical atmosphere through evaporation, and therefore enhance precipitation. A similar trend of absence or weakened activity at the LGM, however, with a later onset or strengthening in the mid-to-late Holocene, is suggested for the ENSO circulation. Analysis of oxygen isotopes of corals in Papua New Guinea (Fig. 2.2) lead Tudhope et al. (2001) to postulate that ENSO persisted over the 130ka time-period studied, although with substantial changes in strength, including a weaker system at 6.5ka (U/Th age). Rittenour et al. (2000) found large ENSO events decreased in number in the studied period from 17ka cal BP to 13.5ka cal BP, as seen in the periodic deposition of varves in a North American glacial lake, postulating a decrease in activity until only smaller events occurred in the early-to-mid Holocene, before a later reinvigoration. The record of storm-deposited lake laminae in tropical Ecuador from 15-7ka cal BP shows a periodicity of around 15 years, with the establishment around 5ka cal BP of a periodicity of years (Rodbell et al., 1999). The authors state that these results show the ENSO circulation was weaker during 15-7ka cal BP, with conditions similar to present established around 5ka cal BP. Pollen data from the Australasian region (McGlone et al., 1992) and synchronicities between Indonesian and South American charcoal records (Haberle and Ledru, 2001) also constrain the onset of the variable climatic conditions characteristic of modern ENSO to around 5ka cal BP. Shulmeister and Lees (1995) cite pollen evidence showing that ENSO was weaker than present in the Australian region during the early-to-mid Holocene, as the region experienced warm conditions 47

63 Chapter 2 Palaeoclimate and Sea-level with little or no annual variability, with variable ENSO conditions established around 4ka BP (uncalibrated 14 C, ~4.4ka cal BP). However, utilising molluscs from archaeological sites, Sandweiss et al. (2001) document the onset of a weak ENSO around 5.8ka cal BP, from little or no activity in the early Holocene. The authors state that modern periodicities only established after ka cal BP. 2.7 Summary The tropical location of the Gulf of Carpentaria places it within an area of study which is still subject to some controversy. The nature and timing of the postglacial increase in precipitation in the region is still under investigation and many conflicting results are yet to be fully explained. The results of these investigations not only provide data on climate reconstructions of the region, but are an integral component of the entire global climatic system. Sea-level data are also needed to add to the present knowledge of the LGM and post-glacial transgression. Foraminiferal assemblage and species data (Chapter 3) are used to determine precipitation and sea-level changes in this thesis, utilising the methodology outlined in Chapter 4. The results of foraminiferal and sedimentary analysis of the Gulf of Carpentaria from the LGM to present are presented in Chapter 5, adding to the knowledge of local precipitation regimes and sea-level change, and global implications (Chapter 6). 48

64 Chapter 3 - Foraminifers CHAPTER 3 Foraminifers 3.1 Introduction In this chapter, features of living foraminifers are presented (Section 3.2), examples of assemblages and species specific to certain habitats are discussed (Sections 3.3 to 3.6), and modifications of these assemblages upon death are noted (Section 3.7). Distribution patterns of modern foraminifers are utilized to interpret the palaeoenvironment of the Gulf of Carpentaria (Chapters 5 and 6). 3.2 General systematics and biology Systematic nomenclature The system of Loeblich and Tappan (1961, 1964, 1987) is considered the standard method of foraminiferal classification. They recognised foraminifers as belonging to the: Kingdom Protista, Subkingdom Protozoa, Phylum Sarcomastigophora, Subphylum Sarcodina, Superclass Rhizopoda, Class Granuloreticulosea, Order Foraminiferida. A recent publication (Loeblich and Tappan, 1994) has raised the ranks of the major categories such as the Order Foraminiferida and suborders within the order, and recognises the new Class Foraminiferea of Lee (1990). This thesis follows the general classification of Loeblich and Tappan (1994) Features of foraminiferal tests Foraminiferea are unicellular organisms which produce a protective shell, or test. The test may be composed of a proteinaceous organic substance, foreign particles may be cemented to the test, or the test may be of biogenically secreted calcium carbonate. A useful characteristic of most foraminifers, for micropalaeontology, is the good preservation potential of their tests. Features of the tests, such as composition and morphology, are used to distinguish taxa for systematic classification. 49

65 Chapter 3 - Foraminifers The five main structural types of test wall are: 1. Proteinaceous a thin, tectinous layer (mucopolysaccharide), not readily preserved in the fossil record, and not found in the Gulf of Carpentaria material. 2. Agglutinated cemented particles such as sand grains, sponge spicules and other foraminiferal shells (cement may be organic or calcareous). Depending on the size of material agglutinated, the test may appear slightly rough under incident light to having identifiably separate grains. Illustrated in Table 3.1a. 3. Microgranular secreted calcium carbonate: calcite grains cemented with calcium carbonate, the inner layer of calcite granules is more regularly arranged. This type is most common in the Palaeozoic, and is not in the Pleistocene-Holocene sediments from the Gulf of Carpentaria. 4. Calcareous Porcellaneous secreted calcium carbonate: two layers of calcite arranged parallel with the surface of the test enclosing a third layer of randomly oriented calcite crystals. The test appears shiny and translucent to opaque under incident light. Illustrated in Table 3.1b. 5. Calcareous Hyaline secreted calcium carbonate: calcite or aragonite crystals arranged perpendicular to the surface of the test. The test appears glassy and transparent, and pores in the test may be visible under incident light. Illustrated in Table 3.1c. As the unicellular foraminifer matures, it may outgrow its original test, and new and larger chambers are built to accommodate the developing organism. Each genus has a characteristic pattern of chamber shape and/or arrangement. Broadly, the foraminifer may be unilocular (having one chamber) or multilocular (multi-chambered). The chambers may form many arrangements, including uniserial (one row), planispiral (coiled in one plane), milioline (Table 3.1b chambers arranged in a series where each chamber extends the length of the test, and each successive chamber is placed at an angle of up to 180 degrees from the previous one), or trochospiral (coiled in three-dimensions). 50

66 Chapter 3 - Foraminifers Table 3.1 Current and previous classification of foraminifers based on wall structure a) agglutinated, b) calcareous porcellaneous and c) calcareous hyaline. Wall structure (And an illustration of an example of the type) Previous classification A Suborder of the Order Foraminiferida Current classification An Order of the Class Foraminiferea a) Agglutinated Textulariina Astrorhizida, Lituolida, Trochamminida, Textulariida. b) Calcareous porcellaneous Miliolina Spirillinida, Miliolida. c) Calcareous hyaline Rotaliina Lagenida, Robertinida, Globigerinida, Buliminida, Rotaliida. Only the agglutinated, calcareous porcellaneous and calcareous hyaline walls are utilised in palaeoenvironmental interpretations, since they are found in modern environments and in the fossil record. Previously, hard-shelled foraminifers were subdivided into only three groups, corresponding to the structure of their test wall (Table 3.1). Currently, eleven orders of foraminifers exist, also listed in Table 3.1. The common name of the orders is formed by removing the last letter of their name (e.g. a species of the order Lagenida is often referred to as a lagenid). 51

67 Chapter 3 - Foraminifers The habitat of living foraminifera Foraminiferea are aquatic organisms, and have been found living in environments as diverse as the open ocean and inland lakes. Although foraminifers are termed micro-organisms, they may range in size from less than 63µm (the delineation between silt and sand) to occasional specimens 2.5cm or greater (Murray, 1991). Foraminifers may be benthic (living on or in the sediment floor) or planktonic (floating within the water mass; Fig. 3.1). Of the orders delineated by Loeblich and Tappan (1994), Globigerinida is the only planktonic group. There are many environmental parameters influencing foraminiferal distribution including water depth, salinity, temperature, movement within the water body (e.g. currents, turbidity), sediment type, oxygen levels and amount of organic matter available for food. Most species are specific to a certain range of parameters, with some species more tolerant (and therefore widespread) than others. water surface planktonic foraminifera benthic foraminifera sediment substrate 100 µm Figure 3.1 Schematic representation of planktonic and benthic foraminiferal habitats. The scale bar refers to the foraminifers only. 52

68 Chapter 3 - Foraminifers Foraminifers live at depths ranging from tidally exposed mud flats to deep ocean trenches. Water depth is an almost all-inclusive environmental factor, as changes in water depth may be associated with all of the parameters mentioned previously. It is argued that since changes in depth are accompanied by changes in interrelated parameters, depth itself is probably of little consequence (Reiss and Hottinger, 1984). However, the distribution of some assemblages and species, such as marsh foraminifers, are vertically zoned with respect to tide level and can therefore be used as sea-level indicators (Scott and Medioli, 1980). The majority of foraminifers inhabit waters of around a normal marine salinity (32-37 ). Some foraminiferal taxa can tolerate hyposaline (low salinity, or brackish environments) where salinity is below 32. Boudreau et al. (2001) have found a species of foraminifer in Lake Winnipegosis, Canada, which survives in salinities as low as 1-2. On the other hand, in a hypersaline (high salinity, >37 ) lake in the Coorong District, South Australia, Cann and De Deckker (1981) collected living foraminifers in waters of up to 80 salinity. Many of the foraminifers found in low salinity waters can also tolerate hypersaline waters, as they are highly adaptable to changes in salinity, however other species have a more restricted range. Some species of foraminifers can survive in extremely low water temperatures, such as in saline waters which freeze below 0 C. Murray and Pudsey (2004) collected living foraminifers in waters of -1.9 C at the Larsen Ice Shelf, northern Antarctic Peninsula. At the other extreme, Langezaal et al. (2003) document the occurrence of living foraminifers in temperatures reaching 50 C during summer on the sand flats in Mok Bay, Netherlands. Individual taxa have a more limited range. Water temperature may influence other aspects of foraminiferal distribution, as species typical of the open ocean in tropical areas may be found nearer shore, even in estuaries, in temperate climates (e.g. Albani and Yassini, 1993; Yassini and Jones, 1995). Water movement may create an environment of change in which only species adapted to this change can survive. An example is adaptation to the test 53

69 Chapter 3 - Foraminifers morphology of a particular genus, with thicker robust shells to withstand turbulence in tidal channels (Phleger, 1960). Internal osmotic processes may also be developed to regulate the organism as salinity fluctuates within an estuary due to tidal processes (Debenay et al., 2000). Currents may transport living or dead tests outside their normal habitat, so that the composition of assemblages found in the fossil record will not necessarily reflect the living assemblages. Species may be transported away from their natural habitat, reducing their preservation in some environments, while other areas may record an influx of transported species. Benthic foraminifers may live within the substrate (i.e. infaunally), at the sediment/water interface, or clinging to plants (i.e. epifaunally). It has been shown that some species utilise different environments at different stages in their life cycle e.g. living infaunally but migrating to sea grass beds to reproduce epifaunally (Matera and Lee, 1972). The nature of the substrate has a direct influence on habitats available for the many taxa living in and around the sediments, however, many may be adapted to survive in different substrates. Van der Zwaan et al. (1999) suggest that the parameters discussed above do not play a very important role in foraminiferal distribution, with oxygen and organic flux the major determinants. These authors state that oxygen is a necessity for foraminifers, and it determines the presence or absence of foraminifers in any particular area, although they acknowledge that some species are adapted to oxygen-deficient environments. Organic flux, or food, controls the density of the population which can be established, with competition between species for this food also being important. Although recognising oxygen and organic flux as principal controllers of foraminiferal distribution, Murray (2002) advised that these factors alone are too simplistic to explain the varied distribution of foraminifers in niche habitats. Boltovskoy et al. (1991) note that almost no variables act independently on test morphologies and conclude that there are very few broad trends related to specific environmental parameters. In summary, palaeoenvironmental 54

70 Chapter 3 - Foraminifers reconstructions should be modelled on general environments taking into consideration individual parameters. Palaeoenvironmental reconstructions should not be based on individual species within a population, but on characteristics of the entire assemblage, with particular species and morphology changes noted for the indications they provide. 3.3 Introduction to living assemblages and indicator species Many studies have been carried out on the relationship between foraminiferal species distribution and particular aspects of their environment, such as depth and salinity as mentioned above in Section However, like most microbiota, benthic foraminifers do not display a simple direct relationship with individual environmental variables. They are generalists who, in order to survive, maintain a low degree of specialisation. Broader environmental correlations, such as water mass characteristics (from oceanic to brackish) are of more relevance for palaeontological studies, as they reflect conditions occurring in nature. The environment is a complex interaction of many variables, never just one in isolation. And in this regard foraminifers are useful indicators of the general environment. As shown in Table 3.1, in the past foraminifers were subdivided into only three groups. Figure 3.2 is of a ternary diagram from Murray (1973) illustrating the relationship between environments and the occurrence of the suborders Textulariina (agglutinated), Miliolina (calcareous porcellaneous) and Rotaliina (calcareous hyaline). As Figure 3.2 shows, some general observations can be made about the habitats occupied by these taxa (although it must be noted that isolated water bodies are not included within the diagram). For example, the presence of Textulariina and Rotaliina, with the absence of Miliolina is one diagnostic feature of hyposaline marshes, while the suborders Miliolina and Rotaliina are present in hypersaline lagoons with very few Textulariina. For the purposes of this study, the occurrence of foraminifers in three main environments is examined isolated water bodies (brackish and more saline waters), transitional/estuarine (tidally influenced areas and more restricted 55

71 Chapter 3 - Foraminifers lagoonal) and the marine environment (continental shelf seas and open marine). In each section environmental characteristics are noted and parallels to the expected palaeoenvironments of the Gulf of Carpentaria are indicated. Studies of relevant foraminiferal species and assemblages are cited, generally in order of increasing salinity (or in the order of farthest inland to open ocean), but also in an order that allows similar species and assemblages to be discussed together. Particular species and morphological changes important to this study are further elaborated. Figure 3.2 Summary foraminiferal and environmental ternary diagram from Murray (1973), where Miliolina refers to foraminifers with calcareous porcellaneous tests, Textulariina refers to agglutinated forms, and Rotaliina refers to calcareous hyaline tests. 56

72 Chapter 3 - Foraminifers 3.4 Isolated water bodies Isolated water bodies may range from fresh to extremely saline, and be located inland or on the coast. The saline nature of the water body may derive from residual marine water, salts in the system or from occasional inundations of marine water. Or the salt may be derived from sediments in the catchment area situated away from the influence of the present ocean. If the water body is stratified, benthic foraminifers in the assemblage will reflect the more saline bottom water. A total salt concentration of 3 grams per litre (3 ) or less is considered fresh. The average river water salinity is 0.5 or less, while average ocean salinity is 35, and the normal marine range is considered to be Previous studies have shown that at times an isolated water body was developed within the Gulf of Carpentaria (Nix and Kalma, 1972; Smart, 1977; Torgersen et al., 1983, 1985). Since the initial Lake Carpentaria was composed of marine water trapped in the depression as sea-level fell, some residual marine water is expected to be within the system. However, the existence of overflow channels from the lake through the Arafura Sill (Jones and Torgersen, 1988) indicates the presence of a large amount of water, and an input of fresh water may have flushed the marine influence from the system. The salinity of Lake Carpentaria may have varied from fresh to brackish (De Deckker et al., 1988; Torgersen et al., 1988), and at times the lake may have experienced brief inundations of marine water (McCulloch et al., 1989). In general, foraminifers may colonize an isolated water body through their continued existence in marine water that enters the system, or via avian transport. Studies of foraminiferal assemblages in isolated water bodies are relatively rare (compared to studies of marine and estuarine waters), and reflect the less common occurrence of foraminifers in these environments. In accordance with the rarity of studies, a combined figure showing both brackish and more saline sites of foraminiferal studies on isolated water bodies mentioned in the text is provided at the end of Section

73 Chapter 3 - Foraminifers Isolated brackish waters The term brackish waters is used to indicate water of a salinity between fresh and marine (i.e ), which may also be termed low-salinity. Fresh water, through precipitation (whether directly from rainfall into the basin or from fluvial input) or groundwater constitutes some component of the isolated water body s composition. A general trend is that agglutinated foraminifers are commonly found in lowsalinity waters. The majority of agglutinating foraminifers are considered to be deep infaunal (Buzas et al., 1993). For example, in modern sediments of an ephemeral brackish lake on Kapiti Island, New Zealand (Fig. 3.3), only Trochammina inflata was found by Hayward and Hollis (1994). Throughout the Pleistocene, numerous littoral lakes along the Mediterranean coast were colonized by foraminifera. Only agglutinated forms, such as Trochammina spp., and Jadammina macrescens survived in the very lowsalinity lakes (Usera et al., 2002). However, there is evidence that adaptable calcareous species in the order Rotaliida can survive in fresher waters, especially if the salinities gradually decrease. Cribroelphidium gunteri, a coastal marine foraminifer colonised saline areas of the non-marine Lake Winnipegosis, Canada, during the warmer period of the Holocene (Patterson et al., 1997). C. gunteri is still extant within Lake Winnipegosis and has adapted to salinities as low as 1-2 within the lake (Boudreau et al., 2001). Lake Winnipegosis also has areas of higher salinity, and C. gunteri was found to be most abundant in the higher salinity areas, which also experience a high oxygen content (Patterson et al., 1997). Patterson et al. (1997) demonstrated that C. gunteri is tolerant of changes in salinity and oxygen, but is not adapted to a wide ph range. The calcareous Ammonia beccarii was detected in Little Dip Lake in the Coorong district, South Australia (Fig. 3.3), living in a salinity of 23.8 when samples were collected by Cann and De Deckker (1981). The authors state the fluctuating salinities of Little Dip Lake have not been recorded over A 58

74 Chapter 3 - Foraminifers species of Ammonia has also been recorded in Ongeim'l Tketau (Jellyfish Lake) on Mecherchar Island in the western Pacific Palau Archipelago (Fig. 3.3) in salinities of 27-31, dominating the assemblage with Helenina sp. (Lipps and Langer, 1999). Ammonia beccarii has also been discovered in sediments of a palaeolake existing in southern Jordan during the Last Interglacial (Petit-Maire et al., 2002). The authors identified A. beccarii/tepida (sic), along with charophytes, ostracods and gastropods, suggesting a lake of fluctuating salinities. In the Pleistocene littoral lakes along the Mediterranean studied by Usera et al. (2002), A. beccarii tepida was thought to have colonised areas of slightly higher salinity brackish waters than the agglutinated taxa mentioned previously. A. beccarii has also been found in cores from inland lakes of the Dead Sea rift, in sediments dating back two million years (Almogi-Labin et al., 1995). Almogi-Labin et al. (1995) established that A. beccarii episodically colonised an inland lake of 5-30 salinity, with normal sized specimens of A. beccarii tepida during one phase, and small numbers of dwarf specimens of A. beccarii tepida and A. beccarii cf. parkinsoniana indicating the lower limit of salinity tolerance for an earlier phase. A. beccarii has been identified in the sediments of Lake Clayton, within the Lake Eyre Basin (Fig. 3.3), South Australia (Warren, 1997). The precise salinity and the time of deposition are unknown, although foraminifers are only found in the top 1.5m of the lake sediments. It is noted that A. beccarii experienced a stressful environment, indicated by the small and deformed shells found (Warren, 1997). Ammonia beccarii is a widespread euryhaline species occurring in almost all foraminiferal assemblages in temperate and tropical waters from brackish lakes and estuaries, to marine waters, to hypersaline salt flats. Langer and Lipps (2003) noted that only taxa with short life spans such as Ammonia can survive in harsh environments. A. beccarii grows and reproduces in a salinity between 15 (Cann et al., 2000) and 56 (Almogi-Labin et al., 1992), and is most successful between (Cann and De Deckker 1981; De Deckker 1982). A. beccarii is known to tolerate wide fluctuations in environmental parameters such as salinity which prevent other species from establishing (Murray, 1991), 59

75 Chapter 3 - Foraminifers leading to monospecific foraminiferal assemblages. It inhabits a shallow infaunal habitat, up to a few centimetres below the sediment-water interface (Murray, 2002). A. beccarii is notable for its wide intraspecific morphological variation (Murray, 1991), with dwarf forms considered to be brackish water indicators (e.g. Almogi-Labin et al., 1995; Yokoyama et al., 2000 and 2001a). However, Poag (1978) demonstrated that under stressed conditions (e.g. lower salinity), a morphotype of Ammonia with thicker walls, more chambers, and a larger test will form. Culturing experiments by Schnitker (1974) revealed morphological variations considered to be five separate species may be induced by varying the temperature, demonstrating their existence as formae (morphological variants not genetically distinct) rather than subspecies. However, with the advent of new technology, DNA analysis was able to distinguish at least three species within Ammonia: A. tepida, A. beccarii and A. parkinsoniana (Pawlowski et al., 1995), and recent molecular analysis has revealed twelve distinct species within Ammonia (Hayward et al., 2004b). Not all of these are distinct morphologically and many have not been formally named, so this thesis will continue to refer to A. beccarii and other well known species, naming the morphologically determined formae or subspecies where appropriate. Morphologically quite similar to Ammonia, Helenina is also a cosmopolitan and euryhaline calcareous foraminifera. Helenina anderseni has been reported from brackish ponds to lagoons and mangrove environments (Javaux and Scott, 2003). Helenina sp. occurs in the landlocked Ongeim'l Tketau in the Palau Archipelago (Fig. 3.3), in salinities of 27-31, dominating the assemblage in equal numbers to Ammonia sp. (Lipps and Langer, 1999). Lipps and Langer (1999) stated that this association is typical of mangrove environments like Ongeim'l Tketau. Lipps and Langer (1999) also identify the rotaliid Elphidium sp. within Ongeim'l Tketau. From the illustration in Lipps and Langer (1999) it is difficult to ascertain whether the species is keeled or unkeeled. It has been suggested that unkeeled forms of the Elphidiidae family are able to withstand extreme salinity variations. Normal marine species of Elphidium are typically keeled, whereas unkeeled 60

76 Chapter 3 - Foraminifers species may be found in brackish marshes (Murray, 1973). However, Murray (1976) noted that unkeeled Elphidium spp. in general were found in salinities ranging from 0-70, in tidal marshes, lagoons, and in the nearshore environment. The species Elphidium excavatum is noted as having colonised the more saline areas of brackish Mediterranean lakes in the Pleistocene studied by Usera et al. (2002), although not as abundantly as the dominant Ammonia beccarii. Petit- Maire et al. (2002) also reported E. excavatum (as Cribroelphidium excavatum var. selseyense) from a Last Interglacial brackish palaeolake in Jordan. From the same family as Elphidium and Cribroelphidium (i.e. Elphidiidae), Haynesina germanica is also thought to have lived in the more saline areas of brackish Mediterranean lakes in the Pleistocene studied by Usera et al. (2002). Elphidium excavatum is an unkeeled species, living infaunally in littoral lakes and lagoons with brackish to hypersaline waters (Murray, 1991). It is highly adaptable to changing environmental conditions (Cribroelphidium excavatum of Linke and Lutze, 1993), and is able to survive at least short periods of severe oxygen depletion (Alve, 1995). Haynesina germanica is an infaunal species (with no keel like all Haynesina) which lives in brackish waters between 0 and 30, although it can survive up to 50 (Usera et al., 2002). Summary of brackish isolated water bodies According to the abovementioned studies, isolated water bodies of a brackish nature may be characterised by the presence of certain agglutinated taxa and calcareous hyaline foraminifera of the order Rotaliida. The agglutinated foraminifers which may be present are: Trochammina (T. inflata is mentioned as existing in impermanent waters) and Jadammina (J. macrescens in particular). The calcareous foraminifers which may inhabit such an environment are: Ammonia (A. beccarii, A. beccarii tepida and A. beccarii parkinsoniana in particular), Helenina (H. anderseni is specified), Cribroelphidium (C. gunteri), Elphidium (mainly unkeeled species such as E. excavatum), and Haynesina 61

77 Chapter 3 - Foraminifers (particularly H. germanica). The presence of one or more of these taxa to the exclusion of all other species is a good indication of a water body of brackish or fluctuating salinity, even though all the species listed do occur in other environments. Specific associations recorded include Trochammina and Jadammina in waters of the lowest salinity (Usera et al., 2002). In waters just under normal marine salinity, Ammonia and Elphidium are dominant (Petit- Maire et al., 2002), with either Helenina (Lipps and Langer, 1999) or Haynesina (Usera et al., 2002) also present. Other indicators of brackish waters are particular morphological changes of the foraminifera, such as dwarfism and deformities of A. beccarii due to growth near the limit of its tolerance. Species abundance and diversity are also important, as low-salinity waters typically only support a low number of species and a low number of individuals, with species abundance increasing when salinities are close to marine (e.g. the ideal conditions for A. beccarii s growth and reproduction are in a salinity range of ). The presence of other organisms also assists in palaeoenvironmental reconstructions. Woody fragments may be found in a non-marine unit (e.g. Unit I of Li et al., 2000 in cores from Anderson Inlet, Victoria; Fig. 3.3), and fresh to brackish water charophytes (e.g. Lychnothamnus barbatus, Chara vulgaris and C. zeylanica, Garcia, pers. comm., 2005) and ostracods (e.g. Ilyocypris and Leptocythere, Reeves, 2004) may give an indication of the particular salinity range. Additionally, a commonly used negative indication of an especially low-salinity period in the fossil record is the total absence of any foraminifera (e.g. the nonmarine Units I and IV of Li et al., 2000 from Anderson Inlet). In addition to the palaeoenvironmental salinity indicators mentioned above, a water-body s isolated nature may be signified by the absence of particular species especially the ocean-dwelling planktonic foraminifers. The presence of charophytes also indicates the non-marine attributes of a water body, as they are typically found in springs, rivers and lakes in low energy environments. 62

78 Chapter 3 - Foraminifers 3 4 Brackish and saline isolated water bodies Figure 3.3 Location of brackish and more saline isolated water bodies within the region where foraminifers have been found. Sites are numbered in the order they are discussed within the text. 1: Kapiti Island (Hayward and Hollis, 1994), 2: Coorong District (Cann and De Deckker, 1981), 3: Palau (Lipps and Langer, 1999), 4: Lake Clayton (Warren, 1997), 5: Anderson Inlet (Li et al., 2000), 6: Lake Corangamite (Bell, 2002). 63

79 Chapter 3 - Foraminifers Isolated water bodies of marine salinity and higher Isolated water bodies may have trapped residual sea water, and so be of marine salinity. In some environments, high evaporation rates may remove much of the water, concentrating the salts from the original sea water and/or from salts deposited in the lake from the catchment area to a salinity above the original sea water. The agglutinated taxa Trochammina previously mentioned as occurring in brackish waters was also found living in impermanent non-marine saline lakes in the Coorong District (Fig. 3.3) of South Australia (Cann and De Deckker, 1981). The salinity of the studied lakes fluctuates between 10 (when rainfall is high in the winter) to more than 180 (due to summer evaporative drying of the lake), although Trochammina was only found in lakes with a salinity below 60. With a salinity range of 40-56, the inland Navit pool near the Dead Sea is host to a living population of Ammonia beccarii tepida, which thrives in microbial mats in shallow, often very warm waters (Almogi-Labin et al., 1992). Holocene deposits of a marine saline lake in the Bahama islands were found to contain dominant Ammonia beccarii parkinsoniana, which was replaced over time by an association of Triloculina oblonga and Cribroelphidium gunteri (Dix et al., 1999). Triloculina is a calcareous porcellaneous genus of the Order Miliolida commonly found in hypersaline lagoons (Brasier, 1975). Dix et al. (1999) interpreted the replacement of A. beccari with T. oblonga to indicate the closure of the restricted marine embayment from the rising ocean via a sand barrier, and increasing salinity of the newly created lake. Species of Elphidium (with no keel) dominated the ephemeral non-marine saline lakes in the Coorong District studied by Cann and De Deckker (1981). Some individuals were collected living in lake water of 80 salinity, while others were found in moist mud under the cover of dead vegetation a mechanism for survival of summer evaporative drying of the lake. Elphidium spp. is found infaunally in North American saline lakes, migrating to sea grass beds to reproduce (Matera and Lee, 1972). 64

80 Chapter 3 - Foraminifers A population of the unkeeled Elphidium (E. excavatum excavatum) has been observed in the permanent Lake Corangamite, Victoria (Fig. 3.3), although only as dead tests in the surface sediment (Bell, 2002). Lake Corangamite is the largest permanent inland saline lake in Australia. Its salinity has risen dramatically in the past ten years (50 in 1992, Williams, 1996; over 100 in 2002, Timms, 2003), perhaps explaining why only dead foraminifers were recovered in the 2002 study. Summary of higher salinity isolated water bodies Agglutinated foraminifers are rare in isolated water bodies of marine salinity or greater, only represented by the genus Trochammina in the example cited here, occurring within an ephemeral lake. The rotaliid foraminifers Elphidium (E. excavatum excavatum is named), Ammonia (A. beccarii tepida) and Cribroelphidium (C. gunteri) are also known to inhabit isolated high salinity water bodies. However, all four genera may also occur in isolated water bodies of a brackish nature, and may also be found in marginal marine environments. The miliolid Triloculina oblonga is found to have lived in a saline isolated water body, noting that the lake was derived from marine waters where T. oblonga is also found. Saline isolated water bodies appear to support only one or two species or genera of foraminifer, which may be very abundant. As previously mentioned, the absence of marine species of foraminifers, especially planktonic taxa, may be used to support the hypothesis of an isolated water body, along with the presence of charophytes. The presence of euryhaline charophytes (e.g. Lamprothamnium, Garcia, pers. comm., 2005) and ostracods (e.g. Cyprideis, Reeves, 2004), coupled with the absence of fresher water species may be utilised to confirm the saline nature of the water body. 3.5 Transitional environments The transitional environment is, by definition, the meeting place of marine and continental environments, and salinities may range from higher than normal marine salinity (due to evaporative concentration), to normal marine, to fresh 65

81 Chapter 3 - Foraminifers water. It is a variable environment, affected by diurnal (e.g. tides), seasonal (e.g. precipitation/evaporation), and even longer term changes (e.g. ENSO). Tidally influenced areas are discussed separately to lagoonal environments in the section below. Tidal influence may reach hundreds of kilometres upstream in a tidal river, but is also present in high energy shores subject to periodic tidal inundation, whereas a lagoon is less open to the ocean, being a semi-enclosed to enclosed coastal body of water. However, the same continental-to-marine water gradient occurs in both tidally influenced areas and in lagoons, revealed in similar changes of foraminiferal assemblages in both environments. It should be noted that a bottom wedge of denser, more saline water may cause a vertical difference in salinity at particular locations, with the benthic foraminiferal assemblage reflecting the more saline bottom water. As the sea-level rose from a low-stand of around 125m bpsl after the Last Glacial Maximum, the marine influence would have extended into Lake Carpentaria via channels in the Arafura Sill (Jones and Torgersen, 1988). Tidal channels may have connected the lagoonal Lake Carpentaria with the ocean, creating an intertidal environment, especially on the western margin. As sealevel rose further, it would have flooded into Lake Carpentaria, breaching the 53m bpsl Arafura Sill and creating a semi-enclosed body of mostly marine water. A more fully marine water body would have developed after sea-level rose above the 12m bpsl Torres Strait. This body of oceanic water could have been influenced by terrestrial waters, therefore, also experiencing estuarine conditions Tidally influenced environments Lying between the high and low tide range, the intertidal environment is subject to the continuous presence of tidal currents. Tidally influenced areas of rivers, mangrove and salt marshes, and near-shore environments are included in this definition. A terrestrial influence may be present in episodes of reduced salinity, especially within tidally influenced rivers, and evaporation may increase salinity in the periodically inundated marshes. 66

82 Chapter 3 - Foraminifers In mesotidal and macrotidal areas, such as the present-day Gulf of Carpentaria, there may be large changes for foraminifers inhabiting the intertidal environment during each tidal cycle. Normal marine sea-water advances during flood tide, replaced by fresher water during ebb tide. Salinity may also change due to seasonal fluctuations in rainfall (increases in rainfall lowering salinity) and evaporation rates (increases in evaporation rates increasing salinity). Agglutinating foraminifers are commonly found in extreme or changing conditions, such as reduced salinity, increased turbidity, soft substrate and slightly stagnant bottom conditions (Nagy and Lofaldi, 1981), where organic matter is abundant and ph is low (Petrucci et al., 1983). Many studies have found that in intertidal environments salinity is not always the dominant limiting factor on foraminiferal distribution, as the vertical position of the site within the intertidal zone controls variations in ph, substrate and vegetation (Scott et al., 2001; Horton et al., 2003). Vertical zonation of foraminiferal assemblages is also related to the short periods of stress experienced by the foraminifers due to the alternation of marine and fresh water over the tidal cycle. Species with welldeveloped osmoregulation can survive in this changeable environment (Debenay et al., 2000). In a study of the tidal Cocoa Creek, north Australia (Fig. 3.4), Horton et al. (2003) found agglutinated species such as Miliammina fusca and Trochammina inflata in mangrove-populated salt marshes. The same species have also been found in salt marshes in Hardy Inlet, Western Australia (Quilty, 1977; Fig. 3.4). Along Cocoa Creek, M. fusca and T. inflata occurred above high tide level (elevation m Australian Height Datum, where mean spring high water is 1.21m AHD), and were associated with dense vegetation cover, high elevations, low ph and relatively fine sediment grain sizes (Horton et al., 2003). Agglutinated foraminifers including Trochammina and Ammobaculites agglutinans have also been found in tidal mud flats, in 0-2m water depth, on the edge of a distributary channel within the Mahakam delta, Kalimantan (Fig. 3.4) by Lambert (2003). Debenay et al. (2000) found a similar zonation along the Auray River, France, with T. inflata and Jadammina macrescens occurring along the channel edge in supratidal marshes (above high tide). 67

83 Chapter 3 - Foraminifers Tidally influenced environments Figure 3.4 Location of tidally influenced environments within the region with foraminiferal studies mentioned in the text. Sites are numbered in the order they are discussed within the text. 1: Cocoa Creek (Horton et al., 2003), 2: Hardy Inlet (Quilty, 1977), 3: Mahakam delta (Lambert, 2003), 4: Waitemata Harbour (Hayward et al., 1997, 2004a), 5: Broken Bay (Albani, 1978), 6: Mekong Delta (Debenay et al., 2000), 7: Spencer Gulf (Cann et al., 2002), 8: Cairns (Debenay et al., 2000), 9: Gulf St Vincent (Cann et al., 1988, 1993), 10: Pichavaram (Jayaraju, 2004), 11: Port Hacking (Albani, 1968), 12: Anderson Inlet (Li et al., 2000), 13: Joseph Bonaparte Gulf (Yokoyama et al., 2000 and 2001a), 14: Lake Victoria (Apthorpe, 1980). 68

84 Chapter 3 - Foraminifers However, vertical zonation (and associated parameters) is not the only factor controlling the distribution of these agglutinated species substrates, salinity and water movements may also have an effect. Alve and Murray (1999) documented Jadammina macrescens and Miliammina fusca dominating the landward and seaward sides respectively of marshes along the Skagerrak- Kattegat coast (North Sea). Hayward et al. (2004a) showed that the foraminiferal assemblage composition in the estuarine fringes of Waitemata Harbour, New Zealand (Fig. 3.4), has changed due to human-induced environmental changes. The agglutinated Textularia and Miliammina associations dominate present conditions in variable salinities strongly affected by floods and droughts, compared to the more saline past when Ammonia dominated. In the variable salinity fluviomarine mixing zone of Broken Bay estuary, eastern Australia (Fig. 3.4), Albani (1978) found abundant Textularia candeiana and T. conica. Calcareous species also abundant included the miliolids Quinqueloculina and Triloculina, the rotalids Elphidium and Parellina, and the discorbid Cibicides. Albani (1978) proposed that assemblages in such an area of variable conditions appear to reflect the various water masses i.e. marine and fluvial, while in areas of more stable salinity, the assemblages are controlled by water depth and tidal activity. As previously noted in Section 3.4 on isolated water bodies, Ammonia is a widespread genus adaptable to a range of salinities and environmental conditions. However, Murray (1991) noted that the abundance of A. beccarii tepida is related to the salinity gradient prevailing in estuaries. In a similar finding, Yassini and Jones (1995) ascertained that A. beccarii dominates foraminiferal assemblages on sandy substrates in water depths of less than 1m on the Australian east coast, decreasing in relative abundance from marine lagoons to the open ocean. Within Hardy Inlet, Western Australia (Fig. 3.4), A. beccarii dominates the upstream part of the tidal river, occurring with charophytes (more than one species was noted, but only Lamprothamnium was identified) in fine grained mud and sand (Quilty, 1977). At the sample sites, salinity in the winter may be <5, although a wedge of bottom water may be present with up to 29 salinity, while summer values are (Quilty, 69

85 Chapter 3 - Foraminifers 1977). In the tropical Mekong delta, Vietnam (Fig. 3.4), that is subject to tidal cycles, A. beccarii tepida is dominant where the marine influence is moderated by fresher waters, although it is most common where the salinity is >20 (Debenay et al., 2000). Agglutinated and calcareous hyaline foraminifers are known to dominate brackish marshes (Fig. 3.2). Along brackish intertidal areas where mangroves may be growing, for example in Spencer Gulf, South Australia (Fig. 3.4), Trochammina inflata, Helenina anderseni, Ammonia beccarii and Elphidium cf. articulatum are dominant (Cann et al., 2002). H. anderseni is known to dominate low salinity tropical mangrove swamps around Cairns (Fig. 3.4), northern Australia (Debenay et al., 2000). In the present-day salt marsh around the lagoons of Venice, H. anderseni is also common (Serandrei Barbero et al., 2004). H. anderseni is known to occur in the high tide zone of brackish environments in New Zealand, associated with A. beccarii, E. excavatum and Haynesina (Hayward and Hollis, 1994). H. anderseni has been reported above mean high water (Phleger, 1965; Scott et al., 1991), commonly in areas with higher salinity than where T. inflata occurs (Hayward and Hollis, 1994; Serandrei Barbero et al., 2004). Intraspecific variation is common among the family Elphidiidae, with many formae within Elphidium advenum, E. excavatum and Haynesina depressula recognised by Hayward and Hollis (1994) and Hayward et al. (1997). The family Elphidiidae (and especially the genus Elphidium) is generally a brackish water to continental shelf taxa, living in greatest abundance in intertidal to shallow marine conditions. E. excavatum (also called Cribroelphidium excavatum), as previously mentioned in Section 3.4, is adapted to a wide range of environments, from brackish to hypersaline waters, including tidally influenced areas. E. excavatum occurs over a wide range of tidal sand flats in the upper and middle ranges of a tidal inlet in New Zealand, and with associated H. depressula the assemblage is restricted to intertidal and subtidal areas around the edge of the entrance channel (Hayward et al., 1996). The slightly less cosmopolitan E. advenum may range from an estuarine to deep-water marine environment along the Australian coast (Albani, 1978). Also around Australia, E. 70

86 Chapter 3 - Foraminifers depressulum occurs mainly in the estuarine zone of mixed waters, while E. crispum has been found in shallow-water marine environments and also in estuarine waters (Albani, 1978). E. macellum is commonly a major component of intertidal to shallow subtidal (0-20m) associations on the east coast of Australia (Hayward et al., 1997), while in normal marine intertidal to subtidal assemblages in substrates of clean sand, E. craticulatum is common (Li et al., 2000; Albani and Yassini, 1993). In a study of an estuary on the NSW coast, Australia, the largest populations of Elphidium spp. occurred within sea grass beds on a sandy substrate (Holt, 1997). The preference of Elphidium species for slightly different depth-related habitats was further investigated in Gulf St Vincent, South Australia (Fig. 3.4); with E. crispum most abundant in shallow subtidal waters, while E. macelliforme was found to inhabit deeper waters (Cann et al., 1988, 1993). Cann et al. (1988, 1993) expressed this inverse relationship between the two species numerically and used it to construct a palaeosea-level curve for the area, focussing on fluctuations in sea level from 45-30ka BP. The relationship was noted to be closest to the modelled values in deeper waters, possibly due to transportation effects or wider variations in habitat in the shallower waters (Cann et al., 1993). The association between the family Elphidiidae and Ammonia spp., such as reported by Cann et al. (2002) in Spencer Gulf, has been documented from many environments, especially shallow intertidal flats. In a tidal mangrove estuary along Cocoa Creek, north Australia (Fig. 3.4), Horton et al. (2003) found Ammonia beccarii tepida and Elphidium discoidale multiloculum within an elevation range of 0.16m to 0.35m AHD (i.e. generally between mean high and low water neap tides in the area). This elevation zone corresponded to the seaward fringes of the mangrove community, and the entire mud flats, and was related to barren vegetation, relatively coarse grain size and high ph (Horton et al., 2003). A. beccarii and E. excavatum are found in areas of intertidal sand flats with slightly brackish water and comparatively low nutrients in an inlet in New Zealand (Hayward et al., 1996), whereas within a mangrove community on intertidal mud flats in Pichavaram, India (Fig. 3.4) they are recorded as A. beccarii tepida and Cribroelphidium excavatum (Jayaraju, 2004). Lambert 71

87 Chapter 3 - Foraminifers (2003) recognised a difference in species abundance based on substrate amongst the dominant A. beccarii and the family Elphidiidae in the inner delta front, mouth bar and tidal mud flats of Mahakam delta, Kalimantan (Indonesia), with A. beccarii particularly abundant on a sandy substrate. Around the Lagoon of Venice, Donnici et al. (1997) recognised a distinction between salt marsh, where Haynesina paucilocula dominates, and the lower and less established marsh flat, with dominant Ammonia beccarii and H. paucilocula. However, A. beccarii and H. germanica are widespread in salt marsh and surrounding areas down to 6m water depth on the Skagerrak-Kattegat coast (North Sea), irrespective of mud content of the sediment, whereas E. williamsoni and Miliammina fusca are restricted to sediments with a mud content below 60% (Alve and Murray, 1999). In the intertidal Mok Bay, Netherlands, situated in the North Sea, studies of living foraminifers found the low-diversity foraminiferal assemblage was dominated by Haynesina germanica making up to 98% of the assemblage, with Ammonia, Bolivina, Buliminella, and Elphidium excavatum also occurring (Langezaal et al., 2003). Haynesina germanica is known to be common in marshes and coastal zones (Murray, 1991; Lévy et al., 2000), especially in stations rich in organic matter, leading Cearreta (1988) to conclude that the growth of H. germanica depends on the amount of available food. Tidal range, salinity, and substrate appear to also have an effect on Ammonia- Elphidiidae associations in deeper waters. Salinity appears to determine the relative abundance of species within the tidally influenced Vie Estuary (normal marine to 7 salinity), France. The assemblages are dominated by Ammonia tepida, Cribroelphidium excavatum and Haynesina germanica with the relative abundance of A. tepida and H. germanica increasing upstream, whereas C. excavatum decreases with decreasing salinity (Debenay et al., 2003). Debenay et al. (2000) documented that Ammonia tepida and Haynesina germanica dominate assemblages in deeper intertidal areas of the Auray River (France) especially in the middle reaches of the estuary. However, Port Hacking, eastern Australia (Fig. 3.4), has widespread Ammonia and Elphidium, with Rosalina australis and Discorbis dimidiatus also present throughout the estuary in salinities ranging from and depths up to 20m below high water (Albani, 1968). Within another eastern Australian estuary, Broken Bay (Fig. 3.4), Albani 72

88 Chapter 3 - Foraminifers (1978) found tidal areas on a sheltered side arm of the estuary to include A. beccarii, Elphidiidae (E. advenum, E. crispum, Cellanthecus craticulatus, Cribrononion oceanicus and C. simplex) and Pseudorotalia inflata controlled by water depth (<15m) and distance from the entrance (about 10km). In the intertidal to subtidal (<8m depth) areas of Waitemata harbour, New Zealand (Fig. 3.4), Hayward et al. (1997) found dominant A. beccarii (with Bolivina, Textularia, Haynesina and Elphidium) on muddy sand, in near normal marine to slightly reduced salinities. As well as occurring in the diluted marine waters mentioned above, similar Ammonia-Elphidiidae associations may occur in hypersaline environments where marine water is concentrated by evaporation. In the Casamance River (Sénégal, Africa), studied by Debenay and Pages (1987), the main species within the estuary (in salinities of ) are Ammonia tepida and Cribroelphidium gunteri. Murray (1973) shows that foraminifers possessing all three wall types may occur in hypersaline marshes (Fig. 3.2). In Quaternary sediment, a high percentage of Ammonia beccarii, with relatively abundant E. advenum has been suggested by Li et al. (2000) to indicate an environment of variable salinity in the Anderson Inlet in Victoria, Australia (Fig. 3.4). Whereas the occurrence of dwarf A. beccarii and Elphidium spp. led Yokoyama et al. (2000 and 2001a) to postulate shallow/brackish water conditions in a core in Bonaparte Gulf, Western Australia (Fig. 3.4). In the Marmara Sea (Mediterranean) a Holocene assemblage dominated by Ammonia spp. with Textularia spp. and Elphidium spp., was interpreted by Kaminski et al. (2002) as indicating an environment of shallow and brackish water. In the area of a tidal channel closest to the lagoonal Lake Victoria, southern Australia (Fig. 3.4), with a salinity range of , Apthorpe (1980) found dominant Ammonia aoteanus, while in the semi-marine entrance of the channel, on a silty substrate subject to high current velocity, miliolids (including Quinqueloculina spp., Miliolinella and Triloculina) dominated, with Elphidiidae (E. macellum, E. advena, E. articulatum and C. poeyana) and a small number of planktonic species. Many miliolids, such as Quinqueloculina, are relatively 73

89 Chapter 3 - Foraminifers robust and can withstand the turbulence in high-energy tidal channels (Phleger, 1960). Quinqueloculina spp. are also found in a turbid environment in the Harbour of Alexandria, Egypt, in sandy bottom sediments with some fresh water inflow (Samir et al., 2003). In the more marine influenced areas within the range of tidal influence the diversity of species greatly increases, with many species present that commonly inhabit normal marine waters. In the marginal marine conditions around the mouths of New Zealand estuaries, Hayward and Hollis (1994) found an association containing approximately equal proportions of brackish water and normal marine species. The rotaliids Ammonia beccarii, Elphidiidae (Haynesina depressula, Elphidium advenum and E. charlottensis), the miliolid Quinqueloculina spp. and, from the Order Buliminida, Bolivina spp. are present in a diverse assemblage. Similarly, Ammonia beccarii, Elphidium (E. crispum) and Quinqueloculina spp. are the most abundant species in the intertidal zone along the NSW south coast, Australia, with Rosalina bradyi, and Textularia spp. also abundant (Yassini and Jones, 1995). In tidal channels, near the mouth of the Mekong delta (Vietnam), the assemblage is under marine influence and includes the rotaliids Pararotalia, Asterorotalia and the buliminid Bolivina (Bui Thi Luan et al., 1994; Guélorget et al., 1997; both in Debenay et al., 2000). Species indicating a strong marine influence, such as Pararotalia, Bolivina and Rosalina can occur up to 20km upstream from the river mouth, where salinity is around normal marine in the tropical estuary of the Casamance River (Sénégal, Africa) studied by Debenay and Pages (1987). The farthest seaward of the zones affected by tidal conditions is the near-shore zone. It is populated by a diverse and abundant foraminiferal fauna. Along the near-shore intertidal zone in eastern Australia, calcareous foraminifers dominate; mainly miliolids (commonly prevalent on sandy sea grass beds, Yassini and Jones, 1989) and calcareous hyaline species (although planktonic species are less abundant), with very few agglutinated forms present (Yassini and Jones, 1995). Yassini and Jones (1995) specifically mentioned fauna 74

90 Chapter 3 - Foraminifers including Rosalina spp., Elphidium spp. (E. crispum and E. macellum), Quinqueloculina spp., A. beccarii and Textularia. The shallow marine and intertidal environment is characterized by rapid changes in temperature, salinity, and water movements amongst other parameters which can be responsible for morphological variants in foraminiferal tests. It has been shown that lower water temperatures in winter cause some Elphidium to produce smaller chambers than in the warmer summer, resulting in an uneven peripheral margin (Myers, 1943). Bulgrova (1975) (cited in Boltovskoy et al., 1991) found foraminifers of a smaller size, fragile tests, lacking in ornamentation and of abnormal shapes due to the decrease and fluctuation of salinity in shallower waters. Whereas thicker tests, tighter coiled morphologies, and/or spines may develop in shallow turbulent waters (Wetmore, 1987). Summary of tidally influenced environments Commonly in the intertidal zone, foraminiferal assemblages are composed of a mixture of groups, reworked from both the open marine and estuarine environments by tidal currents (Yassini and Jones, 1995). In intertidal habitats, living foraminiferal distribution is most strongly influenced by salinity and exposure during tidal cycles (e.g. Hayward and Hollis, 1994). The freshwater region of a tidally influenced area, within the fluvial deltaic plain, is commonly barren of all foraminifers and may have associated pyrite and wood remains (e.g. Lambert, 2003). The agglutinated Miliammina fusca, Trochammina inflata, Ammobaculites agglutinans and Jadammina macrescens are characteristic salt marsh species, commonly associated with mangroves and a salinity outside that of normal marine water, and are the major foraminiferal taxa to colonise marsh habitat above the high tide level. Textularia spp. are an agglutinated taxa that can occur in brackish waters below the high tide level. One or more of the rotaliids Ammonia, Family Elphidiidae and Helenina may occur in a wide range of conditions, including brackish waters, fluctuating salinities, and hypersaline waters. The rotaliids Rosalina spp. and Discorbis may occur in intertidal conditions of brackish to around normal marine salinity, while the miliolid Quinqueloculina, buliminids Bolivina and Buliminella, and rotaliids Pararotalia 75

91 Chapter 3 - Foraminifers and Asterorotalia and other species considered marine may be present when the area is subject to a strong marine influence, with only rare agglutinated forms Lagoonal environments A lagoon is a water body separated to some degree from the open ocean. Debenay et al. (2000) classify lagoons as restricted (two or more entrance channels, experiencing tidal circulation) or choked (one or more narrow entrance channels, wind-forcing dominant). The separation of tidally influenced areas and lagoonal environments as presented in this thesis is an arbitrary one, as lagoons are subject to some tidal influence, especially in tidal channels within the lagoon. The following information pertains mainly to the lagoon proper, tidal channels already having been covered in the previous section. As illustrated in Figure 3.2, in the ternary diagram of Murray (1973) within hyposaline/brackish lagoons, mostly agglutinated and calcareous hyaline forms are present. Lloyd and Evans (2002), analysed 24 coastal isolation basins at differing stages of inundation in northwest Scotland, and found foraminiferal distribution to be strongly influenced by salinity and sill height. The authors found an association dominated by Miliammina fusca to signify a basin close to final isolation from marine influence (with a salinity of 25.7 ). It is asserted that marine inundation of basins is signified by a series of five associations dominated by the following foraminifera in order of increasing salinity: Jadammina macrescens and M. fusca; M. fusca and Elphidium williamsoni; E. williamsoni; Haynesina germanica and E. williamsoni; and the highest salinity (37.8 ) Nonion depressulus (Lloyd and Evans, 2002). Similar transitions between landward agglutinated forms and marine calcareous taxa were recognised by Debenay et al. (2000). Jadammina macrescens is present with Miliammina and Trochammina inflata in upstream areas with the lowest salinity in the temperate and tropical lagoons examined by Debenay et al. (2000) as noted in the previous section, all three are known salt marsh species. The authors found that choked lagoons may have the same agglutinated assemblage within their main water body as well. The main water 76

92 Chapter 3 - Foraminifers body of restricted lagoons is commonly dominated by the calcareous Ammonia spp. (A. tepida and A. parkinsoniana) and Elphidiidae (Cribroelphidium gunteri, C. excavatum selseyense and Elphidium limbatum), while other species such as Haynesina germanica and Quinqueloculina seminula may also be present due to the higher marine influence (Debenay et al., 2000). However, the presence of calcareous species, especially the cosmopolitan Ammonia and Elphidiidae, is not necessarily an indicator of increased marine influence on lagoons. Yassini and Jones (1989) attested that in the case of Lake Illawarra, Australia (Fig. 3.5), which varies between salinity, substrate is the controlling factor on foraminiferal assemblages, modifying the influence of salinity. Sandy margins are commonly inhabited by a few agglutinated species (including Miliammina fusca, Textularia inflata and T. porrecta); while in the deeper areas on a silt substrate, A. beccarii and C. sydneyensis are abundant (Yassini and Jones, 1989). Again attesting that calcareous species are not necessarily indicative of an increased marine influence, the well-known transition from agglutinated to calcareous assemblages with increasing proximity to the ocean is not apparent in the Victoria Lakes system, southern Australia (Fig. 3.5). In much of the Victoria Lakes system, which can be classified as choked as it only has one entrance to the ocean, Ammonia and Elphidiidae dominate, although agglutinated forms dominate some low salinity sections (Apthorpe, 1980). At the marine end of Southern Lake Victoria, near the entrance, in an estimated salinity of around 20, Ammonia and E. oceanensis dominate, with agglutinated foraminifers present, while in the peripheral lakes within the Victoria Lakes system where oceanic water has further to travel, agglutinated foraminifers are absent and Cribroelphidium poeyanum and E. articulatum were the only species found (Apthorpe, 1980). Salinity and ph values appear to be the cause of this difference between agglutinated and calcareous forms. Apthorpe (1980) noted that in a ph above 8.5 only the calcareous Elphidium is present, while areas with a ph between contain mixed faunas, and below a ph of 6 agglutinated forms dominate. If Ammonia is present in the latter area it shows severe dissolution effects. 77

93 Chapter 3 - Foraminifers 4 5 Lagoons Figure 3.5 Location of lagoons within the region with foraminiferal studies mentioned in the text. Sites are numbered in the order they are discussed within the text. 1: Lake Illawarra (Yassini and Jones, 1989), 2: Lake Victoria (Apthorpe, 1980), 3: Spencer Gulf (Cann et al., 2002), 4: Cochin Lagoon (Seibold, 1975), 5: Madang Lagoon (Langer and Lipps, 2003), 6: Coorong Lagoon (Cann et al., 2000). 78

94 Chapter 3 - Foraminifers In the lagoonal core sediments of Spencer Gulf, South Australia (Fig. 3.5), Helenina anderseni, with subordinate Ammonia beccarii, and Elphidium cf. articulatum are the pioneer species in the euryhaline setting, replaced by a more agglutinated assemblage as tidal inundation decreases and salinities increase (Cann et al., 2002). When discussing foraminiferal assemblages as indicators of past environments, it must be noted that differences in environments may not necessarily be reflected in the presence of different species, but rather in varying abundances within the assemblage (Albani et al., 1991). Of the main species which are present throughout the choked Lagoon of Venice, Trochammina inflata dominates assemblages in the salt marshes, Haynesina paucilocula dominates in the inner areas affected by pollution and anthropogenic events and Ammonia beccarii is dominant in the marine influenced part of the lagoon (Serandrei Barbero et al., 1997). Around the marine influences entrance other species such as Cribrononion spp., Elphidium spp., Helenina and Quinqueloculina also occur (Serandrei Barbero et al., 1997). Santo André lagoon, Portugal, varies between a stratified closed lagoon and a choked lagoon, due to annual dredging of its entrance. Only Ammonia beccarii, Haynesina germanica and Elphidium oceanensis are present in the fully closed lagoon, which receives mostly freshwater input with occasional storm washovers (Cearreta et al., 2002). When the barrier is breached each year, population numbers increase and Quinqueloculina seminula is added to the assemblage due to the strengthening of the marine influence (Cearreta et al., 2002). Seibold (1975) described a number of foraminifers inhabiting the choked Cochin Lagoon in South India (Fig. 3.5), the dominant taxa being Textularia (T. earlandi, T. foliacea T. oceanica and T. conica), and Ammonia (A. tepida and A. sobrina). Rare (<5%) Pararotalia and very rare (<1%) Rosalina, Asterorotalia, Miliammina, Trochammina and Ammobaculites were also present. However, a similar dominance of Textularia and Ammonia is present in the open reeflagoon system of Moorea Island (in the Polynesian archipelago), the main 79

95 Chapter 3 - Foraminifers difference to the choked Cochin Lagoon being the high abundance of Quinqueloculina in the open reef-lagoon system (Calvez and Salvat, 1980). Murray (1973) has shown that in normal marine lagoons, both calcareous hyaline and porcellaneous foraminifers may dominate, although agglutinated species may also be present (Fig. 3.2). Rare fauna around Moorea Island include Bolivina, Elphidium, Cibicides, Rosalina, and the miliolids Ammomassilina, Spiroloculina and Triloculina (Calvez and Salvat, 1980). The same taxa are present in lagoons around Bermuda: Ammonia beccarii tepida, Textularia, Quinqueloculina, Bolivina, Elphidium, Rosalina and Triloculina along with only a few other species (Javaux and Scott, 2003). Cibicides spp. (including C. refulgens) is present in reefs and lagoonal reefs from the area (Javaux and Scott, 2003). Amongst many of the larger reef foraminifers (not found in the Gulf of Carpentaria cores) Langer and Lipps (2003) documented the occurrence of the agglutinated Planispirinella exigua in the central lagoon floor of the open Madang Lagoon, Papua New Guinea (Fig. 3.5), in a salinity between 23-33%, on a substrate of fine to coarse material (coral debris, mollusc fragments, and larger foraminiferal tests). Bicchi et al. (2002) demonstrate that the aperture of the lagoon and its substrate are the controlling factors in reefal lagoons from the Tuamotu Archipelago, French Polynesia. The central samples from most of the mainly closed larger lagoons had the highest percentage of agglutinated tests, dominated by Textularia, with E. advenum also abundant. Other lagoon centre samples and shallow margins of larger lagoons were either dominated by calcareous hyaline or porcellaneous taxa (including Quinqueloculina spp., Lachlanella and Triloculina). Choked and intermittently closed lagoons may become hypersaline, and are typically dominated by calcareous hyaline and porcellaneous species (Fig. 3.2). In the hypersaline Coorong Lagoon, South Australia (Fig. 3.5), only rotaliids are present. A. beccarii proliferates (associated with the unkeeled Elphidium articulatum), especially in the shallow subtidal waters (Cann et al., 2000). There is a slightly greater diversity of foraminiferal orders present in Araruama, Brazil, one of the largest hypersaline lagoons in the world (salinities ). Debenay et al. (2001) found foraminiferal assemblages to be dominated by Miliolida, (mainly Triloculina oblonga) and Rotaliida (Ammonia tepida and the 80

96 Chapter 3 - Foraminifers less abundant Cribroelphidium excavatum selseyense). Agglutinated species were almost absent, and a large number of the foraminifers found had deformed tests. Vaniček et al. (2000) found dominant H. depressula in the low oxygen, high salinity (up to 38 ) drowned karst lagoons in Croatia. The authors suggest Haynesina is able to withstand variations in temperature, oxygen and salinity, colonising stressed areas, but is not an efficient competitor, as in similar locations with milder conditions other species enter and Haynesina is not found. A. tepida was commonly present with H. depressula, and Quinqueloculina and E. crispum were the other typical colonising species from the area (Vaniček et al., 2000). Summary of lagoonal environments A common, though not infallible, pattern noted in many lagoons, is the change from low diversity assemblages with only agglutinated species present in the areas closest to freshwater input, to diverse assemblages including calcareous forms closest to the ocean. Foraminiferal distribution in shallow water tidally influenced brackish environments marginal to lagoons is controlled by salinity and tidal elevation, although salinity and substrate appear to be most important within the main body of the lagoon (e.g. Yassini and Jones, 1989). Miliammina fusca, Trochammina inflata and Jadammina macrescens may occur in tidal marshes, in areas of a lagoon subject to freshwater input, and even throughout the entire lagoon proper in brackish closed or choked lagoons. Ammonia and the family Elphidiidae are commonly present in all lagoon types, from closed to open, in conditions from hypo- to hyper-saline. It must be noted that Ammonia and Elphidiidae, as previously demonstrated, are cosmopolitan taxa, and their presence amongst many other species is not a diagnostic factor of any particular environment. However, when a species of Ammonia or Elphidiidae is the solitary species present, or combined with only a few others, it is of environmental significance, indicating extreme conditions in which other, less adaptable species cannot survive. Helenina and Textularia may also be found in brackish lagoons in association with Ammonia and Elphidiidae, indicating less harsh conditions, Planispirinella exigua also indicates slightly brackish waters. The presence of calcareous hyaline taxa indicates either close 81

97 Chapter 3 - Foraminifers to normal marine salinity or higher, with Quinqueloculina spp commonly present in tidal channels or within the lagoon when the area is open to marine influence, and Triloculina oblonga found in a hypersaline lagoon. Rare Pararotalia, Bolivina, Rosalina, Asterorotalia, Cibicides and other common marine species such as planktonics may be present in lagoonal waters of around normal salinity and with a connection to the ocean. 3.6 Marine environments Oceanic waters of around normal marine salinity (32-37 ) are included in this category. As sea-level rose after the Last Glacial Maximum, it inundated the former Lake Carpentaria with marine waters, although there was a land barrier along Torres Strait which would have restricted circulation. At a height of around 12m bpsl Torres Strait was breached, establishing the connection between the Pacific and Indian Oceans. As sea level rose further, the connection was strengthened with the deepening waters allowing increased circulation and exchange of waters between the Gulf of Carpentaria and adjacent oceans. As the Gulf of Carpentaria is situated on the Australian continental plate between New Guinea and Australia, it is a continental shelf sea. However, open marine species are present (although not necessarily developed into a true open marine assemblage) since the barrier between the Pacific and Indian oceans was breached Continental shelf environments Continental shelves are discussed in terms of various zones, which range in depth and width due to regional characteristics of each continental shelf system. The Australian shelf varies from 15km wide off the south-eastern coast of Australia to over 400km wide in the Timor Sea. The Australian inner continental shelf extends from the moderate to high energy near-shore coastal zone, sloping down to the midshelf, which is about m deep. Beyond the midshelf is the lower energy outer shelf, which continues to slope down to the 82

98 Chapter 3 - Foraminifers shelf break at around m water depth (globally, the shelf break is in a water depth of between m). Shelf seas display the dynamics of the deep ocean, modified by shallow water depth and the presence of the coast. Wind stress and tidal forces cause moderate to strong water movement, which affects the sea floor in the shallow water depth of shelf seas. Proximity to terrestrial influences may influence environmental parameters of the shelf sea such as salinity and turbidity due to river runoff. On tropical coasts, in the shallow water normal marine salinity area of the inner shelf, foraminiferal assemblages are dominated by abundant calcareous porcellaneous and hyaline taxa (Murray, 1991). Throughout the western section of the Western Australian shelf (Fig. 3.6) the only species which on average exceed 5% of the foraminiferal assemblage are Cibicides refulgens, Globorotalia menardii and Globigerina bulloides, although Elphidium is numerous in the inner shelf zone (Betjeman, 1969). However, porcellaneous species were found to dominate, with hyaline species also abundant, in water depths of 5-30m at Exmouth Gulf on the Western Australian coast (Fig. 3.6) by Haig (1997). Li et al. (1999) found the orders Miliolida (species Quinqueloculina and Triloculina) and Rotaliida (Families Discorbidae and Elphidiidae, with dominant Cibicides refulgens) distinguish the fauna of the inner to middle shelf (<100m) in south Western Australia (Fig. 3.6). Foraminifers of the south eastern Australian inner shelf (Fig. 3.6) extending to 60m depth are mainly Miliolida, Textulariida and Rotaliida (Elphidiidae); with Parellina especially abundant (Yassini and Jones, 1995), although Gallagher et al. (2001) classify Parellina as a middle shelf genus. Calcareous porcellaneous foraminifers are most abundant in muddy sand facies, in shallow, turbid waters (up to and including the intertidal sea grass beds) of Exmouth Gulf, Western Australia (Orpin et al., 1999). However, Haig (1997) noted that seagrass beds were not common throughout Exmouth Gulf, 83

99 Chapter 3 - Foraminifers Continental shelves Figure 3.6 Location of sites within the region mentioned in the text where foraminiferal studies have been undertaken on the continental shelf. Sites are numbered in the order they are discussed within the text. 1: western edge of Western Australia (Betjeman, 1969), 2: Exmouth Gulf (Haig, 1997; Orpin et al., 1999), 3: southern Western Australia (Li et al., 1999), 4: south eastern Australian shelf (Yassini and Jones, 1995), 5: mouth of river Murray (Li et al., 1996), 6: Spermonde Shelf (Renema and Troelstra, 2001), 7: Gulf of Carpentaria (Albani and Yassini, 1993), 8: Cochin Lagoon (Seibold, 1975). 84

100 Chapter 3 - Foraminifers which may explain the small number of larger porcellaneous foraminifers in their studies. Miliolids mostly occur in the shallowest zone (36-87m) of the southern Western Australian shelf studied by Li et al. (1999). Of the order Miliolida, Quinqueloculina philippinensis is best known from tropical shelf seas of the Indo-West Pacific region, as well as its presence within the Exmouth Gulf fauna (Haig, 1988). Quinqueloculina vulgaris is noted as one of the most abundant calcareous species on the western edge of the western shelf by Betjeman (1969), although many miliolids including Q. kerimbatica var. philippinensis are noted as widespread. Orpin et al. (1999) found Rotaliida to be most abundant in gravely-mud sediment, in low energy conditions in the inner shelf environment of Exmouth Gulf, Western Australia. Similarly, Ammonia beccarii occurs in shallow waters, and Li et al. (1996) state that it is present in abundance at <60m around the mouth of the River Murray, South Australia (Fig. 3.6). Within a reef complex on the Spermonde Shelf off Sulawesi (Fig. 3.6), separated from the open ocean, E. craticulatum occurs abundantly on the reef slopes (0-33m deep), reaching its highest density in the deepest samples (Renema and Troelstra, 2001). It is also noted as a widespread species off the western edge of the Western Australian shelf by Betjeman (1969). The present shallow shelf environment (<70m) of the Gulf of Carpentaria (Fig. 3.6) has a relatively simple assemblage amongst the Elphidiidae dominated by E. stratopunctatum, with Parellina hispidula and E. carpentariensis also present (Albani and Yassini, 1993). In the inner to mid shelf of the western edge of the Western Australian shelf, in water depth of fathoms (i.e m), Elphidium spp. (including E. advenum, E. crispum and E. simplex) were present (Betjeman, 1969). In the Elphidiidae family, Elphidium crispum is known to be a cosmopolitan species preferring warm waters of normal salinity (Murray 1991). Along the inner shelf of the Western Australian coast, the rotaliids Pararotalia nipponica (Calcarina calcar of Quilty, 1977) and Asterorotalia gaimardi (Exmouth Gulf, Orpin et al., 1999) are common. Cibicides refulgens is present over the entire area of the westernmost section of the Western Australian shelf studied by Betjeman (1969), except in areas shallower than 10 fathoms (18m), and it is most abundant between fathoms (18-219m). C. refulgens is a cosmopolitan species living over a broad latitudinal 85

101 Chapter 3 - Foraminifers range, which attaches to a substrate (including seagrass) in shelf to upper slope seas (Murray 1991). In the shallow inner shelf in Exmouth Gulf, Western Australia, Textulariida show a preference for gravely-mud substrate, and are especially prevalent in low energy sheltered bay areas (Orpin et al., 1999). Betjeman (1969) recorded Textularia agglutinans and T. foliacea (among others, including T. candeiana) along the western edge of the Western Australian shelf, being most abundant between fathoms (37-73m). Betjeman (1969) noted that agglutinated species are most abundant in the warmer protected waters of Exmouth gulf, on medium grained quartz sands. Global studies of inner shelf seas differ somewhat from Australian shelf seas, due to local effects as well as the influence of different oceanic water masses. On the tidally affected inner shelf of the Gulf of Khambhat (India), the agglutinated Ammobaculites, and the calcareous hyaline Ammonia spp., Asterorotalia and Pararotalia are abundant in the living assemblage but there is a general absence of calcareous porcellaneous taxa (Nigam, 1984). In a similar area, in shallow water up to 35m off Cochin Lagoon on the south coast of India (Fig. 3.6), Seibold (1975) described a number of foraminifers including Ammobaculites, Spiroloculina, Quinqueloculina, Triloculina, Bolivina spp. (including B. striatula), Cancris auriculus, Elphidium spp. (E. aff. discoidale and others), Cassidella, Nonion and Nonionella. Rare Gallitellia vivans occur outside the Cochin Lagoon, India, with rare dead tests inside the lagoon (Guembelitria vivans of Seibold, 1975). In a low salinity coastal zone off Guinea (Atlantic Ocean), studied by Debenay et al. (1987), Ammonia tepida, A. parkinsoniana, and Cribroelphidium gunteri (lower salinity species) are found with Quinqueloculina trigonula and Nonion (inner shelf species). Textularia (T. agglutinans, T. conica and intermediate morphotypes) dominate a nutrient-poor zone where the influence of the Po delta wanes between 21-46m depth on the Adriatic continental shelf, with Rotaliida (of the family Discorbidae and Dibicididae) and Milioliida also present (Donnici and Serandrei Barbero, 2002). 86

102 Chapter 3 - Foraminifers Assemblages on continental shelves are not only very different depending on the area, but they also vary temporally. In the shallow waters (up to 55m) of the inner shelf off California (Pacific Ocean), McGann (2002) documented dominant Elphidiella, with other Rotaliida (including Elphidium excavatum var. selseyensis, Nonionella and Buliminella), and the miliolid Quinqueloculina from a study undertaken in the 1930s. Modern inner shelf fauna down to 30m water depth were dominated by the agglutinated Saccammina spp., with calcareous taxa also present (McGann, 2002). Middle shelf assemblages (40-95m) from the 1930s were dominated by calcareous hyaline species (Elphidiella and Globobulimina), while the modern middle shelf assemblages occurring at water depths of m were dominated by agglutinated species. The outer shelf assemblages varied the least with time, both being dominated by calcareous hyaline taxa including Uvigerina, Cassidulina, Globobulimina, Nonionella, Bolivina, Buccella and Cancris, while agglutinated species were also present in the modern fauna. Murray (1973) depicted most shelf seas as dominated by calcareous hyaline and agglutinated foraminifers (Fig. 3.2). The middle to outer southeastern Australian shelf (60-140m) has abundant calcareous hyaline taxa including Bolivina, Brizalina, Bulimina, Cassidella earlandi, Fissurina, Heterolepa subhaidingeri, Spiroloculina, Lagena and 15-20% planktonic species including Globigerina bulloides (Yassini and Jones, 1995). Of the agglutinated taxa, Textularia spp. are present amongst many that do not occur in the Gulf of Carpentaria material (Yassini and Jones, 1995). Cibicides spp. (including C. refulgens), Textularia agglutinans, Globigerinoides sacculifer, Quinqueloculina and Miliolinella amongst others characterise the mid to outer shelf ( fathoms, i.e m) off the western coast of Western Australia (Betjeman, 1969). Li et al. (1999) noted benthic diversity increased below about 80m towards the outer shelf of southern Western Australia. Li et al. (1999) also found dominant Cibicides on the outer shelf, along with Heterolepa, Bolivina, Uvigerina, Textularia and planktonic species (including Globigerinoides trilobus, Gs. ruber, Globigerina bulloides, and Globorotalia inflata). Li et al. (1996) noted that planktonic species can comprise up to 10% of foraminiferal assemblages at water depths of 100m off South Australia, and in deeper waters can constitute 40-50%. 87

103 Chapter 3 - Foraminifers Summary of shelf environments Inner to mid-shelf assemblages in Australia are dominated by calcareous porcellaneous taxa (especially Quinqueloculina and Triloculina) and calcareous hyaline forms (Elphidium, Parellina, Cibicides, Asterorotalia, Pararotalia). Agglutinated species are present in areas most proximal to the coast, as well as on the westernmost and southeastern middle shelf. The planktonic species that dominate mid to outer shelf assemblages include Globigerinoides, Globorotalia and Globigerina. Other taxa common on the mid to outer shelf include Cibicides, Bolivina, Heterolepa and Textularia, while Brizalina, Bulimina, Cassidella, Fissurina, Spiroloculina, Lagena and Uvigerina may also be present Open marine environments Beyond the Australian continental shelf, the continental slope steeply drops to the abyssal plain where water depths are 4000m or more in the Indian and Pacific Oceans. In this category, the waters are purely oceanic, away from continental influence, with marine salinity (35 ) and subject to global circulation patterns. Loeblich and Tappan (1994) presented a comprehensive report on the foraminifers around the Sahul Shelf and Timor Sea (Fig. 3.7) covering a large range of environments from a sand flat above sea level to shallow coral reefs and to 3200m bpsl (in the Timor Trough). Sample sites were influenced by currents originating in the Pacific, Antarctic and Indian Oceans. The orders Rotaliida account for 24%, Lagenida for 23%, Miliolida for 16%, Buliminida for 15%, various agglutated taxa for 15%, and the planktonic Globigerinida for 5%. In studies of the Australian continental shelf, planktonic taxa dominate the outer shelf and continental slope areas, as well as deeper waters (Fig. 3.7: western edge of Western Australia, Betjeman, 1969; southern Western Australia, Li et al., 1999; southeastern coast, Yassini and Jones, 1995). Bolivina spp., Rosalina spp. and small miliolids are the dominant benthic forms in m water depth along the southern shelf of Western Australia (Li et al., 1999). 88

104 Chapter 3 - Foraminifers Open marine Figure 3.7 Location of sites within the region mentioned in the text where foraminiferal studies have been undertaken on open marine areas. Sites are numbered in the order they are discussed within the text. 1: Sahul Shelf and Timor Sea (Loeblich and Tappan, 1994), 2: western edge of Western Australia (Betjeman, 1969), 3: southern Western Australia (Li et al., 1999), 4: southeastern coast (Yassini and Jones, 1995), 5: Visakhapatnam shelf (Rao et al., 1979), 6: Ashmore Reef (Glenn and Chaproniere, 2001). 89

105 Chapter 3 - Foraminifers Along the continental slope of the southeastern coast Bolivina, Bulimina, Fissurina and Lagena are among the benthic taxa in water depths >140m (Yassini and Jones, 1995). Rao et al. (1979) stated that on the Visakhapatnam shelf (Indian Ocean, Fig. 3.7) the cosmopolitan and abundant Bolivina vadescens is mainly affected by bottom water temperature and currents, as salinity and type of substrate seemed to have little effect on its distribution and population density. This is in accord with the assertion that the families Bolivinidae and Buliminidae are associated with upwelling phenomena (Debenay and Redois, 1997). Denne and Sen Gupta (1991) and Osterman (2003) found Bulimina spp. on the outer shelf, slope and abyssal plains of the Gulf of Mexico, Atlantic Ocean. However, Bolivina may occur in shallower waters, as on the Guadiana shelf (marginal to the Atlantic Ocean), Mendes et al. (2004) found Bolivina spp. correlated to muddy sediments in a low energy environment in water depths between 40-95m around the mid-shelf, and Boltovskoy et al. (1991) report Bolivina is generally restricted to 30-2,000m. The dominant planktonic species on the continental slopes of the Australian shelf include Globigerina bulloides (western edge of Western Australia, Betjeman, 1969; southern Western Australia, Li et al., 1999; southeastern coast, Yassini and Jones, 1995), Globorotalia menardii (western Western Australia, Betjeman, 1969; southern Western Australia, Li et al., 1999), and Globigerinoides ruber (western Western Australia, Betjeman, 1969; southern Western Australia, Li et al., 1999; southeastern coast, Yassini and Jones, 1995), with other species such as Gs. trilobus and rare Tenuitella found in southern Western Australia by Li et al. (1999). Of the planktonic foraminifers, Globigerina bulloides is abundant in polar and transitional regions when present in subtropical and tropical waters it may indicate the influence of the Southern Ocean, and it is also reported as an upwelling species (e.g., Kroon and Ganssen, 1988; Hemleben et al., 1989). Schmuker and Schiebel (2002) found G. bulloides to be most common in the water column between 0-20m, in an average temperature of 26.2 C and 90

106 Chapter 3 - Foraminifers average salinity of 36.4 in the Caribbean Sea. Globorotalia cultrata is known as an equatorial and tropical species, preferring warm sea surface temperatures and normal marine salinities. However, Gr. menardii (considered synonymous with Gr. cultrata) is found in a similar environment to G. bulloides in the Caribbean Sea, in the water column between 0-20m, at an average temperature of 26.3 C and average salinity of 36.1 (Schmuker and Schiebel, 2002). Glenn and Chaproniere (2001) report Gr. cultrata and Globigerinoides ruber from the surface sediments on the reef flat of Ashmore Reef, Western Australia (Fig. 3.7). Gs. ruber and Gs. trilobus are predominantly subtropical species, with Gs. ruber one of the shallowest dwelling planktonic foraminifers (Bé, 1977; Hemleben et al., 1989). Gs. ruber is one of the species that has been found most abundantly in the warm low-latitude oligotrophic waters both north and south of the equator (Feldberg and Mix, 2002). Tenuitella is found in temperate to subtropical regions. Hilbrecht (1996) found T. iota in the subtropical and temperate Indian Ocean. In the Caribbean Sea, T. parkerae is most common at a depth in the water column of 20-40m, at an average temperature of 26.6 C and average salinity of 36.1 (Schmuker and Schiebel, 2002). Globigerina bulloides and Globigerinoides ruber, although common open marine species, are also known to occur in more estuarine environments such as around Windang Island, offshore Illawarra (Yassini and Jones, 1995). Some species occur in the open ocean in tropical latitudes, while in the temperate zone they are found in estuarine environments: Turborotalia inflata (Pseudorotalia inflata of Albani, 1974) and Cribrononion hawkesburiensis (Albani and Yassini, 1993) are both documented from southeastern Australian estuaries. The deeper waters of the Pacific Ocean share many taxa in common with continental shelf areas of Australia (e.g. mid to outer southeastern shelf, Yassini and Jones, 1995). Textularia, Brizalina, Bulimina, Cassidulina, Cibicides refulgens, Fissurina, Lagena, Quinqueloculina, Triloculina, and Uvigerina can be found within seamounts of the central North Pacific at water depths between m (Ohkushi and Natori, 2001). Ammomassilina alveoliniformis, which also occurs in the reef lagoonal system of Moorea Island (Calvez and Salvat, 91

107 Chapter 3 - Foraminifers 1980), has been found rarely in hemipelagic mud beside hydrothermal vents at a depth of 2430m in the northeast Pacific (Jonasson et al., 1995). Summary of open marine environments Calcareous forms dominate open marine areas, in the Sahul shelf and Timor Sea agglutinated species only account for 15% of the total population found. Assemblages on the edges of continental shelves are highly diverse and abundant. Planktonic species are commonly indicative of open marine conditions, dominating assemblages on continental shelves and deeper water. Planktonic species found around the Australian continental shelf include Globigerina bulloides, Globorotalia cultrata, Globigerinoides ruber, Gs. trilobus and Tenuitella. Of the benthic species the calcareous hyaline taxa Bolivina spp., Rosalina spp., Bulimina, Fissurina and Lagena may be present, and small porcellaneous species have also been found. 3.7 Thanatocoenoses As living foraminifers die or reproduce, their tests are deposited in the sediment as thanatocoenoses, or death assemblages. These are necessarily subject to sedimentological processes before being collected and studied. Gooday and Hughes (2002) stressed that differences in the species composition of live and dead assemblages reflect a mixture of taphonomic processes and life processes (particularly differences in production rates). The empty foraminiferal tests that eventually will comprise the fossil assemblage are primarily produced by the living foraminiferal community inhabiting the upper few centimetres of the sediment. Usually the composition of the fossil assemblage, therefore, reflects that of the corresponding live community. However, species not occurring in the live community may be added to the fossil assemblage. For example, bottom currents may carry foraminifera from adjacent but environmentally different areas (Murray et al., 1982), or deepburrowing macrobenthos may mix fossil material into the modern sediment layer. 92

108 Chapter 3 - Foraminifers On the other hand, selective destruction of foraminiferal tests may result in the poor representation or absence of poorly-preserved species in the fossil assemblage. Many arenaceous foraminifera may disintegrate completely after death of the organism. Some tests are only weakly cemented with easily degradable organic cement. Other species use iron-oxide compounds in their cement, which become unstable in the reducing environment below the oxidized surface sediment layer (Schröder, 1988). Calcareous tests are known to dissolve in acidic conditions, whether as acidic pore-water within the sediment or an entire water body of lowered ph. In modern estuarine and sheltered harbour environments, lowered ph commonly occurs due to high levels of dissolved organic matter and consequent lowered oxygen concentrations (Hayward et al., 2004a). Culturing experiments using live Ammonia in normal salinity waters of different ph conditions (normal seawater commonly has a ph of around 8) have shown that decalcification begins below 7.5 (Le Cadre et al., 2003). Most chemical and biological destruction occurs in the top few centimetres of sediment, due to reworking by benthic fauna inhabiting that zone (de Stigter et al., 1999). Deep-infaunal species, which produce tests below the zone of most intense destruction, may be more easily preserved (Loubere et al., 1993). Fragile smaller species of foraminifers are more prone to chemical and mechanical destruction during transport than larger species. For example, the small planktonic genus Tenuitella is commonly abundant in modern sediments, but it is easily removed by winnowing and dissolution (van Eijen, 1995). In a study of the shelf of the south coast of Western Australia, Li et al. (1999) found the most common relict forms are of miliolids (Quinqueloculina), elphidiids (E. advenum), and discorbids (Discorbis dimidiatus, Rosalina australis). Some larger taxa and cibicidids (Cibicides refulgens) may also be found as relicts, but other groups are rarely found relict. Cann and Gostin (1985) stated that tests of smaller foraminifera are easily winnowed from their environments to be deposited elsewhere. In a study of channel deposits along the north Australian macrotidal South Alligator River, 93

109 Chapter 3 - Foraminifers practically all thanatocoenoses were dominated by small-sized specimens (<150µm) with full-marine affinities that are prone to floating (e.g. Schackionella globosa and globigerinids, Wang and Chappell, 2001). The proportion of relatively large and heavy marine forms, such as Quinqueloculina philippinensis, was found to decrease upstream (Wang and Chappell, 2001). Such inland transport of marine species does not occur in the mesotidal McArthur River (Wang, 1990), which flows into the Gulf of Carpentaria, indicating the importance of tidal regimes to thanatocoenoses. In addition, the processes of erosion of the source rock, transportation, sedimentation and preservation, all influence the reworked assemblage; and information may be gained about these aspects by studying the dead assemblage (Harper and Collen, 2002). The influence of water movements on foraminiferal thanatocoenoses can be used to map these movements in palaeoenvironmental studies, as described by Debenay (1988) in a lagoon in New Caledonia. Alve (2003) pointed out that different taphonomic processes (e.g. loss or gain of tests through transport; loss of calcareous tests through dissolution; loss of organo-agglutinated tests through decomposition of the organic cement) operate at different scales in different sites. Estuarine environments are particularly subject to fluctuating conditions, so care must always be taken to interpret thanatocoenoses as only an average assemblage since the factors involved in the formation of the thanatocoenoses are themselves subject to change (Scott and Medioli, 1980). Debenay et al. (1998) demonstrated that interpretation of thanatocoenoses must depend on the environment of deposition when comparing two particular lagoons. In the restricted Cananéia Iguape lagoon (Brazil) the thanatocoenoses reflect conditions over the entire year, while in the Lagoa da Conceicao (Brazil), living populations only develop in the dry season when salinities are favourable, so, therefore, the thanatocoenoses only reflect dry season populations (Debenay et al., 1998). 94

110 Chapter 3 - Foraminifers Jorissen and Wittling (1999) discussed four processes in the transition from living to dead assemblage given by Murray (1991): transport, test disintegration, in-situ mixing of faunas over time, and population dynamics. Focusing on comparing living and dead assemblages to expand the latter point, Jorissen and Wittling (1999) found that deep and intermediate infaunal taxa are present in relatively the same numbers throughout the year, while the more opportunistic epifaunal and shallow infaunal taxa experience a population boom in summer with increased food supply. Therefore, compositional differences between live communities and dead assemblages are likely to occur as a result of differences between the test production rates in species (Stigter et al., 1999). In favour of thanatocoenoses, Murray (2003) mentions the benefit of the cumulative effect of dead assemblages. At any particular time, if sampling living assemblages, rare or ephemeral species will be missed. Rare or ephemeral species are likely to be found in thanatocoenoses, unless dissolved or transported. 3.8 Summary Because foraminifers occupy a wide range of environments they are very useful as environmental indicators. Particular species and assemblages can be used to identify palaeoenvironments ranging from fresh water to the deep sea. The environments detailed above, and their respective foraminiferal associations and indicator species, will be discussed in relation to their occurrence in the palaeoenvironment of the Gulf of Carpentaria (Chapters 5 and 6). Entire foraminiferal species assemblages are the basis for the reconstruction of the palaeoenvironment of the Gulf of Carpentaria, however, various indicator species are considered, and morphological variations and diagenetic processes are also noted for the information they provide. 95

111 Chapter 4 Methods CHAPTER 4 Methods 4.1 Introduction A detailed description is presented of the methods used to obtain and analyse samples for the purposes of this study, from the selection of the core sites (Section 4.2), to the sub-sampling processes (Section 4.3), foraminiferal analysis (Section 4.4), sedimentary analysis (Section 4.5), and dating (Section 4.6). 4.2 Material selection and collection Seismic data A ship-borne seismic survey of the Gulf of Carpentaria was completed by the United States Geological Survey (USGS) in Up to seventeen reflective surfaces were noted (Chivas et al., 2001; Edgar et al., 2003). The reflective surfaces represent unconformities, indicating the subaerial exposure of major transgressive/regressive cycles. Evidence of incised channels can also be observed in the data. These seismic data were used to select sites for the piston core collection. Obvious channels were avoided in order to core relatively undisturbed material. Cores were also taken spanning a cross-section of the basin from the shallow margins to the deeper centre, in order to gain information on palaeo-lake levels through observing subaerially exposed horizons, as well as to ensure the collection of material from some deeper areas that would have remained constantly under water Piston Core collection In 1997 six piston cores were collected from the Gulf of Carpentaria, utilising the research vessel Marion Dufresne. The joint Australian/French/USA operation was largely financed by the Australian Research Council, and as part of the IMAGES III (International MArine Global change Study) program operated by the Institut Français pour la Recherche et la Technologie Polaires. Material from these sediment cores forms the basis of this study. 96

112 Chapter 4 Methods Using PVC as the barrel liner to capture the sediment, and up to ten tonnes of lead weight to drive the corer into the sediment, cores ranging in length from 4.2 to 14.8m were recovered. Onboard the Marion Dufresne, the retrieved cores were sectioned to 1.5m lengths; passed through a MST (multi-sensor track recorder) to determine magnetic susceptibility, P-wave velocity and γ-ray bulk density measurements; split longitudinally and subject to digital photography and digital and Munsell colour measurement, and preliminary core description. These data are reported in Chivas et al. (2001). Once split longitudinally onboard the ship, one split half of the core was labelled as archive material, wrapped in plastic-film and another layer of stronger plastic, and stored in a plastic container. All subsequent activities were undertaken upon the other, working, half. Since their collection, the cores have been maintained whenever possible at 4 C. One core (MD 32) was designated as the IMAGES reference core and initially stored at CEREGE (Centre Européen de Recherche et d'enseignement des Géosciences de l'environnement) in France. The five remaining cores, and the working half of MD 32, are currently stored in a 4 C cool room at the University of Wollongong, Australia. 4.3 Core preparation and sub-sampling For the five cores stored at the University of Wollongong, all core preparation activity (except photography and weighing) was undertaken within the cool room. The working half of MD 32 was segmented in France and transported to the University of Wollongong. The majority of core preparation activity was carried out by the following: Adriana Garcia (Research Assistant/Post-doctoral Fellow), Martine Couapel, Sabine Holt and Jessica Reeves (PhD students). The following people also worked on preparing the core: Allan Chivas (Head of project), Sandrine Pendu (undergraduate student and visitor from the University of Paris) and Grant Pearson (Bachelor of Science Honours student, University of Wollongong). Core preparation activity was as follows: 1. The working half of each core was photographed and a visually-based descriptive log created. 97

113 Chapter 4 Methods 2. Utilising specially prepared stainless steel implements (washed between each use with a cloth and tap water, then rinsed with deionised water) the working half of the core was segmented into 1cm depth-slices (a few slices were larger due to obstructions within the sediment such as concretions larger than 1cm). Every fifth slice (designated a sample slice) was further subsampled. Sample slices were also prepared around horizons of visible change, with the remaining slices treated as archive material. The entire working half of all six cores was segmented into 1cm depth slices. 3. To clean these slices of potential contamination from the PVC coring pipe they were collected in, and also from the face exposed to the longitudinal splitting and the cover of cling-wrap for transportation and storage, 3mm of material was removed from the edges using steel spatulas (washed with a cloth and tap water, and rinsed with deionised water between uses). 4. Initially, cleaned archive slices were wrapped in pre-weighed aluminium foil, weighed, labelled and stored in sealed plastic bags. Later, when glass Petri dishes were used, the original slices were unwrapped, cleaned of any residual aluminium, reweighed and stored flat in the Petri dishes with tape-sealed lids. After the change in methodology, any new cleaned archive slices were then weighed and stored flat in the Petri dishes. The Petri dishes were previously engraved, and in a clean laboratory designed to minimise dust, washed with tap water, rinsed with deionised water, weighed, and dried. 5. The sample slices were further sub-sampled for a variety of purposes (Fig. 4.1). 6. As apparent in Figure 4.1, to obtain the sub-sample for later micropalaeontological analysis, an aluminium ring (28mm in diameter) was inserted into the slice. The aluminium ring was washed with tap water and rinsed with deionised water between each use. The sediment within the ring was transferred to a 250ml glass jar (previously washed, rinsed, dried, labelled, and weighed within a clean laboratory ) with a plastic lid, and weighed immediately. 98

114 Chapter 4 Methods Sample for palynomorph and dinoflagellate identification Sample for organic geochemistry Sample for diatom identification Sample for coccolith identification 95mm 3mm wide layer discarded Sample retained as archive Sample for micropalaeontological analysis Sample for sediment analysis Figure 4.1 Schematic sub-sampling of sample slices (modified from Chivas et al., 2001). 7. This sediment aliquot was dried (within jar with lid unscrewed but still covering jar) overnight in an oven between C in the clean laboratory. Subsequent weighing was used to determine moisture loss (i.e. the sediment s original water content). 8. Deionised water was added to the jar and the sediment left to disaggregate for at least 15h. Although every effort was made to process samples quickly, some were left standing in water for a period of months. Later ph analysis of the water failed to differentiate between recently hydrated samples and older samples, so it is assumed to have had little or no effect. 99

115 Chapter 4 Methods 9. The water and sediment were washed through nylon-meshed plasticsupported sieves with deionised water within the clean laboratory. Initially, a tier of three separate mesh sizes was used: 150µm, 105µm and 63µm. 10. Sediment was collected in three separate (previously washed with water and/or alcohol, rinsed with deionised water) ceramic crucibles (i.e. size fractions of sediment >150µm, µm and µm) and dried in an oven overnight at between C. 11. Sediment from each of the three size fractions was then transferred (with a nylon paintbrush) onto previously weighed paper, weighed, and transferred into a bottle. Initially the bottle was plastic, but later, glass bottles were used with plastic lids having an aluminium coating. The >150µm size fraction was further split using a stainless steel micro splitter (washed with tap water, rinsed with deionised water, dried) and weighed as necessary to produce a subsample of about 0.2g. All sub-samples were stored separately in bottles. 12. Later, a methodology change dispensed with the tier of three separately sieved size fractions washing sediment through only one nylon sieve of 63µm size. 13. The >63µm fraction was processed as the >150µm was previously (weighed, split, weighed, stored in glass bottles). Additional steps were taken to recombine the earlier work of separately storing fractions in plastic/glass bottles. Where it was possible (with larger samples) the three size fractions were split in half, recombined (to make a sub-sample of sediment >63µm), which was weighed, and split and weighed until a further sub-sample of about 0.2g was isolated. 4.4 Foraminiferal analysis Samples from each 5cm of the top 1.5m of all six cores were studied, as well as from horizons of visible change that were also sampled in the initial process (step 2 of Section 4.3). From step 13, above, the 0.2g sediment sample (>63µm) was poured into a black painted picking tray and examined under a 100

116 Chapter 4 Methods Zeiss SV-8 research stereomicroscope (with an equivalence of 8 to 64x total magnification). A representative selection was sought by the pouring and picking methodology. Sediment was poured in vertical lines, while it was viewed for picking in horizontal lines. Sample-size permitting, the first 300 foraminifers viewed in this manner were picked using a synthetic-haired paintbrush and tap water. Further viewing of the picking tray ensured isolated foraminifer species not represented in the first 300 were also picked and noted. A few samples had less than 300 foraminifers, and in these cases all the foraminifers were picked. All ostracods and all charophytes were picked. Representatives of other biological remains (e.g. molluscs, echinoderms) and mineral grains (e.g. quartz grains), were also picked. Microfossils and other material from larger samples (i.e. those which were split to give a final >63µm sub-sample of 0.2g) were glued (using a cinnamon-laced tragacanth gum) onto plastic or cardboard slides with glass covers. Upon inspection of small samples (i.e. the entire >63µm sediment sample weighing less than 0.2g and therefore not split), an effort was made to reduce contamination to cater for future geochemical analysis. Microfossils from these single samples were placed in plastic or cardboard slides, unglued. The identification of the microfossils was made using a variety of sources, mainly Loeblich and Tappan (1994), but also including Albani and Yassini (1993), Yassini and Jones (1995), Warren (1975), Hayward et al. (1997), and referring to Loeblich and Tappan (1987). Scanning Electron Microscope images of foraminifers and other selected material from the sediment sample were taken using a Lecia Cambridge 440 SEM at the University of Wollongong s Faculty of Engineering. Gold-coating of the material was undertaken by David Carrie from the University of Wollongong s School of Earth and Environmental Sciences (SEES). 101

117 Chapter 4 Methods 4.5 Sedimentological analysis Certain sedimentary components were investigated further. The Leica Cambridge 440 SEM at the University of Wollongong s Faculty of Engineering, equipped with EDAX (energy dispersive analysis x-ray) capabilities, was utilised to analyse the composition of green clays suspected to be glauconitic grains. Carbon coating of the grains was carried out by Nick Mackie of the Faculty of Engineering at the University of Wollongong. Sediment particle size analysis was performed on the University of Wollongong s Faculty of Engineering Malvern Mastersizer 2600 by Adam Switzer (University of Wollongong SEES) utilising the method outlined in Chivas et al. (2001), with the results also displayed in Chivas et al. (2001). 4.6 Radiocarbon dating Selection of material Upon inspection under a microscope, it is clear that a proportion of the calcareous microfossils in the uppermost marine unit has been reworked from the underlying non-marine units, as indicated by the presence of reworked and calcified non-marine foraminifers (e.g. Ammonia-ooids, see Fig. 5.3) and ostracods (e.g. Ilyocypris and Cyprideis). Care was taken in the selection of material for AMS (Accelerator Mass Spectrometry) radiocarbon dates. Most samples chosen were well preserved, clean and without evidence of reworking. Those of questionable origins were noted, and all specimens were photographed. Where applicable, marine species were chosen from the marine horizon to be dated, rather than other species of a non-marine affinity occurring in the same horizon. Bivalve, gastropod and ostracod shells were used. Dr Winston Ponder is gratefully acknowledged for identifying the molluscs dated, although responsibility for any errors rests with the author. Where possible, material for dating was chosen from split fractions which had not been picked of microfossils. A dry nylon brush, at times dampened with deionised water, or cleaned (rinsed with deionised water) steel implements were used to transfer the shell. The shell was photographed under an incident light stereomicroscope then washed for one second in an ultrasonic bath within 102

118 Chapter 4 Methods a clean plastic vial containing deionised water. The shell was left to dry in a cleaned (alcohol and deionised water) plastic tray in a clean laboratory at room temperature overnight, weighed in an aluminium boat, and stored again in the plastic tray. Usually greater than 1mg of calcium carbonate shell was supplied for radiocarbon dating Dating method In the ANSTO (Australian Nuclear Science and Technology Organisation) facility the carbonate shell material was dated by AMS (Accelerator Mass Spectrometry).* The shell material was reacted with acid to convert the carbonate to CO 2, which was reduced over iron to produce graphite, which is the target material used in the accelerator (Lawson et al., 2000). Conventional radiocarbon ages are quoted, with an error of 1σ, in years before present (BP), where present is 1950 AD, and were calculated using the Libby half-life of 5568 years on the assumption that the production of 14 C in the past has been constant (Stuiver and Polach,1977) Calibration of dates All dates younger than 20, C yr BP on Gulf of Carpentaria material studied in this thesis are quoted as calibrated ages, and are presented in Table 5.3 of Chapter 5. Material older than 20, C yr BP cannot be calibrated using the available datasets and is reported in conventional radiocarbon years. The dates were calibrated using the program Calib v4.4 (Stuiver and Reimer, 1993). For lacustrine samples the intcal98.14c dataset was used (with no reservoir correction); marine samples older than C yr BP used the marine98.14c dataset which includes a mean global reservoir correction of 402 years for AD A R (the difference in reservoir age of the local region and the model ocean) of 50±31 years (the regional mean for northeast Australia in the calibration data of Stuiver et al., 1998) was applied to the inbuilt marine reservoir effect for marine samples (i.e. a total reservoir effect correction of 452±31 years). * supported by AINSE (Australian Institute of Nuclear Science and Engineering) grants 98/155R, 01/032 and 02/

119 Chapter 4 Methods 4.7 Summary The working half of all six cores collected in the Gulf of Carpentaria was cut into 1cm slices and samples were taken from every fifth slice with additional samples around areas of special interest. Utilising about 20g dry weight of sediment >63µm, 300 foraminifers were picked and identified from every 5cm depth increment (and additional samples) of the six cores to a depth of 1.5m. SEM images were taken of microfossils and other components of the sediments >63µm fraction. The water content of the sediment was determined by oven drying and EDAX analysis was performed to identify glauconite. Reference is also made to sedimentary particle size analysis. Carbonate microfossils were radiocarbon dated, and these were calibrated utilising the program Calib v

120 Chapter 5 Results CHAPTER 5 Results 5.1 Introduction Utilising sedimentological (Section 5.2) and micropalaeontological (Section 5.3) analysis, combined with 14 C dating (Section 5.4), five environmental facies (Section 5.5) were assigned to the studied levels (0-150cm depth) of cores MD 28 to MD 33. The results from the cores are presented here (Section 5.6) in order from west to east as shown in Figure 5.1, corresponding generally to an increasing proximity to the palaeolake depocentre and therefore to an increasing coverage and depth of water. West (Gove) East (Weipa) water depth (m) (N) MD 29 MD 31 MD 30 (N) MD 28 MD 32 depocentre MD km -80 VE = 7607 Figure 5.1 Schematic cross-section of the Gulf of Carpentaria, showing the position and length of cores MD Core sites MD 29 and MD 28 are projected from the North (N) in their correct water depths for comparison with the remaining cores. 105

121 Chapter 5 Results 5.2 Sedimentology For this study, lithological logs of the cores were created, based on visual features such as sediment colour and visible shells (note that sediment colours in the lithological logs, although based on actual sediment colours, are exaggerated in order enhance visible features), and are shown in Section 5.6 with a number of tabulated sedimentological and micropalaeontological features in the >63µm sediment fraction. These features include foraminiferal abundance and preservation, microscopic (<1mm) bivalve and gastropod abundance and preservation, macroscopic (>1mm) bivalve and gastropod abundance and preservation, ooid abundance, echinoid fragment abundance, glaucony abundance, pyritised plant fragment abundance, quartz type, matrix composition, and others such as carbonate and iron oxide nodules, and particular minerals in the sediment. SEM images of sedimentary components are shown in Figure 5.2. A brief explanation of their significance follows Carbonate Ooids Ooids form by the accretion of calcium carbonate around a nucleus via chemical precipitation, mechanical accretion or biologically mediated growth. The classic interpretation of the environmental significance of ooids is of an area of high energy and/or agitated warm shallow water saturated with CaCO 3. Ooids tend to form in high energy environments and/or where waters of different salinity or temperature mix, typically in intertidal environments. They have also been documented from continental saline lakes, such as in <5m water depth in the Great Salt Lake, USA (Eardley, 1938). In the Gulf of Carpentaria cores the nucleus is typically a lacustrine foraminifer (Figs 5.2a, 5.2b), which may alter the environmental interpretation slightly, although ooids in the Gulf of Carpentaria are also commonly found around quartz sand grains (Fig. 5.2c). The lightweight foraminiferal shell, compared to other typical nuclei such as mineral sand grains, may collect calcium carbonate in a lower energy environment as it is more easily agitated by shifting currents. In a survey of the surficial sediments (>63µm) of the Gulf of Carpentaria, Jones (1987) found relict ooids occurring in water depths of >25m bpsl. 106

122 Chapter 5 Results a a b b c c 50 d 100 e e 50 g g h h i 100 j 100 f f 100 k 100 Figure 5.2 Scanning electron photomicrographs of sedimentary components in the Gulf of Carpentaria cores. Material is taken from the upper 1.5m of cores MD 28, 29, 32 and 33. Scale bars are in µm (a, b, d, f, g, h, I, j, k are 100µm and c, e are 50µm). 107

123 Chapter 5 Results Key to Figure 5.2 The SEM images and reflected light image opposite illustrate the sedimentological and micropalaentological features used in facies determination of the Gulf of Carpentaria cores. a. ooid formed around rotaliid foraminifer, broken, showing foraminifer inside b. whole foraminiferal ooid c. ooid formed from quartz grain, broken, showing quartz grain inside d. echinoid spine e. glaucony grain. Originally a dull grey-green colour f. pyritised plant fragment g. unidentifiable rotaliid. An example of foraminiferal preservation: very reworked h. Elphidium reticulosum. An example of foraminiferal preservation: noticeably reworked i. Elphidium advenum. An example of foraminiferal preservation: slightly reworked j. Helenina anderseni. An example of foraminiferal preservation: well preserved k. Elphidium carpentariensis. An example of foraminiferal preservation: very well preserved 108

124 Chapter 5 Results l l m m n. n o o p r p q r s s t t u u v w w x 500 Figure 5.2 (cont.) Scanning electron photomicrographs of sedimentary components in the Gulf of Carpentaria cores. Material is taken from the upper 1.5m of cores MD 28, 29, 32 and 33. Scale bars are in µm (l, m, n, o, p, t, u, v, w are 100µm and q, r, s are 50µm). Image x is a reflected light photograph, scale bar is 500 µm. 109

125 Chapter 5 Results Key to Figure 5.2 (cont.) The SEM images and reflected light image above illustrate the sedimentological and micropalaentological features, referred to in Table 5.1, used in facies determination of the Gulf of Carpentaria cores. l. Bivalve, family Corbulidea. An example of bivalve preservation: very reworked m. Bivalve, family Corbulidea. An example of bivalve preservation: noticeably reworked n. Bivalve, family Corbulidea. An example of bivalve preservation: slightly reworked o. Bivalve, family Corbulidea. An example of bivalve preservation: well preserved p. Bivalve, family Corbulidea. An example of bivalve preservation: very well preserved q. quartz grain of type: angular r. quartz grain of type: rounded s. quartz grain of type: frosted t. pyrite grain, as seen when found free in sediment. Originally a shiny black colour u. partially pyritised foraminifer. Note the third chamber in particular which has broken, showing the pyrite infilled within v. calcareous grain. Originally cream coloured, but commonly found red stained as is this specimen w. iron oxide stained fragment. Originally a rust red stained colour x. macroscopic bivalve shell, family Corbulidea. Specimen from core MD 29, 20cm 110

126 Chapter 5 Results Echinoid fragments In the material studied for this thesis, the echinoid spines (Fig. 5.2d) and plates found are considered to have originated from individuals inhabiting a marine environment. Throughout the studied material from the Gulf of Carpentaria cores, the well preserved appearance of the typically fragile spines indicates that the echinoids died close to their site of deposition. Yokoyama et al. (2000) utilise the presence of ubiquitous broken echinoid spines to define a shallow marine facies for water depths less than 5m, within a tidally influenced zone. Echinoderms, amongst bivalves, foraminifers and ostracods, are listed by Jones (1987) as one of the principal components in the >63µm fraction of modern surficial sediments in the Gulf of Carpentaria Glaucony Odin and Matter (1981) proposed the term "glaucony for the family of glauconitic minerals which shows two end minerals: a potassium-poor glauconitic smectite and a potassium-rich glauconitic mica. The term glaucony is used to indicate a series of related minerals with a broader range of composition and morphology than the specific mineral glauconite. The most common habit is granular, since the green glaucony clay develops within a substrate which is usually granular itself (Odin, 1998). Figure 5.2e is of a glaucony grain from the Gulf of Carpentaria cores. The environmental interpretation of the presence of glaucony is a shallow marine environment of formation, usually on the continental shelf. It forms in cool reducing conditions, with slow sedimentation rates (Selley, 1976). Glauconite is stable in sea water, and although it may be reworked on to beaches, it is rapidly oxidized by subaerial weathering (Selley, 1976), so, nevertheless, its presence indicates the influence of marine waters. Glaucony may accumulate during transgressions (Loutit et al., 1988), and its presence in the sediment has been used as an indicator of the LGM sea-level low stand (e.g. Rao et al., 1993) and the following transgression (e.g. Lee et al., 2002). Glauconite was found in the sand size fraction of the surface samples from the Gulf of Carpentaria examined by Jones (1987). Glauconite was also found infilling foraminiferal or echinoderm tests, as casts of these tests and as broken grains from these casts (Jones, 1987). 111

127 Chapter 5 Results Scanning electron microscope EDAX scans were taken of individual carboncoated green clay grains from various samples of the Gulf of Carpentaria material to confirm their status as glaucony. Table 5.1a lists the oxides and their concentration in ten representative samples (G1-G10). Table 5.1b presents the composition (as percent oxides) of glauconite samples 4-9 of Newman and Brown (1987), utilised as a comparison to samples G1-G10 from the Gulf of Carpentaria. The water content was not analysed in all of the samples reported in Newman and Brown (1987), and it was not analysed in the Gulf of Carpentaria material. Sodium, present in small quantities in the samples of Newman and Brown (1987), was not able to be analysed via the EDAX method used on the Gulf of Carpentaria material, as Na peaks corresponded to interference patterns. The small amounts of sulfur and copper present in the Gulf of Carpentaria samples may be impurities in the material, or may be artefacts of the analysis method (Cu especially is commonly only found outside the 2σ confidence range). Not all of the analyses in Newman and Brown (1987) reported iron separately as FeO and Fe 2 O 3 (sample 8 is combined), nor was it analysed separately in the Gulf of Carpentaria material, and is expressed as FeO initially. For the adjusted average for the Gulf of Carpentaria samples, the total iron has been partitioned between FeO and Fe 2 O 3 using the average Fe 2 O 3 /FeO ratio from Newman and Brown s analyses. Comparing the average glauconite values of Newman and Brown (1987) and the adjusted average of samples G1-G10, the major difference is the higher amount of calcium found in the Gulf of Carpentaria samples (especially G2 and G10), possibly due to mixing in the calcium carbonate rich environment. Magnesium is higher in the Gulf of Carpentaria material, and potassium and iron are lower. Potassium and iron are lost via weathering (Velde and Meunier, 1987). The analysis indicates a similarity between the Gulf of Carpentaria material and glauconite, but it is not exact. As Newman and Brown (1987) state, glauconite aggregates and pellets are often heterogeneous. Therefore, the green clay mineral found in the Gulf of Carpentaria cores is referred to as the less specific glaucony throughout the thesis. 112

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129 Chapter 5 Results Quartz grains Overall the sediment samples show very poor sorting, with the quartz, percentage in the lacustrine section usually ranging from 0 to 10% (with a peak of 50%). The quartz grains found in these samples were divided into 3 arbitrary groups angular (Fig. 5.2q), rounded (Fig. 5.2r) and frosted (Fig. 5.2s). The angular quartz is presumed to have undergone little reworking, and probably consists of fluvially transported sand. The rounded grains are understood to have undergone transport and/or reworking resulting from wave action along a palaeo-shoreline. The frosted grains are presumed to have been transported for a considerable period by wind. Quartz grains ranging from clear to translucent, and with angular to rounded morphologies were noted by Jones (1987) as a principal component of the >63µm fraction of the surficial sediment in the Gulf of Carpentaria Pyritised fragments The pyritisation of plant fragments (Fig. 5.2f) and microfauna, the presence of pyrite framboids in the sediment (Fig. 5.2t) and the inclusion of pyrite within foraminiferal tests (Fig. 5.2u), requires the presence of plentiful organic matter in a reducing (water-logged) environment with a high sulfide content. Anaerobic conditions are necessary i.e. stagnant water at the sediment/water interface. However, this anoxic hypolimnion need not be large and does not require a deep lake. Once formed, pyrite is readily oxidised upon sub-aerial exposure Calcareous nodules Calcareous nodules are formed from the cementation of calcium carbonate, the continuation of this process in sub-aerial environments leads to the formation of calcrete horizons. In the studied material from the Gulf of Carpentaria there is no evidence for the full development of this process into thick and laterally extensive palaeosols. The evaporation of carbonate-rich waters such as occurs with the drying of lake waters may also lead to the precipitation of calcareous material. Calcretes may also form from sub-aerial modification of marine carbonates. The two main possibilities for the formation of the calcareous nodules (Fig. 5.2v) in the Gulf of Carpentaria are via groundwater movement or pedogenesis, both indicating the absence of a large permanent 114

130 Chapter 5 Results water body above the sediment. Nanson et al. (1991, 1993, in press) found well developed calcrete layers, forming at similar times to the ferricrete horizons, in Late Pleistocene deposits from the Gilbert River, and suggested that these calcretes resulted from seasonal variations in groundwater associated with the wet-dry monsoon climate. Jones et al. (2003) noted that most of the floodplain soils of the upper delta plain of the Gilbert River are oxidised, and may contain microscopic calcareous or ferruginous nodules. Relict calcium carbonate pisoliths are present in the >63µm fraction of surface sediment in the Gulf of Carpentaria (Jones, 1987) Iron Oxides The presence of iron oxide nodules or stained material (Fig. 5.2w) is interpreted as a possible indicator of sub-aerial exposure. Jones (1987) notes the presence of relict ferruginous pisoliths in the >63µm fraction of surficial sediments in the Gulf of Carpentaria. Iron oxide can form upon deposition when undergoing penecontemporaneous diagenesis under oxidizing conditions (i.e. the pores in the sediment open to air). Under these conditions, the organic matter is destroyed and iron compounds are changed to ferric oxides (Selley, 1976). Iron oxide may also form with the oxidation of sulfides upon drainage after deposition. It may also be transported from on-shore ferricrete formed in lateritic profiles. Nanson et al. (1991, 1993, in press) discussed the formation of extensive ferricrete and calcrete horizons within soil profiles of flood plain alluvium in rivers draining into the Gulf of Carpentaria (especially the Gilbert River). The authors reveal that thick ferruginous induration of alluvium occurred during the Late Quaternary. 5.3 Micropalaeontology This chapter outlines how facies were defined based upon the dominant foraminiferal species present, their abundance and preservation, and the presence of rarer indicator species (introduced in Chapter 2, Foraminifers), as well as by sedimentological features. SEM images of key taxa are shown in Figure 5.3. Further SEM images of the four species found that were identified only to genus level, and are possibly new species, are presented in Appendix A. 115

131 Chapter 5 Results a b c a c d e f 100 g d d e f g h i i j j k k 50 m n l m n o o Figure 5.3 Scanning electron microscope images of foraminifers from the Gulf of Carpentaria cores. Specimens are taken from the upper 1.5m of cores MD 28, 29, 32 and 33. Scale bars are in µm (a, b, c, d, h, n, o are 100µm and e, f, g, i, j, k, l are 50µm). 116

132 Chapter 5 Results 100 p q p q r r s s t u u v v w x x Figure 5.3 (cont.) Scanning electron microscope images of foraminifers from the Gulf of Carpentaria cores. Specimens are taken from the upper 1.5m of cores MD 28, 29, 32 and 33. Scale bars are in µm (p, q, r, s are 100µm and t, u, v, w, x are 50µm). Key to Figure 5.3 a. Textularia sp.1, side view. See Appendix A for more images. b. Ammomassilina alveoliniformis, side view c. Lachlanella compressiostoma, side view d. Quinqueloculina philippinensis, side view e. Gallitellia vivans, side/lateral view showing aperture f. Bolivina glutinata, side view 50 g. Bolivina vadescens, side view h. Loxostomina costatapertusa, side view i. Schackionella globosa, umbilical view j. Murrayinella murrayi, umbilical view k. Pararotalia calcariformata, spiral view l. Pararotalia sp.1, spiral view. See Appendix A for more images. m. Helenina anderseni, spiral view n. Ammonia beccarii, spiral view o. Ammonia convexa, spiral view p. Asterorotalia gaimardi, spiral view q. Elphidium advenum, side view r. Elphidium carpentariensis, side view s. Elphidium reticulosum, side view t. Haynesina depressula simplex, side view u. Tenuitella parkerae, umbilical view v. Tenuitella sp.1, umbilical view. See Appendix A for more images. w. Rosalina sp. 1, spiral view. See Appendix A for more images. x. Globigerina bulloides, umbilical view 117

133 Chapter 5 Results Although 300 individuals were picked and identified from each sample where present for use in facies analysis, for simplicity only species occurring in an abundance greater than 2% in any sample are shown in the following facies diagrams (Figs 5.6, 5.8, 5.10, 5.12, 5.14, 5.16). In the facies diagrams, the species are shown in taxonomic order, with agglutinated species on the left of the graph, moving through calcareous porcellaneous to calcareous hyaline on the right. A list of species found is provided in Appendix B. Appendix C contains the data used to construct the facies diagrams (i.e. the core depth and abundance of those species forming greater than 2% of the population). In the facies description of each core that follows in this chapter, foraminiferal species abundance is given as a percentage of the entire assemblage, including reworked but still identifiable forms. The terms rare (0-5% of the assemblage), present (5-10%) and abundant (10-100%) are used to describe their abundance. 5.4 Dating All 14 C dates were undertaken by Accelerator Mass Spectrometry at ANSTO (see previous chapter), and are presented throughout this thesis as calibrated 14 C ages where calibration is possible. Table 5.2 summarises each dated sample s depth, conventional age with errors, calibrated age, ANSTO code and the species of mollusc or ostracod which was used. Looking further at the columns from left to right, the first column contains the core number and depth within the core of each sample dated. The conventional radiocarbon ages provided by ANSTO (Stuiver and Polach, 1977), with an error of 1σ, were calibrated using the program Calib v4.4 (Stuiver and Reimer, 1993) and are shown using the probability summing tool within the same program. Correction for reservoir effect was made for marine samples, while no correction for reservoir effect was made on lacustrine samples. No calibration is possible using Calib v4.4 on samples older than 20, C yr BP. 118

134 Chapter 5 Results 119

135 Chapter 5 Results 120

136 Chapter 5 Results 121

137 Chapter 5 Results The ANSTO reference number is provided in Table 5.2, and the species which provided the material for dating are listed (bivalve molluscs are identified to the family level, gastropods and ostracods are identified to the genus level), as well as the total weight of the sample dated. Some depth horizons (MD 30, 65cm; MD 31, 0cm; MD 31, 55cm; MD 33, 0cm and MD 33, 20cm) have had a repeat radiocarbon analysis performed due to suspicions about the reliability of the original date. Two samples are composed of numerous ostracod valves (MD 28, 15cm and MD 33, 77cm). Samples from these same depth horizons were dated twice in order to verify the applicability of ostracods to radiocarbon dating the Gulf of Carpentaria cores. The remaining forty-four samples are mollusc shells. Calibrated ages are plotted against depth in Figure 5.4. The dotted line on MD 28 is a linear extrapolation of the age series, the equation for which is on the graph. Note that this equation is an average of the data and does not take into account changes in sedimentation rates. Core MD 29 has only two data points no extrapolation was attempted with these limited data. Also, core MD 29 appears similar to core MD 28 sedimentologically, palynologically and with respect to foraminiferal assemblages, and therefore their age structure is assumed to be broadly similar. Cores MD 31 and MD 33 contain age data collected outside the 150cm depth range of this study (MD 31 at 320cm and 330cm, and MD 33 at 210cm, both listed in Table 5.3) in order to show the actual trends in age versus depth. 5.5 Facies The sedimentological and micropalaeontological characteristics mentioned above, constrained by radiocarbon dating, were used to define five facies, recognisable in the majority of the cores studied. The facies diagrams for each core are coloured in order to represent the designated facies: dark blue is used for Brackish Lake, light blue for Saline Lake, brown for Exposed, pink for Transitional, and yellow for Marine. Specific sub-facies are also described below, and are mentioned in the classification where sufficient evidence is present. Note that none, one, or more of the sub-facies may characterise an 122

138 Chapter 5 Results environment at any given time. Here follows a brief description of the designated facies and their broad environmental interpretation: Brackish Lake facies The Brackish Lake facies is shaded dark blue in the facies diagrams of each core that follows in Section 5.6. This facies is characterised by the occurrence of the foraminifers Ammonia beccarii and Helenina anderseni. These foraminifers may be very abundant, forming foraminiferal sand. Abraded lacustrine foraminiferal tests are rare throughout most of this facies, but may be abundant on the lake margin sub-facies. Other taxa may be present including charophytes, ostracods and gastropods. Of the charophytes, Lychnothamnus barbatus (fresh water, 0-3 salinity), Chara zeylanica and C. vulgaris (3-20 salinity) may occur (Garcia pers. comm., 2005). The freshwater ostracods Limnocythere and Cyprinotus may also occur, as do the fresh to saline Leptocythere lacustris, Ilyocypris australiaensis and Cyprideis australiaensis. (Reeves, 2004). The freshwater gastropod Planorbidae may also be present. The presence of these fresh to brackish water charophytes, ostracods and molluscs distinguishes this facies from the saline lake facies. The sediments are typically consolidated and dark-grey coloured, with varying levels of sand in a clay-silt matrix. This assemblage indicates non-marine waters, which may fluctuate in salinity from fresh to brackish. Brackish lake margin This sub-facies is distinguished by the presence of many abraded tests from the assemblage mentioned above, as well as a few better preserved specimens. A few abraded transitional/marine foraminifers may be present including Elphidium spp., Pararotalia spp. and Asterorotalia gaimardi. Calcareous nodules and iron oxide nodules are also present. 123

139 Chapter 5 Results The environment of deposition is indicated to be within shallow water on the brackish-water lake edge. Therefore, it would have been subject to wave currents and have been at times a high-energy environment. Some washing in of sediment from the lake-shore is indicated, depositing reworked fauna. Some sub-aerial exposure also would have occurred as the lake levels fluctuated, accounting for the iron oxide and calcrete nodules. Tidally influenced brackish water Within this sub-facies, marine species of foraminifer occur; most are well preserved. The planktonic Gallitellia vivans occurs, as well as Pararotalia spp. and Elphidium spp. (of these E. reticulosum is most abundant). Other estuarine and marine species such as A. gaimardi and Rosalina sp.1 may also occur. The Brackish Lake assemblage of foraminifers and other taxa are present. The sedimentary matrix consists of broken shells, rare to abundant ooids and rare echinoid fragments. Proximity to a channel from the brackish lake to the ocean, subject to some tidal influence, is indicated. The sub-facies may have been deposited in a lagoonaltype of channel as described along the Mary River (north-western Australia) by Woodroffe and Mulrennan (1993). A series of disconnected channels and billabongs proceed to the coast, flooding and connecting in the wet season (Woodroffe and Mulrennan, 1993). A combination of high tides and storm surges may have transported marine water and organisms into the area, or the site where this sub-facies was deposited may have been on a floodplain, where water spilling over tidal channels pooled. Some horizons may contain no well preserved marine species, only abraded forms, and hence indicate a deposit of reworked marine sediment in the channel or floodplain between the brackish lake and the ocean. Brackish swamp This sub-facies is characterised by the abundance and dominance of well preserved H. anderseni. Other Brackish Lake taxa may be present, and there are abundant pyritised plant fragments. Very few abraded tests are found. 124

140 Chapter 5 Results A permanent body of fresh water is indicated, with vegetation growing around it, and some vegetation within the water. The conditions at the sediment-water interface would have been at times anoxic and it was, therefore, a low-energy environment. Abundant shell population This sub-facies is characterised by the dominance of one commonly well preserved macroscopic bivalve of the family Corbulidae. The foraminifers Ammonia beccarii and H. anderseni are very abundant, along with abundant Cyprideis australiaensis (Reeves, 2004), forming a microfossil sand. The Brackish Lake charophytes, ostracods and gastropods may be present. The sediments are characterised by a broken shell matrix, and the presence of ooids, glaucony and echinoid fragments in the upper part of the sequence. This facies indicates a change in conditions, brought about by an influx of fresh water into the palaeolake as precipitation increased and the lake expanded. However, estuarine gastropods may also be present (Ponder, pers. comm., 2003), indicating that any influx of fresh water was moderated at first. The marine indicators in the upper part of the sequence were deposited when the lake reached its maximum water depth, overflowing via the Arafura channels into the sea, at the same time as sea-level was rising and allowing tidal exchange of waters with the (still brackish) lake body Saline Lake facies The Saline Lake facies is shaded light blue in the facies diagrams of each core that follows in Section 5.6. This facies is characterised by the occurrence of foraminifers Helenina anderseni and Ammonia beccarii with none of the freshwater indicator species that are present in the Brackish Lake facies. The ostracod Cyprideis australiensis (Reeves, 2004) may be present. The sediment matrix is typically a light grey coloured consolidated clay-silt, with more clay and less sand than the Brackish Lake facies. 125

141 Chapter 5 Results The environment of deposition is suggested to be non-marine waters, which are slightly saline, and the lake is not filled to maximum capacity. Saline lake margin This sub-facies is indicated by the presence of many abraded tests in the foraminiferal assemblage, including lacustrine species and the estuarine/marine species Elphidium spp., Pararotalia spp. and Asterorotalia gaimardi, together with calcareous and iron oxide nodules. A few well preserved specimens of H. anderseni and A. beccarii occur. The environment of deposition is indicated to be within shallow water on the saline-water lake margin. It would have been subject to wave currents and may at times have been a high-energy environment. Some washing in of sediment from the lake-shore is indicated, depositing reworked foraminiferal tests. Some sub-aerial exposure also would have occurred as the lake levels fluctuated, producing the iron oxide and calcareous nodules. Saline swamp This sub-facies is characterised by the dominance of well preserved H. anderseni and abundant to rare pyritised plant fragments. There are very few abraded tests. A semi-permanent saline body of water is indicated, with vegetation growing around and throughout, being at times anoxic and, therefore, a low-energy environment. Due to the shallow water level, it may have dried out at times Exposed sediments Exposed sections of the core are shaded brown in the facies diagrams of each core that follows in Section 5.6. This designated facies is not a facies in the strictest sense, rather an overprint of exposure upon a non-marine facies. This sub-aerial exposure partly obscures the original facies, but also provides some environmental indications. 126

142 Chapter 5 Results Rare abraded microfossils are present, predominantly non-marine (Helenina anderseni and Ammonia beccarii), but a few larger robust marine species such as Asterorotalia gaimardi are also present. Calcareous and iron oxide nodules are abundant in the sediment. No well preserved tests of any microfauna or macrofauna occur. The consolidated sediments have a yellow hue; orange coloured (iron stained) mottling and hard calcareous horizons may be present. Total sub-aerial exposure of the site at times is indicated, possibly soon after the sediments were deposited - as occurs in a lake-edge environment Transitional facies The Transitional facies is shaded pink in the facies diagrams of each core that follows in Section 5.6. This facies is a transitional between the non-marine and marine facies, containing both non-marine and marine fauna. Sediments are typically the consolidated lacustrine dark grey clayey-silt. Closed to choked lagoon This sub-facies is dominated by the well preserved non-marine indicator species A. beccarii except in MD 28 where well preserved H. anderseni dominates. Abraded lacustrine tests also occur. The euryhaline genus Elphidium (E. reticulosum is most abundant of the Elphidium) is present. Marine/transitional taxa which may be present include A. convexa, Textularia spp. (including T. foliacea), Ammomassilina alveoliniformis, Pararotalia spp., Asterorotalia spp., Rosalina sp.1 and rare miliolids. Planktonic species may occur, either Gallitellia vivans or Tenuitella spp. The easily transported Schackionella globosa may also occur. Ostracod fauna are much more diverse than in the lacustrine facies. As sea levels rose, the infiltration of marine waters into the Carpentaria basin through channels in the Arafura Sill is indicated. Connection to the ocean was not maintained as the sea-level was not high enough. Sediment may have been 127

143 Chapter 5 Results washed into the channels with the initial influx, preventing further marine waters entering, or the first infiltration of marine waters may have been through large storm events. These marine waters were mixed with the brackish Lake Carpentaria. Restricted to open lagoon Well preserved H. anderseni and A. beccarii are present within this sub-facies, but do not dominate; some abraded lacustrine tests are present. The assemblage is slightly more diverse than the choked lagoonal sub-facies, and includes the species mentioned in that facies in greater abundance, as well as Loxostomina costatapertusa, Bolivina spp. and Murrayinella murrayi. The ostracod assemblage is similarly diverse. As sea levels rose, the infiltration of marine waters into the Carpentaria basin through channels in the Arafura Sill continued. These marine waters mixed with the brackish lake and pooled within topographic depressions, maintaining connection with the ocean via channels in the Arafura Sill and ensuring estuarine conditions prevailed Marine facies The Marine Facies is shaded yellow in the facies diagrams of each core that follows in Section 5.6. Abundant and diverse foraminifers characterise this facies. Waters of normal marine salinity are indicated. Sediments are unconsolidated green-grey silty clay, with more sand present typically than other facies. Well preserved non-marine species (H. anderseni and A. beccarii) are present but not dominant, abraded tests of these species also occur, and abraded brackish water charophytes and ostracods may also occur. The assemblage is composed of diverse species, including all the species mentioned in the Transitional restricted lagoonal facies in greater abundance, and other species including Nubeculina advena. The ostracod assemblage is correspondingly abundant and diverse. 128

144 Chapter 5 Results Marine waters flooded the Gulf of Carpentaria over the Arafura Sill and sealevel continued to rise, breaching the Torres Strait and connecting the Gulf of Carpentaria to the Pacific Ocean. This marine environment continues in the Gulf of Carpentaria. 5.6 Palaeoenvironmental data from each core The results of this study are presented for each core, in the order west to east as indicated in Section 5.1, and specific characteristics introduced in Sections 5.2 to 5.5 are discussed for each core. A summary of sedimentary features visible to the naked eye (before segmentation for sampling) of each core is given in the section on sedimentology below. This is also displayed as a lithological log. Also in the diagram are the sedimentological features discussed in Section 5.2: abundance of foraminiferal tests, bivalve shells, ooid grains, echinoid spines, glaucony grains and pyritised organic fragments. The preservation of foraminiferal tests and bivalve shells, the presence of pyrite, calcareous or iron oxide nodules and the presence and type of quartz sand are also displayed. The key to these sedimentological diagrams is shown on page 131 and SEM images are displayed in Figure 5.2. The relative abundance of foraminiferal species (at >2% abundance) at 5cm intervals down each core, as discussed in Section 5.3, is graphically displayed, with facies denoted by shading (Section 5.5) and explained further in the section on facies below. The individual samples chosen for radiocarbon dating (Section 5.4), and their ages, where anomalous or important to the facies interpretation, are discussed further under the dating section below. The facies (Section 5.5) are described in detail for each core, beginning at the base of the studied section of the core (i.e. 150cm depth). 129

145 Chapter 5 Results Core MD As shown in Figure 5.1, core MD 29 is situated closest to the Arafura Sill and furthest from the palaeolake depocentre. It is also the furthest away from the influence of Australian rivers in their present configurations, as seen in Figure 1.1. MD 29 Sedimentology A key to all sedimentological figures (Figs 5.5, 5.7, 5.9, 5.11, 5.13, 5.15) is shown on the following page. The sedimentary characteristics of core MD 29 are shown in Figure 5.5. The entire lacustrine section of the core (150-20cm) has a yellow hue (due to oxidised iron), and is composed of hard dry sediments. Sediment colour lightens upwards, from 150 to 26cm, to the darker green-greybrown less-consolidated marine section at 26-0cm. There is evidence of brief periods of sub-aerial exposure throughout the lacustrine segment of the core, with a sustained period of pedogenesis from 119 to 108cm, and another overprint of sub-aerial exposure from 66 to 63cm. From 117 to 70cm lighter coloured laminae up to 1cm thick are evident, indicating seiche-deposited material. The lighter colour laminae indicate the deposition of more oxidised material (i.e. less organic matter darkening the sediment colour). Sediment was transported into the shallow lake margin by wind induced wave action (the dominant quartz grain type is frosted to rounded) or episodic flooding events (around 20cm fluctuation in lake-levels, the sediments immediately above this succession contain evidence of deeper waters). Evaporation of the seichedeposited shallow water on the margin was a low-energy event, preserving the laminae, while the shallow fluctuating environment restricted the occurrence of fauna, preventing bioturbation. Possible bioturbation at the boundary between lacustrine and marine sediment is indicated by the presence of clasts of marine sediment within the lacustrine succession, up to 55cm below the erosional bioturbated boundary between the two units. 130

146 Key to Figures 5.5, 5.7, 5.9, 5.11, 5.13 and 5.15 Chapter 5 Results Key to lithological log Key to clay/silt/sand graphs Shell Bioturbated clast Lamination Pedogenesis Mottling Gradual boundary Sharp boundary clay silt sand Key to sedimentological graphs The presence of iron oxide grains (Fig. 5.2w) at 5cm depth intervals is shown by I, calcareous nodules (Fig. 5.2v) are denoted by C. The percentage of clay/silt/sand is shown cumulatively. Foraminiferal abundance is the number of individuals per gram of dry sediment plotted on a logarithmic scale. Microscopic bivalve and gastropod abundance (<1mm) is shown on a relative scale where 0 = zero, 1 = rare = 1-2 shells, 2 = present = 3-4 shells, 3 = abundant = >5 shells. Macroscopic bivalve and gastropod abundance (>1mm) is shown on a relative scale where 0 = zero, 1 = rare = 1-2 shells, 2 = present = 3-4 shells, 3 = abundant = >5 shells. Ooid abundance (Fig. 5.2a-c) is shown on a relative scale where 0 = zero, 1 = rare = 1-10 ooids, 2 = present = up to 25% of the sediment, 3 = abundant = > 25% sediment. Echinoid fragment abundance (spines and plates, Fig. 5.2d) is shown on a relative scale where 0 = zero, 1 = rare = 1-10 fragments, 2 = present = up to 25% of the sediment, 3 = abundant = > 25% sediment. Glaucony abundance (Fig. 5.2e) is shown on a relative scale where 0 = zero, 1 = rare = 1-10 grains, 2 = present = up to 25% of the sediment, 3 = abundant = > 25% sediment. Pyritised plant fragment abundance (Fig. 5.2f) is shown on a relative scale where 0 = zero, 1 = rare = 1-10 fragments, 2 = present = fragments, 3 = abundant = >20 fragments. Foraminiferal preservation (Fig. 5.2g-k) is a relative measure where 0 = not present, 1 = very reworked = only genera are recognizable, 2 = noticeably reworked = tests are broken but the species are recognizable, 3 = slightly reworked = easy identification of species, 4 = well preserved = tests are slightly dull, 5 = very well preserved = not visibly reworked. Microscopic bivalve and gastropod preservation (Fig. 5.2l-p) is a relative measure where 0 = not present, 1 = very reworked = none whole, pinkish colour, 2 = noticeably reworked = broken or cracked, slight pinkish colour, 3 = slightly reworked = frosted, 4 = well preserved = slightly dull, 5 = very well preserved = not visibly reworked. Macroscopic bivalve and gastropod preservation is a relative measure where 0 = not present, 1 = very reworked = none whole, pinkish colour, 2 = noticeably reworked = broken or cracked, slight pinkish colour, 3 = slightly reworked = frosted, 4 = well preserved = slightly dull, 5 = very well preserved = not visibly reworked. Note: Figure 5.2l-p, of microscopic shells, demonstrates the same scale used for macroscopic shells (Fig. 5.2x). Quartz type (Fig. 5.2q-s) is categorized as 0 = not present, 1 = angular grains, 2 = round grains, 3 = round and frosted grains. 131

147 Chapter 5 Results 133

148 Chapter 5 Results 134

149 Chapter 5 Results MD 29 Micropalaeontology Species occurring, displayed in Figure 5.6, include: Textularia agglutinans, T. foliacea, T. secasensis, Textularia sp.1, Ammomassilina alveoliniformis, Lachlanella compressiostoma, Quinqueloculina crassicarinata, Q. incisa, Q. parvaggluta, Q. philippinensis, Q. tropicalis, Q. tubilocula, Facetocochlea pulchra, Gallitellia vivans, Bolivina glutinata, B. vadescens, Loxostomina costatapertusa, Schackionella globosa, Pararotalia calcariformata, Pararotalia sp.1, Heterolepa subhaidingeri, Cibicides refulgens, Helenina anderseni, Ammonia beccarii, A. convexa, Ammonia spp. (reworked), Asterorotalia gaimardi, A. milletti, Elphidium advenum, E. carpentariensis, E. reticulosum, E. simplex, Haynesina depressula simplex, Globigerina bulloides and Globigerinoides trilobus. MD 29 Dating Radiocarbon dating was performed on marine mollusc shells at 20cm and 5cm. A single valve of a bivalve was used at 20cm, giving an age of 10.5ka cal BP. At 5cm the age was determined to be 410a cal BP also from a single valve of a bivalve. The mollusc used at 5cm is from the family Lucinidae. Species from this family are known to be deep burrowing, which is perhaps responsible for the younger than expected age at 5cm. However, MD 29 may display an undisturbed marine sequence, being situated far from the influence of rivers and erosion, and may have retained the uppermost section of the marine sequence that appears to be missing in cores MD (Fig. 5.4). MD 29 Facies A summary of the micropalaeontological and sediment characteristics of each facies for core MD 29 follows: cm: Brackish Lake including tidally influenced water at 145cm Ammonia beccarii is dominant (55-75%); Helenina anderseni is also abundant (12-40%). Abraded tests are only rarely present (never above 5%). At 145cm a few small marine taxa are present and have not been visibly abraded: Cassidelina sgarrellae and Bolivina spp. and the planktonic Globigerina bulloides, Globigerinoides trilobus and Gallitellia vivans. A marine species not 135

150 Chapter 5 Results recorded above 2% abundance in any of the cores, Cancris auriculus, also occurs. Rare, well preserved, echinoid spines also occur between 145 and 140cm. Up to 15% sand is contained in the matrix of clay and silt; the fraction above >63µm is mainly quartz grains of a mixture of morphologies; angular, rounded and frosted. Carbonate nodules, some iron oxide stained, are present. At 150cm and 140cm rare pyritised plant fragments occur cm: Exposed This interval is mostly barren of all microfossils, although rare reworked specimens are found. The matrix is composed of mainly clayey silt, with the >63µm fraction a mixture of quartz sand morphologies angular, rounded and frosted. Carbonate nodules are present with iron oxide staining. At 135cm very rare pyritised plant fragments occur. Iron oxide nodules are added to the >63µm fraction between 120 and 100cm, and at 95cm iron oxide nodules dominate with some quartz grains. At 90cm the >63µm matrix is purely quartz sand of angular and rounded morphologies. Seiche deposits and exposure surfaces are noted in the lithological log as lighter coloured lenses and horizons 1-2cm thick (Fig. 5.5). 85cm: Saline Lake lake margin Only one unabraded test of H. anderseni was found, only 3 other tests were found in the sub-sample (reworked Asterorotalia gaimardi, Ammonia beccarii and H. anderseni). The matrix is silty-sand-clay, and the >63µm fraction is composed of a mixture of quartz sand (angular and rounded morphologies) and carbonate nodules (with iron oxide staining). 80cm: Brackish Lake lake margin A. beccarii dominates at 80cm (70%) although only 10 foraminifers were found in the sub-sample, equalling 50 tests per dry gram of sediment. The reworked robust marine species Asterorotalia gaimardi is abundant (10%), as well as abraded lacustrine assemblage species (Ammonia beccarii and H. anderseni). The matrix is silty-sand-clay, and the >63µm fraction is composed of a mixture of quartz sand (angular and rounded morphologies) and carbonate nodules 136

151 Chapter 5 Results (with iron oxide staining). The lithological log (Fig. 5.5) shows some seiche deposits as lighter coloured lenses 1cm thick around this level. 75cm: Exposed The matrix is silty-clay, with the fraction >63µm consisting of a mixture of quartz grains (angular and rounded morphologies) and carbonate nodules (some iron oxide stained). 70cm: Brackish Lake lake margin A. beccarii is dominant (55%) and H. anderseni present (5%), although less than 300 foraminifers were picked from the sub-sample (750 per dry gram of sediment). Abraded lacustrine taxa are abundant (37%), and the euryhaline genus Elphidium spp. (2%) is also present. This is the beginning of a section of sediments lighter in colour, representing an erosional boundary, as seen on the lithological log (Fig. 5.5). 65cm: Exposed No foraminifers occur. The sand fraction of the silty clay matrix is dominated by iron oxide nodules and quartz grains (angular and rounded), while carbonate nodules (iron oxide stained) are also present. An exposure overprint of light coloured yellow mottling is noted on the lithological log (Fig. 5.5) cm: Brackish Lake tidally influenced waters Abraded lacustrine taxa (greater than 50% of the species present) dominate the assemblage. Of the unabraded foraminifers, A. beccarii is mostly dominant (0-32%) although fewer than 300 specimens per sub-sample were recovered (up to 1000 per dry gram of sediment). At 50cm, a reduction in morphotype variation is noted most specimens of A. beccarii are medium-sized. At 35cm a similar restricted range of morphotypes (small-to-medium) occur. Elphidium spp. are also abundant in this facies, mostly E. reticulosum (0-15%), with the added presence of rare marine species: Bolivina spp., Loxostomina costatapertusa, Asterorotalia gaimardi, Pararotalia spp., Facetocochlea pulchra, Cassidelina sgarrellae, Cancris auriculus, Spiroloculina excisa (a species never occurring at more than 2% abundance throughout the cores) and Globigerinida. 137

152 Chapter 5 Results The best preserved marine fauna are found at 55cm, with more abraded forms present toward the top of the facies. The matrix fines upwards with increasing clay content to 45cm, where up to 10% sand is added to the clayey-silt. From 60 to 35cm the >63µm fraction consists of approximately half angular quartz grains and half carbonate nodules. At 30cm the >63µm fraction is entirely composed of carbonate nodules. Iron oxide nodules are present throughout. 25cm: Transitional choked lagoon Ammonia beccarii dominates (65%), with abundant Elphidium spp., mainly E. reticulosum (12%) and diverse marine taxa (5% in total), including species mentioned in the previous facies, and two species susceptible to transport via floating in the water column: the planktonic Gallitellia vivans and Schackionella globosa. One specimen of Quinqueloculina parvaggluta was found. The matrix is composed of clayey-silt, the >63µm fraction consists of carbonate nodules, foraminifers and shell pieces. Pyrite is present in foraminifers. Rare echinoid fragments occur. The sharp change in colour that accompanies the boundary between marine and non-marine sediments is seen on the lithological log between 27 and 24cm (Fig. 5.5). 20-0cm: Marine - fully marine A diverse and abundant foraminiferal assemblage is present. A. beccarii, Asterorotalia gaimardi, Pararotalia spp., Elphidium reticulosum are all abundant (10-20% each). Ammonia convexa, E. carpentariensis, Bolivina spp., Textularia spp. and Quinqueloculina spp. are also present (each around 10%), as well as G. vivans (0.5-5%). The matrix consists of sandy silty slay. The >63µm fraction is composed of shell pieces and angular quartz grains. From 20 to 10cm, pyrite occurs in foraminifers. At 10cm and 5cm there are rare iron oxide grains in the sediment, and at 0cm iron oxide grains are present in greater quantity. Abundant glaucony, abundant echinoid fragments, and rare microscopic and macroscopic bivalve and gastropods occur. Diverse marine ostracods are present throughout the facies (Reeves, 2004). 138

153 Chapter 5 Results Core MD The site of core MD 31 is situated within sinuous contour lines on the bathymetric map (Fig. 1.1), indicating possible channels in the vicinity. MD 31 Sedimentology Figure 5.7 shows the sedimentary characteristics of core MD 31. The consolidated lacustrine sediments become progressively darker from 150cm to the transition to very unconsolidated green-grey marine sediments. Shells are scattered throughout the section from 150 to 105cm. The contact between lacustrine and marine sediments is angled, spanning from 70 to 60cm depths, and appears erosional and bioturbated. Bioturbation is also apparent 25cm below the transition as a clast of marine coloured sediment within the lacustrine sequence. MD 31 Micropalaeontology Species occurring, displayed in Figure 5.8, include: Textularia secasensis, T. foliacea, Textularia sp.1, Ammomassilina alveoliniformis, Lachlanella compressiostoma, Quinqueloculina philippinensis, Facetocochlea pulchra, Gallitellia vivans, Bolivina glutinata, B. vadescens, Loxostomina costatapertusa, Schackionella globosa, Murrayinella murrayi, Pararotalia calcariformata, Pararotalia sp.1, Helenina anderseni, Ammonia beccarii, Ammonia convexa, Ammonia spp. (reworked), Asterorotalia gaimardi, Pseudorotalia angusta, Elphidium advenum, Elphidium carpentariensis, Elphidium reticulosum, Haynesina depressula simplex, Rosalina sp.1, Tenuitella parkerae and Tenuitella sp. 1. MD 31 Dating Included in Table 5.3 and Figure 5.4 are the radiocarbon results from lacustrine gastropods from 320cm and 330cm. A lacustrine bivalve was chosen from the shell hash layer at 70cm, and a lacustrine gastropod was dated from the lacustrine facies at 65cm, both displayed ages consistent with other data (12.2ka cal BP and 12.3ka cal BP respectively). Marine bivalves were dated 139

154 Chapter 5 - Results 141

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156 Chapter 5 - Results from 60cm, 55cm, 30cm, 10cm, 5cm and 0cm and do not show a clear relationship between age and depth (Fig. 5.4). The transition between lacustrine and marine sediments is angled, between 70-60cm, and 60cm is a transitional facies while the marine facies actually begins at 55cm. The marine mollusc dated at 60cm was of a younger than expected age, being 340a cal BP. Repeat samples were taken from 55cm (7.3ka cal BP, 1.4ka cal BP) and 0cm (8.2ka cal BP, 7.3ka cal BP), as the ages appeared anomalous. The entire unconsolidated marine section was probably subject to disturbance, perhaps from cyclones or trawling ships while on the sediment floor, but also likely during coring. MD 31 Facies A summary of the micropalaeontological and sediment characteristics of each facies for core MD 31 follows: cm: Saline Lake lake margin Helenina anderseni (13-57%) and Ammonia beccarii (10-22%) dominate, abraded tests (including slightly abraded marine species such as Asterorotalia gaimardii, Elphidium reticulosum, Tenuitella spp., Bolivina spp., Schackionella globosa) contribute up to 60% of the total assemblage. Although less than 300 specimens were picked in the sub-sample (up to 1100 per dry gram of sediment), foraminifers are more abundant at this site than at the previously described MD 29. At 105cm a few less-reworked smaller marine species are present. The matrix is composed of varying levels of clay silt and sand. The >63µm fraction is composed of quartz grains (angular, rounded and frosted), broken shell pieces and carbonate nodules. Macroscopic shells are present at 150cm and 120cm. Rare ooids occur at 130cm, at cm and 100cm. At 120cm large reworked rotaliids (cf. Challengerella) and the gastropod Planorbidae are present. Angular and frosted quartz are the dominant forms from 120 to 105cm. Rare pyritised plant fragments occur from 150 to 105cm. At 100cm massive pyrite is present. From 150 to 105cm this facies is composed of light grey sediments, apparently oxidised, shown in the lithological log (Fig. 5.7). The boundary between adjoining facies at 100cm is gradual and the colour change is hardly visible. 143

157 Chapter 5 - Results 95-65cm: Brackish Lake There are a greater abundance of foraminifers (although less than 300 specimens were found in the sub-sample (up to 1250 per dry gram of sediment), abraded taxa are less common than in the previous facies (<30%), and Ammonia beccarii dominates (22-65%) over H. anderseni (14-42%). At 90cm mainly medium sized forms of the two dominant species occur. Also at 90cm a few less-reworked marine and euryhaline species are present, including Elphidium carpentariensis, Textularia spp. and Pararotalia sp.1. Charophytes occur from 90 to 65cm, identified as mainly Chara zeylanica, although a few individuals of C. vulgaris are also present (Garcia, pers. comm., 2005). At 70cm the fresher water ostracod Ilyocypris is present, with many instars present suggesting a life assemblage. The more estuarine ostracod Cyprideis is rare at this level. The matrix is composed of varying levels of sand in clay and silt throughout the facies. Within the >63µm fraction, quartz (angular, rounded and frosted from 90 to 85cm and at 70cm; angular for the rest of the facies), broken shell pieces and carbonate nodules occur. A shell hash layer is centred at 72cm (encompassing 85-70cm), composed of many macroscopic reworked bivalve shells. As yet unidentified estuarine gastropods are present in the shell hash layer at 80cm, with more brackish forms (Iravadinae) present at 70cm (Ponder, pers. comm., 2003). The horizon at 70cm, influenced by the shell hash layer, is different from the surrounding material. At 70cm, large reworked rotaliids (cf. Challengerella) and gastropods similar to Planorbidae are present in a matrix dominated by shell pieces with quartz sand, some carbonate and iron oxide nodules. At 70cm well preserved microscopic shells are present, and slightly reworked macroscopic shells are abundant. From 70 to 65cm other abraded marine foraminifers occur, including Cibicides, Asterorotalia, Pararotalia, Clavulina, Textularia and Quinqueloculina. From 90cm (abundant) to 80cm (rare) pyritised plant fragments occur. Pyrite is found in the sediment along with pyrite in foraminifers at 95cm, 95cm (abundant), 80cm and 65cm. From the sedimentary data, the horizon at 65cm appears to be transitional; with rare echinoid fragments, ooids, and abundant macroscopic (very reworked) and microscopic (well preserved) shells. But only lacustrine foraminifers are present, also well preserved, in this section. On the lithological log, this facies is seen as a uniform section of dark grey sediments (Fig. 5.7). 144

158 Chapter 5 - Results 60cm: Transitional choked lagoon This section is dominated by reworked lake assemblage foraminifers (40%), with well preserved Ammonia spp. and Elphidium spp. also abundant (30% and 15% respectively). Well preserved marine and euryhaline taxa, including Gallitellia vivans, Ammomassilina alveoliniformis, Pararotalia spp., Rosalina sp.1, Quinqueloculina spp., Textularia spp., Asterorotalia gaimardi, Nubeculina advena and Spiroloculina excisa are present, but in total form less than 20% of the assemblage. The matrix is equally dominated by clay and silt, with very little sand. The >63µm fraction consists of ooids, shell pieces and quartz sand (angular, rounded and frosted). Echinoid fragments are abundant and glaucony is present. Pyrite occurs in the sediment and in foraminifers; iron oxide nodules also occur. The lithological log (Fig. 5.7) shows the boundary between the lacustrine and marine sediments occurs over 68-59cm, so some mixing of marine and non-marine sediments (and associated fauna) may have occurred in the sampling process between 65cm and 60cm cm: Transitional restricted to open lagoon Abraded lacustrine tests account for up to 50% of the total assemblage. The dominant unreworked species is A. beccarii (7-15%), Elphidium reticulosum and Rosalina sp.1 are present (5-10%), while other rare euryhaline and marine taxa occur, including Asterorotalia gaimardi, Pararotalia spp., Loxostomina costatapertusa, Bolivina spp., Murrayinella murrayi, Ammomassilina alveoliniformis, Textularia spp., E. carpentariensis, E. advenum, Haynesina depressula simplex, and the planktonic Gallitellia vivans and Tenuitella spp. The matrix is of clayey-silt, with a decreasing amount of sand toward the top of the facies. The >63µm fraction is composed of ooids, broken shell pieces, and quartz sand (angular, rounded and frosted). Glaucony is rare, pyrite is present in foraminifers and in the sediment, microscopic shells are present and macroscopic shells and echinoid fragments are abundant in unconsolidated green-grey marine sediments. 145

159 Chapter 5 - Results 40-0cm: Marine - fully marine More diverse than core MD 29, the marine assemblage in core MD 31 also has a higher number of true marine foraminifers including abundant G. vivans (2-15%). The other dominant species A. beccarii, Rosalina sp.1 and Murrayinella murrayi are present up to around 10% each. The matrix is composed of clayeysilty-sand, with the sand fraction varying from less than a percent to 25% of the total matrix. The >63µm fraction is composed of ooids, broken shell pieces, and quartz sand (angular). Pyrite is present in foraminifers from 40 to 35cm, with pyrite occurring in the sediment from 40 to 30cm. Glaucony and microscopic shell abundance varies from rare to abundant; while echinoid fragments and macroscopic shells are present or abundant Core MD In common with core MD 31, core MD 30 is also positioned near where contour lines curve on the bathymetric map (Fig. 1.1), indicating its proximity to possible channels. MD 30 Sedimentology Figure 5.9 shows the sedimentary characteristics of core MD 30. The consolidated dark grey lacustrine sediments are lighter between 150 and110cm, and include some shells. The sediments darken from 110cm before the transition to very unconsolidated green-grey marine sediments. The contact between marine and lacustrine sediments appears sharp, erosional and bioturbated, and it is sharply angled, spanning the depths between 93 and 67cm. Bioturbated sediment is noted 7cm below the lowest part of the angled transition. MD 30 Micropalaeontology Species occurring, displayed in Figure 5.10, include: Textularia agglutinans, T. foliacea, Textularia sp.1, Ammomassilina alveoliniformis, Lachlanella compressiostoma, Quinqueloculina philippinensis, Planispirinella exigua, Facetocochlea pulchra, Cassidelina sgarrellae, Gallitellia vivans, Bolivina glutinata, B. vadescens, Loxostomina costatapertusa, Schackionella globosa, 146

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162 Chapter 5 - Results Murrayinella murrayi, Pararotalia calcariformata, Pararotalia sp.1, Helenina anderseni, Ammonia beccarii, A. convexa, Ammonia spp. (reworked), Asterorotalia gaimardi, A. milletti, Elphidium advenum, E. carpentariensis, E. reticulosum, Haynesina depressula simplex, Rosalina sp. 1, Tenuitella parkerae and Tenuitella sp.1. MD 30 Dating Lacustrine gastropods were dated at 150cm and 80cm (13.9ka cal BP and 12.5ka cal BP respectively). The gastropod Hydrobia at 150cm was broken, and the younger than expected date is not considered to reflect the age of the sediment around it, rather, it may be a remnant of reworking of younger material into the older sediment below (note the clast shown at 125cm in the lithological log). Two bivalve shells were also dated from the shell hash layer at 90cm, displaying an age within the expected range, 12.8ka cal BP (Fig. 5.4). Four lacustrine gastropods were used to determine the age of the transition, at 70cm. Unfortunately, although appearing slightly reworked, they were the only available material from that horizon, so their age of 12.4ka cal BP is probably a little older than the actual transition. Marine bivalves were dated at the following levels: 65cm, 60cm, 30cm, 5cm and 0cm and do not show a consistent trend (Fig. 5.4). The very unconsolidated nature of the marine section, and the suspiciously homogenous marine foraminiferal assemblage (Fig. 5.10), makes it likely that the materials selected for dating had been reworked. MD 30 Facies A summary of the micropalaeontological and sediment characteristics of each facies for core MD 30 follows: cm: Saline Lake lake margin Around 90% of the assemblage is composed of abraded foraminifers, with the only unreworked species being Helenina anderseni (around 10%). There are less than 500 foraminifers per dry gram of sediment. The matrix is composed of clay and silt, with a few percent sand. The >63µm fraction consists of carbonate nodules, angular quartz grains, and broken shell pieces. Pyritised plant fragments are rare from 150 to 120cm, and increase slightly in abundance from 150

163 Chapter 5 - Results 115 to 110cm. Rare ooids occur at 145cm and from 115 to 110cm. The lithological log (Fig. 5.9) shows that the section from 150 to 115cm is light grey in colour, with a gradual change to dark grey at 114cm cm: Brackish lake lake margin - including tidally influenced waters at 90-85cm and at 75cm. There are a high number of foraminifers, approximately 500,000 per dry gram of sediment, in a low diversity assemblage. Ammonia beccarii dominates the assemblage (40-90%), H. anderseni is also abundant (0-15%). The assemblage includes up to 45% reworked lake assemblage taxa and also the euryhaline Elphidium spp., mainly E. reticulosum (0-5%). From 90 to 85cm a few fairly wellpreserved marine species are present, including Gallitellia vivans, Murrayinella murrayi, Bolivina spp., Loxostomina costatapertusa, Elphidium advenum, E. carpentariensis and Cassidelina sgarrellae. At 75cm some Pararotalia spp., Textularia spp. and Spiroloculina excisa occur. Reeves (2004) notes the betterpreserved lacustrine section from 105 to 70cm has an increasing diversity of freshwater ostracods. Pyritised plant fragments are present to abundant (especially abundant at 95cm), along with pyrite in foraminifers and pyrite in the sediment. The matrix is clayey-silt-sand. At 105cm the sand sized fraction consists of carbonate nodules, angular quartz grains and broken (and pink coloured) shell pieces. From 100 to 80cm the >63µm fraction is composed of extremely abundant foraminiferal tests combined with broken shell pieces and angular quartz sand. At 90cm a number of characteristics indicative of marine influence occur abundant echinoid pieces (first occurrence, present to abundant throughout the rest of the core except at 85cm), and the presence of ooids (also occur rarely at 85cm) and glaucony (first occurrence, present to abundant throughout the rest of the core). A shell hash layer is centred at 77cm, encompassing 85-75cm. At 80cm, bivalves, broken shell pieces and freshwater gastropods (Planorbidae) are present (all pink coloured). The charophyte Chara zeylanica is present (Garcia, pers. comm., 2005) at 80cm. At 75cm the sand fraction consists of dominantly foraminiferal tests and angular quartz grains, with some pink broken shell pieces, and more brackish mollusc taxa are present (Ponder, pers. comm., 2003). The angled transition from lacustrine to marine is shown in the lithological log (Fig. 5.9), and due to the nature of sampling, from 151

164 Chapter 5 - Results 90 to 75cm the sampling would have been from the lacustrine sediments around the transition. 70cm: Transitional choked lagoon There is a dominance of Ammonia spp. and reworked lake assemblage forms (30% each), Elphidium spp. (mainly E. reticulosum) is also present (10%). Other euryhaline and marine forms contribute in total about 20% to the foraminiferal assemblage. They include: Textularia spp., Asterorotalia spp., Pararotalia spp., Ammomassilina alveoliniformis, Ammonia convexa, Loxostomina costatapertusa and Murrayinella murrayi. There are around 50,000 foraminiferal tests per dry gram of sediment. The matrix is clayey-silty-sand, with broken shell pieces and angular quartz grains present in the >63µm fraction. Ooids, glaucony, echinoid spines, and pyritised plant fragments are abundant, as well as pyrite in foraminiferal tests and pyrite within the sediment. However, Chara zeylanica is also present (Garcia, pers. comm., 2005). Iron oxide nodules also occur cm: Transitional restricted to open lagoon Reworked lake assemblage foraminifers dominate, forming up to 25% of the total assemblage. The planktonic Gallitellia vivans is abundant, with Rosalina sp.1, A. beccarii, A. convexa and Bolivina spp. present (3-10%). Asterorotalia spp., Pararotalia spp., Elphidium spp., Textularia spp., Quinqueloculina philippinensis, Lachlanella compressiostoma, and Spiroloculina excisa also occur. There are approximately 50,000 foraminifers per dry gram of sediment. The matrix is clayey-silty-sand, with sand content gradually increasing toward the top of the facies to reach 20% of the matrix. The >63µm fraction is composed of broken shell pieces and angular quartz sand. Ooids, glaucony and echinoids are abundant, microscopic shells are rare to present, macroscopic shells are present to abundant. At 65cm iron oxide nodules occur. Pyrite-filled foraminiferal tests are present throughout the facies. Pyrite stained ostracods occur from 65 to 60cm, and rare pyritised plant fragments occur at 60cm. 152

165 Chapter 5 - Results 50-0cm: Marine - fully marine The foraminiferal assemblage is diverse, and the population is numerous (about 50,000 foraminifers per dry gram of sediment). G. vivans, Murrayinella murrayi, Bolivina spp., Ammonia spp., Elphidium spp., and Rosalina sp. 1 are abundant, up to 15% each. All species mentioned in the transitional facies occur. The matrix is clayey-silty-sand; there is a sharp decrease in sand content at 40cm with less sand from 40 to 0cm. The >63µm fraction is composed of broken shell pieces and quartz sand. From 50 to 45cm, and at 35cm, 25cm, 15cm and 10cm the quartz grains are angular. At 40cm, 5cm and 0cm the quartz grains are a mix of angular, rounded, and frosted morphologies. At 30cm and 20cm the quartz grains are angular and rounded. Ooids and echinoids remains are abundant throughout the facies. Glaucony is abundant throughout, except at 10cm where it is only present. Microscopic shells are rare (45-30cm and 15cm) to abundant (25cm, 20cm, 5cm and 0cm). Macroscopic shells are present to abundant (50cm, 35-20cm and 0cm). From 50 to 25cm pyrite is present in foraminiferal tests, and between 15 and 10cm it is rare. Iron oxide nodules are very rare (35-20cm), rare (15-10cm) to abundant (5-0cm) Core MD Although shown in the centre of the transect in Figure 5.1, core MD 28 is transposed from around 150km north and therefore is not necessarily in as central a position as indicated. It is distal from present outflows of Australian rivers. Species and sedimentology data indicate its sub-aerial exposure and, therefore, at times its occupation of a position close to the lake margin. MD 28 Sedimentology Figure 5.11 shows the sedimentary characteristics of core MD 28. The basal section from 150 to 140cm is dark grey consolidated, but not hard, sediment. From 140 to 72cm the sediment colour lightens, is quite dry and hard, has a yellow hue from an overprint of mottling, and shows evidence of bioturbation. There is a possibly organic-rich dark horizon at 130cm. From 72 to 55cm the sediments are dark and sticky, with a visible shell layer within lighter sediments at 70-65cm. Green-grey-brown marine sediments occur overlying the lacustrine 153

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168 Chapter 5 - Results section with a bioturbated erosional boundary around 60-55cm. There is also a slightly visible bioturbated boundary between two sections of marine sediments at around 30-24cm. MD 28 Micropalaeontology Species occurring, displayed in Figure 5.12, include: Textularia sp.1, Ammomassilina alveoliniformis, Lachlanella compressiostoma, Quinqueloculina crassicarinata, Q. philippinensis, Q. parvaggluta, Q. tubilocula, Gallitellia vivans, Bolivina glutinata, B. vadescens, Loxostomina costatapertusa, Schackionella globosa, Murrayinella murrayi, Pararotalia calcariformata, Pararotalia sp.1, Heterolepa subhaidingeri, Cibicides refulgens, Helenina anderseni, Ammonia beccarii, A. convexa, Ammonia spp. (reworked), Asterorotalia gaimardi, A. milletti, Elphidium carpentariensis, E. reticulosum, Haynesina depressula simplex, Rosalina sp. 1 and Tenuitella sp.1. MD 28 Dating Core MD 28 displays a relatively consistant record of age versus depth in the lacustrine section, probably due to its position distal from Australian rivers, in an environment of slow, low-energy sediment deposition. Lacustrine bivalves were radiocarbon dated at the following levels within the lacustrine facies: 75cm, 70cm, 66cm and 60cm. The shell layer is visible to the naked eye from 70-65cm, at 75cm only microscopic shells are present, and they are very abraded. Although the best preserved was chosen from this level, it had a broken edge, and the anomalously young date (17.2ka cal BP compared to 17.9ka cal BP at 70cm) may indicate some disturbance of the sediment. However, the dates overlap at an error of 2σ, so the difference may not be significant. The lacustrine bivalve at 66cm was also 17.2ka cal BP. In the micropalaeontologically transitional facies at 60cm a lacustrine bivalve was used (12ka cal BP). Marine bivalves were dated at the following levels: 50cm, 35cm, 15cm and 0cm. At 50cm the mollusc used was from the family Lucinidae, species of which are known to be deep burrowing, the age was within the expected range (Fig. 5.4), although slightly younger (10.1ka cal BP). Sixty ostracod valves were also dated at 15cm, within the fully established marine facies, and both ostracods and the marine bivalves were of similar ages 157

169 Chapter 5 - Results (ostracods 2.6ka cal BP, bivalves 2.2ka cal BP). The dates at 35cm (10.6ka cal BP) and 0cm (350a cal BP) are also within the expected range. MD 28 Facies A summary of the micropalaeontological and sediment characteristics of each facies for core MD 28 follows: cm: Brackish lake lake margin The assemblage consists of abraded tests (80-90%), with the only unreworked species being Helenina anderseni (10-15%). There are about 500 foraminifers per dry gram of sediment. The matrix is clayey-silt, with a small amount of sand. The >63µm fraction is composed entirely of quartz grains (angular, rounded, and frosted), some iron-stained quartz grains occur. Pyritised plant fragments and iron oxide grains are rare. At 150cm and 145cm carbonate nodules are present. From 150 to 140cm the sediments are dark grey, as shown in the lithological log (Fig. 5.11) cm: Exposed No foraminifers are present. The matrix is clayey silt, with only quartz grains (angular, rounded, and frosted) present in the sand fraction, some iron-stained quartz occurs. Pyritised plant fragments and iron oxide grains are rare. This unit forms a dark layer visible in the lithological log (Fig. 5.11) cm: Saline Lake lake margin There is evidence of fluctuating water around this site, with reworked lake assemblage foraminifers contributing 55-90% of the total assemblage. Foraminiferal abundance is specimens per dry gram of sediment. H. anderseni is the dominant unreworked foraminifer in all levels (15-40%). From 115 to 105cm, marine forms are rare (<5%), and most are obviously reworked. The matrix is clayey-silt, with a few percent sand. The >63µm fraction is solely quartz sand (angular, rounded, and frosted), some iron-stained quartz occurs. Iron oxide grains are present (rare) throughout the assemblage, and pyritised plant fragments rarely occur. From 125 to 105cm carbonate nodules are present. Sediments are light grey, as shown in the lithological log (Fig. 5.11). 158

170 Chapter 5 - Results cm: Brackish lake lake margin - including swamp from 100 to 90cm and 80 to 75cm; and - including abundant shell population from 70 to 65cm. There are more well preserved than abraded foraminifers (13-39% abraded). Well preserved H. anderseni dominates from 100 to 90cm and 80 to 75cm (50-65%). At 85cm and from 70 to 65cm, A. beccarii dominates (39-80%), with H. anderseni abundant (7-22%). From 100 to 90cm and at 75cm, marine forms, mostly abraded, are abundant (10-15%). Foraminiferal abundance is around tests per dry gram of sediment for cm, and the dominant species occur in a range of sizes, from small to large tests. Only 15 foraminifers per dry gram of sediment occur at 80cm, and abundance increases from 500 to 250,000 specimens per dry gram of sediment towards the top of the facies. At samples taken at 70cm and 69cm depths, abraded and broken fragments of charophytes are present. Throughout the shell population (samples examined at 65cm, 66cm, 69cm and 70cm), many instars of the euryhaline ostracod Cyprideis are present, suggesting a life assemblage. The matrix is silty-clayeysand, with a large peak in the sand fraction around the abundant shell population caused by shell fragments. From 100 to 75cm the >63µm fraction consists of solely quartz sand (angular, rounded and frosted). From 70 to 65cm the sand fraction is composed of foraminiferal tests and quartz grains (angular, rounded and frosted). Pyritised plant fragments, pyrite in the sediment, and pyrite-stained ostracods are rare throughout the assemblage. Iron oxide grains occur (rare) from 100 to 70cm. At 75cm macroscopic bivalves occur rarely, at 65cm they are abundant. From 100 to 95cm carbonate nodules are present. As is seen in the lithological log (Fig. 5.11), there is an exposure surface at 102cm, and the sediments are mottled yellow from 100 to 75cm. There is also some bioturbation. The anomalous abundance of A. beccarii (compared to H. anderseni) at 85cm may be due to bioturbation of the overlying transitional facies. At 75cm the dark lacustrine sediments begin, but the macroscopic shell layer (72-65cm) occurs within a section of lighter sediment (Fig. 5.11). 159

171 Chapter 5 - Results 60cm: Transitional choked lagoon H. anderseni dominates (around 40%), although most specimens have quite small tests. Elphidium reticulosum is present (8%). Well preserved euryhaline and marine taxa including Gallitellia vivans, Tenuitella spp., Rosalina sp.1 and Pararotalia spp. occur, although rarely. The lithological log shows the beginning of the marine sediments with a bioturbated erosional boundary around 60cm (Fig. 5.11) cm: Transitional fluctuating between closed/choked lagoon and restricted/open lagoon At 55cm there occurs an almost typical marine assemblage, including most of the marine species found in the Gulf of Carpentaria cores, with only slightly more A. beccarii and H. anderseni than commonly found in the marine section. However, at 50cm the assemblage is most lake-influenced, with little reworking (<10%), containing 25% H. anderseni and 15% A. beccarii (some deformed with changes in the plane of coiling). Abundant euryhaline Elphidium spp., mainly E. reticulosum (45%), and rare marine taxa (<2%) also occur at 50cm. Towards 30cm the assemblage gradually gains more marine species (up to 12%) and Elphidium spp. is reduced to about 10%, while the percentage of abraded foraminifers increases to nearly 50%. The transition from lacustrine to marine ostracods assemblages occurs at 30cm (Reeves, 2004). Foraminiferal abundance is around 50,000 (at 55cm) to 5000 per dry gram of sediment. The matrix is clayey-silty-sand, with the most sand (around 30%) in the sample at 50cm, along with ooids and echinoid fragments (rare). Sand content decreases and varies throughout the rest of the facies. The >63µm fraction is composed of broken shell pieces and angular and rounded quartz grains. Pyrite is present in the sediment, along with pyritised plant fragments. Iron oxide grains occur between 50 and 45cm, pyrite is present in foraminifers between 40 and 35cm, with pyritised ostracods at 35cm. Microscopic and macroscopic shells are present to abundant, and are well preserved to very well preserved. The lithological log shows the grey-green unconsolidated marine sediments for this section (Fig. 5.11). 160

172 Chapter 5 - Results 25-20cm: Transitional restricted lagoon (permanent connection) A large percentage (25%) of this transitional facies assemblage is of marine species that do not comprise more than 2% of the total assemblage. Three species known to be selectively transported via water movement, Tenuitella spp., Gallitellia vivans and Schackionella globosa are present at 25cm, but decrease in abundance sharply to none or rare tests at 20cm. The abundance of Quinqueloculina spp. displays the opposite trend occurring rarely at 25cm and present at 20cm. The abundance of A. beccarii and Helenina anderseni also increase slightly at 20cm. Rosalina sp.1, Elphidium spp., Loxostomina costatapertusa and Pararotalia spp. occur, amongst other species. There are approximately 50,000 foraminiferal tests per dry gram of sediment. The matrix is of almost equal proportions of clay, silt and sand at 25cm, with sand content decreasing sharply at 20cm. The sand size fraction is composed of broken shell pieces and angular quartz. Pyrite is present in the sediment and in foraminifers throughout the assemblage, also occurring as pyritised plant fragments at 25cm. Echinoid fragments, glaucony and ooids are rare. As shown on the lithological log (Fig. 5.11), marine coloured sediments overlie similar marine coloured sediments with a visible bioturbated boundary between them at 25cm. 15-0cm: Marine fully marine The uninterrupted marine assemblage begins at 15cm, with a large percentage of the assemblage (40%) comprising species that are present in a relative abundance of less than 2% these occur in large numbers throughout the assemblage. The assemblage fluctuates slightly in percentage compositions throughout. Reworked forms, Rosalina sp.1, Ammonia spp. and Quinqueloculina spp. also reach percentages about 10% each. Foraminiferal abundance is around 500,000 tests per dry gram of sediment. The matrix is of broken shell pieces and angular quartz. Pyrite occurs in foraminifers at 15cm and pyrite is present in the sediment from 15 to 5cm. Microscopic and macroscopic shells are present to abundant, and are mainly slightly reworked. Echinoid fragments are present. Glaucony and ooids are rare at 15cm, and present from 10 to 0cm. 161

173 Chapter 5 - Results Core MD As shown in Figure 5.1 and Figure 1.1, core MD 32 is situated almost in the centre of the palaeolake, but not in the depocentre. The present Queensland coast is the closest terrestrial influence. MD 32 Sedimentology Figure 5.13 shows the sedimentary characteristics of core MD 32. Dark sticky consolidated lacustrine sediments occur from 150cm; overlain by lessconsolidated green-grey marine sediments at 38cm. Shell layers are visible from 145 to 137cm, and 103 to 97cm, in lighter grey bands within the lacustrine sediment. MD 32 Micropalaeontology Species occurring, displayed in Figure 5.14, include: Textularia agglutinans, T. foliacea, Textularia sp.1, Bigenerina aspratilis, Nubeculina advena, Ammomassilina alveoliniformis, Lachlanella compressiostoma, Quinqueloculina crassicarinata, Q. parvaggluta, Q. philippinensis, Q. tubilocula, Facetocochlea pulchra, Gallitellia vivans, Bolivina glutinata, B. vadescens, Loxostomina costatapertusa, Schackionella globosa, Murrayinella murrayi, Pararotalia calcariformata, Pararotalia sp.1, Heterolepa subhaidingeri, Cibicides refulgens, Helenina anderseni, Ammonia beccarii, A. convexa, Ammonia spp. (reworked), Asterorotalia gaimardi, A. milletti, Elphidium advenum, E. carpentariensis, E. reticulosum, Haynesina depressula simplex, Rosalina sp.1, Tenuitella parkerae, Tenuitella sp.1 and Globorotalia cultrata. MD 32 Dating Lacustrine bivalves were radiocarbon dated from 150cm, 145cm, 120cm, 100cm and 75cm, and at 70cm and 40cm lacustrine gastropods were used, all samples within the lacustrine facies, and all displayed ages within the expected range (Fig. 5.4). From 150cm to 100cm the lacustrine bivalves were from the shell layer, and had an age ranging from 18.4ka cal BP to 17ka cal BP. The bivalve shell dated at 75cm was visibly abraded, and yet was of an age 162

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177 Chapter 5 - Results compatible with the rest of the shell layer (17.3ka cal BP). A lacustrine bivalve was dated in the transitional facies at 35cm, so its age of 12.4ka cal BP is indicative of the older lacustrine facies, not necessarily the transition. Marine bivalves were radiocarbon dated from the following levels: 20cm, 10cm and 0cm. At 20cm the age is anomalously young (1.3ka cal BP), but around 20cm a large gastropod was noted when sampling (necessitating the collection of only a small sample due to its presence). The gastropod may have been responsible for some bioturbation. The ages at 10cm and 0cm are both 10.4ka cal BP, indicating reworking of the unconsolidated marine section, or the absence of the top sediments of the core. MD 32 Facies A summary of the micropalaeontological and sediment characteristics of each facies for core MD 32 follows: cm: Brackish lake - including abundant shell populations from 150 to 135cm and at 100cm; and - including swamp from 90 to 55cm. From 150 to 110cm the assemblage is temporally very homogeneous the relative percentages of foraminifers, and their morphology and low degree of reworking, appearing very similar to each other. Both A. beccarii and H. anderseni occur in approximately equal dominance, and are numerous. In this homogeneous section very few abraded tests occur (around 2%). From 105 to 40cm there is a greater percentage of abraded foraminifers (up to 27% in the last 10cm before the marine assemblage), and although both Ammonia beccarii and Helenina anderseni are numerous, H. anderseni dominates (fluctuating from 42% to almost 80%). From 150 to 120cm foraminiferal abundance varies from 250,000 to 25,000,000 tests per dry gram of sediment. Over the depths cm, foraminiferal abundance decreases from 20,000 to 5,000 tests per dry gram of sediment. At 40cm there are only 250 foraminifers per dry gram of sediment. From 110 to 40cm, Ilyocypris and Cyprideis alternate in dominance of the ostracod assemblage, and there is an increased abundance of fresher water species including Leptocythere hartmanni lacustris, Limnocythere and Cyprinotus (Reeves, 2004). From 85 to 40cm the charophytes Chara zeylanica 166

178 Chapter 5 - Results and C. vulgaris occur, becoming more abundant toward the top of the facies (Garcia, pers. comm., 2005). The charophyte Lychnothamnus barbatus is also present towards the top of the facies (Garcia, pers. comm., 2005). The matrix is silty-clay with only a few percent sand. From 150 to 115cm the >63µm fraction is composed entirely of foraminifers and ostracods, forming a microfossil sand. From 110 to 40cm the >63µm matrix consists of broken shell pieces; while pyrite, calcite, carbonate nodules and a red lithic fragment also occur. Angular quartz grains also occur from 110 to 40cm, except from 90 to 70cm where the grains are rounded. Pyritised plant fragments occur from 130 to 105cm and 95 to 40cm. From 130 to 95cm the plant fragments are rare, at 90cm they are present, and from 85 to 40cm they are abundant. From 95 to 50cm ooids occur rarely. From 150 to 130cm microscopic shells are present to abundant and well to very well preserved. At cm, 135cm and 100cm macroscopic shells are abundant (extremely abundant at 145cm) and well to very well preserved. At other horizons (except , 115 and 105cm) macroscopic shells are rare to present, grading from well preserved in the lower horizons, to very abraded at the top of the facies. This distribution also occurs with the microscopic shells which also range from very abundant to rare (except at 125cm where there are no shells). Only two major shell layers are shown in the lithological log (Fig. 5.13), from 145 to 137cm and from 103 to 97cm, occurring within lighter sediment. Otherwise the lacustrine sediments are dark, with a relatively horizontal and unbioturbated boundary between them and the overlying marine sediments at 38cm (Fig. 5.13) cm: Transitional restricted to open lagoon The assemblage at 35cm appears very similar to the typical marine assemblage in the Gulf of Carpentaria, except for a relatively high proportion of the small species Bolivina vadescens (8%), B. glutinata (5%) and Tenuitella parkerae (5%). Ammomassilina alveoliniformis also occurs in higher numbers than usual (4%). Ammonia convexa and Asterorotalia gaimardi dominate the assemblage at 35cm (around 10%). At 30cm Ammonia beccarii is dominant (almost 20%), while the relative abundances of the other species mentioned all decrease. Foraminiferal abundance is around 25,000 per dry gram of sediment. Based on the ostracod assemblage, Reeves (2004) recognises the transition to marine at 167

179 Chapter 5 - Results 30cm, with fauna from that layer containing a diverse range of marine ostracods. The matrix is clayey-silt, with the few percent of the sand size fraction composed of broken shell pieces and angular quartz (some iron stained). Echinoid fragments are abundant, both microscopic and macroscopic shells are present and very well preserved. Ooids and glaucony also occur. 25-0cm: Marine - fully marine Ammonia convexa, A. beccarii, Elphidium spp., Bolivina spp., Gallitellia vivans, and Asterorotalia spp. dominate the assemblage (up to around 10%). Foraminiferal abundance is around 25,000 per dry gram of sediment from 25 to 15cm, and around 125,000 per dry gram of sediment from 10 to 0cm. The >63µm fraction is composed of broken shell pieces and angular quartz (some iron stained). Pyritised plant fragments and echinoid fragments occur rarely. Glaucony is rare to abundant. Ooids are present. Microscopic shells are rare to present, and are well to very well preserved. Macroscopic shells are present and well preserved. At 5-0cm iron oxide nodules occur in the sediment Core MD As shown in Figure 5.1, core MD 33 is within a depression and therefore closest to the probable depocentre of the palaeolake. It is also proximal to the terrestrial influence of the present Queensland coast. MD 33 Sedimentology Figure 5.15 shows the sedimentary characteristics of core MD 33. From 150 to 65cm the consolidated lacustrine sediments are light grey. From 110 to 100cm there is evidence of flooding, with darker bands in the light grey sediments. There is a shell layer at 82-75cm. From 75 to 65cm the sedimentary structures show evidence of flooding (lighter horizons in dark grey sediments). From 75 to 42cm the sediments are dark and smooth textured, with some bioturbation, and identifiably marine shells occur in the top 5cm of this section. Sticky dark heterogeneous lacustrine sediments overlie these smooth lacustrine sediments on an erosional boundary, and appear reworked up to the transition to lessconsolidated green-grey marine sediments around 25-20cm. 168

180 Chapter 5 - Results 169

181 Chapter 5 - Results 170

182 Chapter 5 - Results 171

183 Chapter 5 - Results MD 33 Micropalaeontology Species occurring, displayed in Figure 5.16, include: Textularia agglutinans, T. foliacea, Textularia sp.1, Bigenerina aspratilis, Nubeculina advena, Ammomassilina alveoliniformis, Lachlanella compressiostoma, Quinqueloculina crassicarinata, Q. parvaggluta, Q. philippinensis, Q. tubilocula, Facetocochlea pulchra, Gallitellia vivans, Bolivina glutinata, B. vadescens, Loxostomina costatapertusa, Schackionella globosa, Murrayinella murrayi, Pararotalia calcariformata, Pararotalia sp.1, Heterolepa subhaidingeri, Cibicides refulgens, Helenina anderseni, Ammonia beccarii, A. convexa, Ammonia spp. (reworked), Asterorotalia gaimardi, A. milletti, Elphidium advenum, E. carpentariensis, E. reticulosum, Haynesina depressula simplex, Rosalina sp.1, Tenuitella parkerae, Tenuitella sp.1 and Globorotalia cultrata. MD 33 Dating A lacustrine bivalve dated at 40ka BP from 210cm is included in the data displayed in Table 5.2 and Figure 5.4. A comparison of two radiocarbon-datable materials was performed on core MD 33: lacustrine bivalves and lacustrine ostracods, from the lacustrine mollusc and microfossil-rich shell layer at 77cm. The two bivalve valves displayed a date of 17.5ka cal BP. The 135 ostracod valves returned a comparable age of 18.8ka cal BP. A marine bivalve was dated from the sedimentologically non-marine horizon at 30cm, however, micropalaeontologically this section is transitional, and marine shells occur in the sediment from 45cm. The shell at 30cm was 410a cal BP, and is younger than expected (Fig. 5.4). Marine bivalves were radiocarbon dated within the marine facies at 20cm, 10cm and 0cm age reversals are noted. Repeat analysis was performed at 0cm (2.2ka cal BP, 1.5ka cal BP) and 20cm (480a cal BP, 290a cal BP), due to suspected anomalous ages. The marine foraminiferal assemblage does not shown any clear evidence of deepening water, indicating possible reworking of the unconsolidated marine sediment. MD 33 Facies A summary of the micropalaeontological and sediment characteristics of each facies for core MD 33 follows: 172

184 Chapter 5 - Results cm: Saline Lake - swamp Helenina anderseni dominates (95-100% of well preserved forms), and is numerous. From 115 to 100cm A. beccarii occurs rarely (1-2%). Very few abraded foraminifers are apparent (around 2%). The assemblage is homogeneous in relative foraminiferal abundance and the low degree of reworking. However, most samples appear bleached (i.e frosty, white tests) to varying degrees. At cm and at 90cm they are the most bleached, with a slight bleach apparent from 140 to 130cm and 105 to 100cm. The test size of H. anderseni also varies, with all sizes present throughout most of the facies, except at 130cm, cm and at 90cm, where only small to medium tests occur. Total foraminiferal abundance varies from 10,000 to 5,000,000 tests per dry gram of sediment. However, an increasing diversity of freshwater ostracods is noted above 140cm (Reeves, 2004). The matrix is mostly clayey-silt with only a few percent sand. The >63µm fraction is mainly composed of foraminiferal and ostracod tests. At cm this microfossil sand includes broken shell pieces. At 135cm and 90cm the sand fraction also contains quartz grains (angular, rounded and frosted). Particle size analysis reveals a peak in sand content at 95cm (Fig. 5.15). No sand at all was observed in the micropalaeontological sub-sample studied at 95cm, however, the sediment and micropalaeontological sub-samples were taken from separate areas of the sample slice and it is assumed that the peak recorded at 95cm correlates with the sand recorded at 90cm. From 150 to 140cm, at 130cm and at 115cm angular quartz grains occur rarely. Iron oxide grains occur from 150 to 95cm. Pyritised plant fragments are present to abundant from 150 to 130cm, and are rare from 125 to 120cm, 110 to 105cm and at 90cm. From 150 to 110cm and 100 to 90cm the sediments are a uniform light grey, with no visible bioturbation or erosion, as seen in the lithological log (Fig. 5.15). Between 110 and 100cm there are darker grey bands in the light grey sediment cm: Brackish lake swamp -including abundant shell population from 85 to 75cm. H. anderseni dominates (80-90%), while A. beccarii fluctuates between 2-10% relative abundance. Abraded foraminifers are abundant (up to 15%). At 80-70cm (including samples examined at 77cm, 68cm, 67cm and 66cm) the test 173

185 Chapter 5 - Results sizes of the two dominant species are mainly medium to large. At cm and 60cm, most instars of the euryhaline ostracod Cyprideis are present. Foraminifers are very abundant initially (about 450,000 per dry gram of sediment) at the base of the facies, decreasing upwards (about 1500 per dry gram of sediment). The matrix is mainly silty clay. The >63µm fraction is partly composed of broken shell pieces; in the interval 85-70cm it consists of mainly foraminiferal tests with some angular quartz grains, and from 65 to 55cm angular quartz grains comprise half of the matrix >63µm. In the interval 85-80cm microscopic and macroscopic shells are present to abundant, mostly abraded. Echinoid fragments occur rarely at 80cm. From 65 to 55cm pyritised plant fragments occur, along with pyrite in the sediment. At 60cm is the first occurrence of ooids (abundant) and glaucony (rare), which are then present throughout the remainder of the core. The shell layer is shown in light grey sediments in the lithological log from 82 to 75cm (Fig. 5.15). In the interval 75-62cm light grey bands occur, within a section of darker sediment from 75 to 55cm. 50cm: Transitional Foraminifers are mostly reworked (about 90%). Of the well preserved species, about 5% are A. beccarii with a few percent each of Asterorotalia gaimardi, Loxostomina costatapertusa and Textularia agglutinans. Foraminiferal abundance is one of the lowest in the core (about 1500 per dry gram of sediment). The matrix is silty clay with less than 1% sand. The >63µm fraction consists of an equal proportion of broken shell pieces and angular quartz sand. Pyritised plant fragments occur, along with pyrite in the sediment. Ooids are abundant, while glaucony occurs rarely. One charophyte oogonia, Chara zeylanica (Garcia, pers. comm., 2005), was found in the micropalaeontological sample. The rare microscopic shells which occur are reworked, while macroscopic shells are not present. Green-grey lenses are shown in the lithological log (Fig. 5.15), in dark grey lacustrine sediments cm: Transitional restricted lagoon Reworked lacustrine taxa dominate this assemblage (35-95%). The well preserved foraminifers vary markedly in relative abundance. Ammonia beccarii 174

186 Chapter 5 - Results dominates at 40cm (about 10%), but is present at less than 10% for the rest of the assemblage. Bolivina spp., Rosalina sp.1, Asterorotalia gaimardi, Quinqueloculina spp., Cibicides refulgens, Heterolepa subhaidingeri, Elphidium spp. and Ammomassilina alveoliniformis all occur rarely throughout the facies. The planktonic Tenuitella spp. is present to rare at 35cm and 45cm respectively. This coincides with an increase in the abundance of Gallitellia vivans from rare to present at 35cm, and abundant at 45cm. The matrix is composed primarily of equal amounts of clay and silt, the small sand fraction consists of broken shell pieces and angular quartz grains. Ooids are present, pyritised plant fragments, glaucony, small shells and echinoid fragments mainly occur rarely throughout the facies. Macroscopic marine bivalve shells are visible throughout most of this facies on the lithological log, although only rare to present in the >63µm fraction utilised for micropalaeontological analysis. At 45cm, marine molluscs are present, while at 40cm estuarine species occur (including an undescribed gastropod found to inhabit sea grass, Ponder, pers. comm., 2003). The lithological log shows this facies occurs within crumbled, heterogeneous dark grey lacustrine sediments (Fig. 5.15). There is a possible erosional boundary at the base of this facies between smoother lacustrine sediments below 42cm. 25-0cm: Marine - fully marine Ammonia spp., G. vivans, Asterorotalia spp., Elphidium spp., Rosalina sp.1 and Bolivina spp. are present (around 10%). Foraminiferal abundance increases towards the top of the facies (to around 500,000 tests per dry gram of sediment). The transition to a marine ostracod assemblage occurs after 30cm (Reeves, 2004). The matrix is primarily clay and silt at the base of the facies, sand content increases toward the top of the facies to a maximum of 20%. This sand fraction almost entirely consists of broken shell pieces, with only a few grains of angular quartz sand. Ooids, glaucony, echinoid fragments, microscopic and macroscopic bivalve and gastropods all occur, generally increasing in abundance and/or preservation towards the top of the facies. The lithological log (Fig. 5.15) reveals that the base of this facies (25cm) is within heterogeneous lacustrine dark grey sediment, with a sharp boundary between the unconsolidated grey-green marine sediments above (at 20cm). 175

187 Chapter 5 - Results 5.7 Summary Sedimentological and micropalaeontological data have been used to delineate the changing environments of the Gulf of Carpentaria, from around 35ka cal BP to the present. No species or technique shown here is able to reconstruct palaeoenvironments in isolation, but utilised together they support each other, and provide valuable information that would not have been available singly. Chapter 6 will present the data collected from the six cores in a unified history of the Gulf of Carpentaria, and discuss the implications of the data presented for regional and global palaeoclimatic framework. 176

188 Chapter 6 Discussion CHAPTER 6 Discussion 6.1 Introduction The previous chapter presented the micropalaeontological and sedimentary characteristics of all the cores MD 28 to MD 33 from the Gulf of Carpentaria, treating each core separately. In this chapter, a brief overview of the spatial distribution of the Gulf of Carpentaria core locations and the consequent effect on environmental facies is first presented (Section 6.2). The designated facies are explained, and a general palaeoenvironmental history of the Gulf of Carpentaria from before the LGM to the present is advanced (Section 6.3). The reconstruction is based on the results of the previous chapter, but also refers to pollen analysis of cores MD 28 to MD 33 from Chivas et al. (2001).* Correlation between regional and global environments is noted, and implications for the terrestrial (Section 6.4) and marine (Section 6.5) history are given. 6.2 Palaeoenvironments in relation to core location In order to describe the environments of the Gulf of Carpentaria, the large distances and consequently differing environments among cores must be borne in mind. When Lake Carpentaria was at its maximum extent, its width was ~300km, and its length was ~500km. The maximum lateral distance separating any two cores is ~250km between MD 29 and MD 33, which is reflected in their simultaneous recording of very different environments. The minimum distance separating two cores is about 30km between cores MD 31 and MD Northern lake edge cores MD 28 and MD 29 The absence of New Guinean tree and fern pollen from the top 150cm of all cores (Chivas et al., 2001) indicates that Australian rivers were the dominant source of water and sediment for Lake Carpentaria. Cores MD 29 and MD 28 are situated on the northern limits of the palaeolake, and being furthest from the *A pollen diagram for core MD 32 was kindly provided by Sander van der Kaars, as it was not previously published in Chivas et al. (2001). 177

189 Chapter 6 Discussion influence of these Australian rivers have the lowest sedimentation rates. A diagram of depth versus age (Fig. 5.4a and b in the previous chapter) demonstrates this low sedimentation rate In general, the cores from this area also have the lowest amount of pollen and foraminifers per gram of dry sediment, and the highest number of reworked foraminiferal tests, as well as numerous calcareous and iron oxide nodules, indicating periods of sub-aerial exposure. Core MD 29 is located closest to the Arafura Sill, ensuring that it records possibly three brief periods of marine influence (in the form of marine foraminifers, including some small and fragile species). A less obvious record of marine incursions is found in core MD 28 (only the more robust foraminiferal species are present, and only two occasions are discernable). The low representation of Chenopodiaceae (saltbush) in both cores MD 28 and MD 29 (Chivas et al., 2001) may indicate fresher water conditions than in the other cores. Core MD 28 was collected in deeper water (63m bpsl) than core MD 29 (59m bpsl), and MD 29 experienced more extensive sub-aerial exposure than MD 28. The foraminiferal population, although similar, was more abundant and better preserved in MD 28, indicating its position in a greater depth of water. The pollen assemblages also indicate that core MD 28 may have been in proximity to slightly deeper water. Both cores contain pollen from grasses, while a higher amount of aquatic ferns and Botryococcus (fresh-to brackish water algae) occur in MD 28 (Chivas et al., 2001), suggesting a greater coverage of water. In core MD 28 Typha is also present (a fresh-to brackish shallow water/swamp taxa), while the presence of Anthoceros in MD 29 (Chivas et al., 2001) may indicate a low energy swampy environment. 178

190 Chapter 6 Discussion The very gentle gradient of the basin means that even when the palaeolake was developed to near maximum water levels, extensive areas, including around cores MD 28 and MD 29, would have mainly existed as a shallow lake environment. For example, at maximum lake conditions the postulated water level is approximately 58m bpsl, therefore only about 1m of water would cover the area around MD Tidal channel influence cores MD 30 and MD 31 Cores MD 30 and MD 31 are relatively close to each other, collected 60m and 59m below present sea-level, respectively. Both cores are proximal to sinuous contours on a bathymetric map (Fig. 6.1), which suggest they are situated near palaeochannels. The continuous presence of broken shells within the sedimentary matrix (unique to these two cores) indicates the cores were either subject to tidal influence, or in the depositional area for shells broken on the lake shore and/or reworked from older marine deposits on the lake shore. The rare presence of ooids in various stages of development throughout the lacustrine sequence also indicates some tidal influence. Marine waters have influenced cores MD 30 and MD 31 on at least two occasions. The greatest marine influence is visible in core MD 30 as a higher abundance of planktonic foraminifers and other small fragile marine species, over a greater depth in the core than MD 31. Of the two cores, MD 30 is situated in closest proximity to a palaeochannel. The gradient of the basin is also shallow around these cores, like MD 28 and MD 29, causing the development of a large shallow water margin environment when the lake waters reached this area. Pollen analysis indicates shallow-water lake conditions only ever deepening slightly, with grasses and sedges on the periphery while the fresh-to-brackish Pediastrum algae inhabited the lake (Chivas et al., 2001). 179

191 Chapter 6 Discussion GULF Figure 6.1 Sketch of facies diagrams showing the position of cores in the Gulf of Carpentaria. Refer to original facies diagrams in Chapter 5 for detailed foraminiferal assemblage data upon which these facies are based. 180

192 Chapter 6 Discussion Lake centre cores MD 32 and MD 33 Core MD 33, near the lake depocentre (collected 68m bpsl), is also the most proximal to the present-day coastline (about 150km from the Queensland coast). Core MD 32 was collected from shallower water (64.5m bpsl), and is more distal from terrestrial influence than core MD 33. Located near the depocentre and close to the north Queensland rivers which provided most sediment and water to the palaeolake, MD 32 and MD 33 have some of the highest sedimentation rates of the Gulf of Carpentaria cores (Fig. 5.4e and f). The gradient of the basin, although gentle throughout, steepens in this area, so when a lake is developed in the Carpentaria basin, this area is noticeably the deepest. The foraminiferal assemblages all contain a minimum number of reworked tests, suggesting the area was under water continuously for the studied period. Pollen analysis confirms that somewhat deeper lake conditions prevailed in the studied period. The fresh-to-brackish water algae taxa (especially Pediastrum and Botrycoccus) are best represented in these cores (Chivas et al., 2001; van der Kaars, pers. comm., 2004). And although the pollen record of all six Gulf of Carpentaria cores is dominated by swamp and grassland species, a greater percentage of woodland tree and shrub taxa occur in cores MD 32 and MD 33 (Chivas et al., 2001; van der Kaars, pers. comm., 2004). Both foraminiferal and pollen assemblages show the highest abundance and best preservation of microfossils in these cores, indicating an environment apparently conducive to habitation by foraminifers and not subject to much reworking. The homogeneous nature of the lacustrine foraminiferal assemblage through time also indicates a body of permanent water, large enough to provide relatively stable conditions. Unlike all the other Gulf of Carpentaria cores, MD 32 and MD 33 never display a marine influence (i.e. the presence of marine foraminifers) in the lacustrine phase, due to their position distal to the Arafura Sill. 181

193 Chapter 6 Discussion The presence of pollen from the terrestrial saline-water plants of Chenopodiaceae (Chivas et al., 2001) in cores MD 32 and MD 33 indicates some residual salt may have remained in the catchment area surrounding the lake margins when sea-level fell and isolated the basin to form Lake Carpentaria. 6.3 Palaeoenvironments through time Six cores within the Gulf of Carpentaria were studied to a depth of 150cm. The environmental facies of each core (Section 5.6 of this thesis: Figs 5.6, 5.8, 5.10, 5.12, 5.14 and 5.16) have been combined (Fig. 6.1) into a description of the palaeoenvironment of the Gulf of Carpentaria as a whole, beginning around 35ka cal BP and ending with the present surficial sediments. The condensed facies diagrams within Figure 6.1 and the following Figures are simplified versions of Figs 5.6, 5.8, 5.10, 5.12, 5.14 and 5.16, with the base of the core (150cm) at the base of the vertical image. Blue shading indicates lacustrine facies (darker blue for brackish waters, lighter blue for saline waters), brown shading represents sub-aerial exposure, pink shading indicates transitional facies, and the yellow shading represents the marine facies. The same colour scheme is used to map the lateral extent of the facies at various critical stages throughout the studied period Brackish lake: around 35-23ka cal BP The Brackish Lake facies (Fig. 6.2) is the first recorded facies and is based only on the two cores which display the oldest age at the base of their studied section (MD 28 and MD 29; Figs 5.2a and b). Although only two radiocarbon ages have been determined for core MD 29, it is presumed to be similar to core MD 28, also on the northwestern edge of the Gulf of Carpentaria. In comparison to core MD 28, MD 29 is considered to have had a slightly slower sedimentation rate and therefore be slightly older at its base. 182

194 Chapter 6 Discussion Figure 6.2 Brackish Lake Carpentaria ~35-23ka cal BP. The dark blue shaded area represents the extent of the lake at the beginning of the facies, the yellow shaded marine area with associated yellow arrow represents a brief marine influence. The facies diagrams for the relevant cores are shown and the section referred to is circled in red on the facies diagrams. 183

195 Chapter 6 Discussion The dominance of well preserved Ammonia beccarii and the presence of Helenina anderseni at the base of core MD 29 indicates near-lake-full conditions, at least up to the -58m contour line (possibly a metre higher) as the well preserved foraminiferal assemblage found 59m bpsl was deposited in at least a metre water depth. The small number of tests present in the core (about 100 per dry gram of sediment) and the dominance of A. beccarii indicates low or fluctuating salinities at the limit of foraminiferal tolerance. A brief marine influence is visible within the Brackish Lake facies in core MD 29, with the rare occurrence of well preserved marine foraminifers, including the planktonic Gallitellia vivans. An extreme climatic event may have deposited marine fauna in the brackish lake, via channels in the Arafura Sill, with sea-level possibly a few metres below the lake level. Cyclone activity and associated storm surges may temporarily raise sea-level by up to 6m (Hopley, 1987). Transport of planktonic species into the palaeolake via a tidal channel is also a possibility, with sea-level at around lake height (e.g. Wang and Chappell, 2001). The large brackish Lake Carpentaria may have been connected to the ocean via channels up to 75m deep found in the Arafura Sill (Jones and Torgersen, 1988), although Jones and Torgersen (1988) note that they find no evidence for the opening of the -75m channels in their core data (beginning 34ka BP). Jones and Torgersen (1988) state that water levels within the channels could have been around 5m deep, resulting in a very low lake level around the -70m contour or higher for the -75m channels. The -62m channels found by Jones and Torgersen (1988) to be active in the last glacial cycle would entail lake levels around 57m bpsl or higher. This event is not well constrained in MD 29, and could have occurred from 30ka cal BP, to up to 40ka years ago (Fig. 5.4a and b). A simple linear extrapolation of the AMS dates (Fig. 5.4a) from core MD 28 gives an age of about 30±5ka cal BP for the base of the studied section. Reeves (2004) notes a Lake Carpentaria facies from cm in MD 28, with a basal date of around 40ka BP (uncalibrated 14 C). At this stage the presence of some well preserved H. anderseni (in an otherwise reworked assemblage) indicates the site was near the lake edge. H. anderseni occurs in 184

196 Chapter 6 Discussion marshes and swamps (Debenay et al., 2000; Serandrei Barbero et al., 2004) and is also found above high-tide level (Phleger, 1965; Scott et al., 1991). The presence of pyrite attests to a swampy environment. Therefore, the lake surface was around the -62m contour. No pollen is preserved in this facies from MD 29. However, the pollen data from core MD 28 confirm the very low water level around the site. Zone 3 at the base of MD 28 in Chivas et al. (2001) is composed of grasses, sedge, Typha (a fresh/brackish shallow water/swamp taxa), aquatic ferns (including Ceratopteris and Azolla) and Botrycoccus (fresh-to-brackish water algae). A swampy environment is indicated by the presence of lake-edge and swamp species, while the presence of algae implies a freestanding body of water existed, although of a low level. The occurrence of preserved plant fragments in this early Brackish Lake facies in cores MD 28 and MD 29 is an indicator of a low-energy environment. Anaerobic conditions were necessary for the preservation of pyritised plant fragments i.e. stagnant water around the sediment-water interface. The anoxic conditions may have begun just below the sediment-water interface, allowing shallow infaunal and epifaunal foraminifers to populate the area above, or at the sediment-water interface. Anoxic conditions commonly occur at the sediment-water interface, due to a totally stagnant water body, or a stratified water mass separating the stagnant bottom water from the overlying habitable water body. Anoxic conditions within the sediment, or a stratified system is postulated, as the presence of foraminifers indicates an area of non-stagnant water inhabited by a sustained population. If stratified, the stratification must only have been in the immediate vicinity of the sediment-water interface, perhaps less than 1cm thick (and not necessarily constant), as a water body as shallow as Lake Carpentaria would be subject to extensive mixing, especially in lake marginal areas. 185

197 Chapter 6 Discussion The dominance of A. beccarii in the deeper waters around MD 28 indicates brackish conditions. Although H. anderseni (most suited to a saline environment) dominates the swampier area around MD 29, it has been documented from brackish waters (Lipps and Langer, 1999), and residual salt in the Gulf of Carpentaria soils would have ensured the shallow marginal environments were not purely freshwater. Pollen evidence also indicates fresher water in the area at this time, compared to other cores in the gulf at other time periods (although it must be noted that core MD 28 shows consistently fresherwater pollen assemblages than the younger successions in cores MD 30-33). The remainder of the Brackish Lake facies in cores MD 28 and MD 29 has been sub-aerially exposed, indicating lake-levels fell to around the -63m contour. A lake level beginning to contract to the -63m contour after 27ka BP (uncalibrated 14 C) is postulated by Torgersen et al. (1988). Like the Brackish Lake facies of this study, the lacustrine Unit III of their investigation is characterised by dominant Ammonia beccarii (formae tepida) and the ostracod Cyprideis. This unit is only present in the deeper cores of Torgersen et al. (1988) Saline lake: ka cal BP During the LGM (centred 22-19ka cal BP), the main lake level contracted to its lowest level, around the -63m contour line (Fig. 6.3). It must be noted that the lake level was not at 63m bpsl for the entire period; this figure represents the minimum extent of the lake that occurred during the time of deposition of this facies. A plot of radiocarbon age versus depth (Fig. 5.4d) gives an age of around 23ka cal BP for the base of the studied section in core MD 31, although it must be noted that the non-linear distribution of the data points makes any supposition regarding ages tentative in this core. Core MD 29 does not contain any pyritised organic fragments during this period (around 20ka cal BP and immediately after), while pyritised plant fragments occur rarely throughout the facies of core MD 28. Both cores MD 28 and MD

198 Chapter 6 Discussion GULF Figure 6.3 Saline Lake Carpentaria ka cal BP. The light blue shaded area represents the minimum extent of the lake. The facies diagrams for the relevant cores are shown and the section referred to is circled in red on the facies diagrams. 187

199 Chapter 6 Discussion have episodes barren of all microfossils (of the unreworked taxa Helenina anderseni dominates, with reworked marine and lacustrine species most common). Carbonate nodules and iron oxides are also present. Periods of subaerial exposure are indicated for cores MD 28 and MD 29, as lower lake levels meant they were situated on the dry bank at the lake edge. Periodic inundation of the area, creating a swampy environment conducive to the survival of H. anderseni is indicated. During subsequent periods of sub-aerial exposure and pedogenesis most foraminiferal tests would be abraded and possibly destroyed, while the pyrite previously formed would be oxidised. Although displaying many indications of reworking, such as carbonate and iron oxide nodules, the environment around cores MD 30 and MD 31 was not consistently sub-aerially exposed for the length of time required for the development of laterally extensive horizons with soil carbonates. Well preserved foraminifers (dominantly H. anderseni, with Ammonia beccarii present in MD 31) and pyritised plant fragments occur in both cores throughout the Saline Lake facies, suggesting coverage of water at times. The indications of reworking together with the well preserved foraminifers and pyritised plant fragments point to a lake-edge or isolated pond environment. The continuous matrix of broken shell pieces and intermittent occurrence of ooids in both cores suggests that runoff into the area reworked previous marine (and possibly lacustrine) deposits. Core MD 30, although collected in water depth a metre greater than MD 31, contains better preserved foraminifers, suggesting the development of an isolated water body in this area associated with a palaeochannel. This facies is not present in the studied section of core MD 32; however, in MD 33, abundant H. anderseni dominate the foraminiferal assemblage and only rare tests of A. beccarii are found. Pyrite is also commonly present in this facies in MD 33, and abraded reworked tests are very rare in the foraminiferal assemblage. The deepest areas of the lake were continuously underwater. 188

200 Chapter 6 Discussion In all cores (MD 28, 30, 31, 33), the palynology (Chivas et al., 2001) reveals abundant grasses (Poaceae), commonly decreasing in abundance toward the top of the facies. Some sedges (Cyperaceae) are also present, commonly increasing in abundance toward the top of the facies. The ratio between sedge to grass has been utilised as a climatic indicator (Turney et al., 2004), where an increase in sedge signifies a wetter period, and vice versa. Therefore, the climate was dry at the beginning of sedimentation, but experienced increasing rainfall toward the end of the facies. An abundance of Typha in MD 28 indicates that the surrounding area was swampy and may have been more brackish than the rest of the lake, perhaps akin to a wetland environment with annual inundation (e.g. McGlone, 2002). The low representation of algal species in all cores except MD 33 suggests low lake levels. A peak in the sand fraction, composed of frosted quartz grains (presumed to have undergone transportation via wind) occurs in this facies in core MD 33, and indicates aridity on the margins of the palaeolake. A similar peak in windblown dust particles was found by De Deckker (2001) in the Gulf of Carpentaria core GC 2, situated near MD 33, and is dated to 21.5ka cal BP. This lake phase is of a higher salinity than the previous Brackish Lake facies (demonstrated by the dominance of H. anderseni in all cores). The higher salinity is most likely due to a contraction of lake levels and, therefore, a higher concentration of ions in the remaining water. No outside influence such as connection to the ocean is necessary to cause the increase in salinity Brackish Lake: ka cal BP A radiocarbon date of 18.8ka cal BP within a shell layer (77cm) at the base of this facies in MD 33 is the oldest date constraining the brackish lake facies (Table 5.2). A lake level around the -62m contour is indicated (Fig. 6.4) as the shell layer is only present in the deeper, easternmost cores (MD 28, 32 and 33). 189

201 Chapter 6 Discussion GULF Figure 6.4 Brackish Lake: abundant shell population ka cal BP. The dark blue shaded area represents the extent of the lake. The facies diagrams for the relevant cores are shown and the section referred to is circled in red on the facies diagrams. 190

202 Chapter 6 Discussion Bivalves (Corbulidae), foraminifers (Helenina anderseni and Ammonia beccarii) and ostracods (Cyprideis) are very abundant, and estuarine gastropods may also be present within the shell layer. The macroscopic bivalves form a visible layer in the lithological log (Section 5.6), within a microfossil sand matrix. H. anderseni dominates the easternmost core, MD 33, while H. anderseni and A. beccarii are in almost equal abundance within MD 32, and A. beccarii dominates the westernmost MD 28 (Figs 5.12, 5.14 and 5.16). The dominance of H. anderseni indicates low energy swamp conditions around the eastern margin of the lake, although no pyrite is present in MD 33 at the base of the facies, suggesting a deeper coverage of water at the actual site. Pyrite is also absent from the base of the facies within MD 32, indicating that the waters there were subject to some circulation. Partway through the facies in MD 32 the intermittent absence of bivalves, the occurrence of pyrite and the slightly chalky appearance of the microfossil assemblage indicate stagnant bottom waters with some breakdown of organic matter, as the tests were etched by the acid byproduct of organic matter breakdown. Supporting the foraminiferal evidence of a freshening of the saline lake (increased abundance of A. beccarii), freshening water is also indicated by the presence of the charophytes Chara spp. and Lychnothamnus barbatus (Garcia, pers. comm., 2005), and ostracod assemblages containing fresh to brackish taxa such as Leptocythere and Ilyocypris (Reeves, 2004). The low diversity and high abundance of the macro and microfauna also indicate a change in conditions from the previous Saline Lake facies. In the large area of the Gulf of Carpentaria it is possible fresher water conditions existed in some areas closer to streams or in pools on the flat lake margin floor. Within the Gulf of Carpentaria cores, the shell layer ranges from a layer 15cm thick (MD 28 and MD 33) to up to 50cm thick in the deepest core (MD 32). In MD 32 separate episodes of abundant shell production are shown on the lithological log (Fig. 5.13; cm and cm) and through micropalaeontological analysis ( cm, 120cm and cm). The first visible shell layer in MD 32 was deposited ka cal BP, the second at 191

203 Chapter 6 Discussion 17ka cal BP. Figure 5.4e illustrates the rapid sedimentation rate in MD 32, during the deposition of this layer: around 3.6mm yr -1. During deposition of the shell layer, the sedimentary matrix of the lake edge cores MD 28 and MD 33 consists almost entirely of broken shell pieces, which continue in the sand fraction of MD 28 and MD 33 until the present day. The abundance of broken shells may be from increased fluctuations in lake levels and a higher wave energy disturbing the bivalve communities on the lake edge, or increased lake levels reworking the older marine sequences deposited on the lake margin. The broken shells and whole shells from the shell layer appear to be of the same species, discounting the latter possibility as a major source of shell fragments. In MD 33 rare echinoid fragments occur associated with the shell layer. A penecontemporaneous influx of marine sediments can be discounted as no marine or estuarine foraminifers are present where the echinoid spines occur. The echinoid fragments may indicate reworking of older marine deposits, although they appear well preserved. All three cores (MD 28, 32 and 33) recorded an algal bloom at the base of the facies, then a sharp decrease in abundance of algae for the duration of the shell layer, which increases sharply again after the shell layer (Chivas, et al., 2001; van der Kaars, pers. comm., 2004). A sustained period of warmer weather may have caused the algal bloom, and/or a freshening of the lake due to increased precipitation. The sharp decrease in abundance of algae may have been due to an explosion of predators (including foraminifers), or onset of variable climatic conditions. The eastern cores (MD 32 and MD 33) have an approximately equal amount of grasses and sedges, while the western Gulf (MD 28) is dominated by grasses (Chivas et al., 2001; van der Kaars, pers. comm., 2004). The peak abundance of trees and shrubs (both lowland and woodland taxa) in the lacustrine section of MD 32 occurs in this facies (van der Kaars, pers. comm., 2004). A peak in the woodland tree Casuarina is common to both MD 32 and MD

204 Chapter 6 Discussion (Chivas et al., 2001; van der Kaars, pers. comm., 2004). Reinvigorated streams or easterly winds may have transported the pollen from new growth in the catchment area of northern Queensland to this eastern section of the Gulf of Carpentaria. A shell layer of predominantly Americanna, Corbulina and the gastropod Gyraulus is described by Torgersen et al. (1988) in core material from the Gulf of Carpentaria, however, it is dated at 27-35ka BP (uncalibrated 14 C). The shell layer noted in the present study is broadly similar, with Corbulidae and Planorbidae (most likely Gyraulus). Although it is clearly a later event, similar conditions are postulated to have occurred ka cal BP. Brackish conditions persisted after the demise of the abundant bivalve and foraminiferal population, from 17ka-12.2ka cal BP (Fig. 6.5). The shallower cores (MD 29, 30 and 31) do not contain this shell layer as they were above the level of the lake on the shallow lake margin at the time of the abundant shell population. MD 29 was sub-aerially exposed. It is possible MD 31 and MD 30 were situated in shallow pools, not completely connected to the main lake body, but not drying out. Broken shells are abundant throughout the core from possible lake-marginal deposition; however, some well preserved foraminifers are present, although the assemblage is dominated by reworked tests. The first indication of higher lake levels in MD 31 and MD 30 in the sedimentary record is an increase in foraminiferal abundance and preservation, as a greater water depth allowed a greater abundance of foraminifers to inhabit the area, and less sub-aerial exposure occurred. MD 32 and MD 33, however, record a penecontemporaneous increase in reworked tests in the foraminiferal assemblage (along with an increase in rounded quartz grains). Greater fluvial input is indicated, transporting foraminifers to the low-energy lake centre and freshening the lake waters. Expansion of the lake margin leading to reworking of previous sub-aerially exposed sediments is also likely. 193

205 Chapter 6 Discussion GULF Figure 6.5 Brackish Lake Carpentaria ka cal BP. The dark blue shaded area represents the extent of the lake, the yellow shaded marine area with associated yellow arrow represents a marine influence. The facies diagrams for the relevant cores are shown and the section referred to is circled in red on the facies diagrams. 194

206 Chapter 6 Discussion In core MD 32, the swamp-inhabiting H. anderseni increases in abundance to dominate the foraminiferal assemblage (after the equal abundance of H. anderseni and A. beccarii during the shell layer) before decreasing toward the end of the facies. Core MD 33 has dominant H. anderseni throughout the entire lacustrine facies. However, A. beccarii comprises a large percentage of the foraminiferal assemblage of all the other cores, indicating brackish conditions for the majority of the lake. Pyrite reaches its maximum abundance in MD 30, 31 and 32, indicating quiet water environments such as swamps (producing reducing conditions) surrounded the lake. A signal of fluctuating climate is noted after the shell layer, constrained to 17-13ka cal BP. Pollen data (Chivas et al., 2001; van der Kaars, pers. comm., 2004) indicate a progressively deepening lake and/or increasing precipitation. The proportion of sedge pollen (in comparison to grasses) increases toward the top of the facies in MD 28, 30 and 31. However, the ratio of sedge to grass remains constant in MD 29 and MD 32, while decreasing in MD 33, as vegetative development around the lake was not uniform and the north-eastern areas may have been drier. An increase in Typha pollen is noted in MD 32 and MD 33, indicating freshening of the lake waters, and/or greater annual inundation of the wetlands surrounding the lake. However, significant periods of drier climate also occurred. The ostracods Cyprideis and Ilyocypris dominate the assemblage at alternating levels in this section of MD 32 suggesting fluctuating periods of drier and wetter periods respectively (Reeves, 2004). Fresh water continued to fill the brackish lake, expanding it to around the -59m contour (Fig. 6.5). Cores MD 28 and MD 29 have an increase in foraminiferal abundance and preservation from earlier levels, although individuals are still not very abundant and the tests tend to be reworked, indicating a position on or near the lake edge. Core MD 29 (situated on a bank in the northwestern area) displays evidence of seiches from wind-blown water into the area, or from changes in precipitation/evaporation causing fluctuations in the lake level in the order of tens of centimetres. In MD 33 lighter-coloured laminations (of more oxidised material, indicating more flooding) occur immediately after the shell layer, suggesting variable climatic conditions. 195

207 Chapter 6 Discussion Although the lake was composed of brackish waters, at 12.7ka cal BP (radiocarbon dated from 90cm of MD 30; Table 5.2) a marine influence is visible in the presence of planktonic marine foraminifers in the three western-most cores (MD 29, 30 and 31). These species have been shown to undergo transport for long distances in tidal rivers (Wang and Chappell, 2001). A sealevel around 60m bpsl, enabling marine waters to infiltrate the -62m deep channels in the Arafura Sill possibly only at the highest tides and/or associated with storm surges is postulated. A catastrophic event such as a cyclone or tsunami also may have transported the foraminifers into the lake. The lake level was around the -58m contour, and drained through these channels to the Arafura Shelf, so tidal exchange was possible. A shell hash layer (of Corbulidae) occurs just after the marine influence, at 12.4ka cal BP (radiocarbon dated from 80cm of MD 30; Table 5.2) in cores MD 29, 30 and 31, indicating their lake-edge position at the time, when lake levels were around the -58m contour. Associated with this shell hash in cores MD 30 and MD 31 are ooids, glaucony and echinoid spines and estuarine molluscs. The marine influence of 12.7ka cal BP remained as sea-level rose to around 58m bpsl. The shallowest core, MD 29, was proximal to the lake margin at the time when the transition to marine conditions began. Foraminifers are few and badly preserved, pyrite is absent, and calcareous nodules and iron oxides occur. In all other cores, just before the transition to a marine environment, over half of the foraminiferal assemblage is composed of well preserved individuals indicating lake levels remained around the -58m contour. Pollen evidence (Zone 2 of all cores in Chivas et al., 2001; van der Kaars, pers. comm., 2004) indicates Lake Carpentaria was of relatively fresh water (the presence of Typha and algae). A swampy phase is noted for MD 29 (mainly grasses and sedges) and a shallow lake environment for all other cores (with algae and aquatic ferns as well as grasses and sedges). 196

208 Chapter 6 Discussion In all cores except MD 32 and MD 33 some iron oxide nodules occur towards the transition to marine. It is unlikely that the bulk of the formation of iron oxides was in situ, due to the presence of pyrite within the sediment and within microfossils in most cores at this level. The iron oxides were probably reworked from the lake shores by the expanding lake waters. The maximum documented extent reached by Lake Carpentaria is around the -58m contour (i.e. 12m water depth), as MD 29 has well preserved foraminifers indicating some coverage of water (in the order of a metre). It may have been about a metre more, but no cores were taken in shallower water depth than 59m bpsl. The precipitation/evaporation regime did not allow a greater amount of water to collect in the basin, and the outflow channels to the Arafura Sill were active with about 4-5m water depth in the 62m bpsl channels when Lake Carpentaria was at its maximum extent. A date of 12.2ka cal BP marks the end of the brackish lake facies (radiocarbon dated from MD 31 at 65cm and 70cm; Table 5.2). Torgersen et al. (1989) noted the presence of a fresh-to-brackish Lake Carpentaria, with a gradual transition to marine conditions. In Unit II of their study, a typically lacustrine fauna (Ammonia and Cyprideis and Ilyocypris) was found at the base (27ka BP uncalibrated 14 C). A gradual increase in species diversity (including Pseudorotalia inflata, Heterolepa subhaidingeri and Elphidium hispidulum) occurred near the top (8.5ka uncalibrated 14 C, 9ka cal BP) Transitional: ka cal BP Lagoonal conditions occurred in the Gulf of Carpentaria around 12.2ka cal BP as the sea-level rose higher than the water level of the brackish lake (58m bpsl), allowing marine water to flow into the lake via the -62m channels in the Arafura Sill (Fig. 6.6). Sand bars may have at times developed in the channel, temporarily closing the connection to the ocean, but later washed out due to storm activity and the rising sea-level. This cyclic build up and destruction may 197

209 Chapter 6 Discussion GULF Figure 6.6 Transitional environment of the Gulf of Carpentaria ka cal BP. The pink shaded area represents a transitional environment, the yellow shaded marine area with associated yellow arrow represents a marine influence. The facies diagrams for the relevant cores are shown and the section referred to is circled in red on the facies diagrams. 198

210 Chapter 6 Discussion be observed in Lake Illawarra (100km south of Sydney) in modern times (Brian Jones, pers. comm., 2003). Sea-level remained below the height of the -53m Arafura Sill, and a semi-permanent connection between the large estuarine water body and the ocean was established. The foraminiferal assemblage data of core MD 29 show a rapid transition to a marine environment. The typical lacustrine Ammonia beccarii dominated assemblage containing only reworked robust marine species, is overlain by a choked lagoon assemblage still dominated by A. beccarii but also including Elphidium spp. and containing a few marine species such as Gallitellia vivans, Bolivina vadescens, Schackionella globosa and Pararotalia sp.1. The next sample is a diverse marine assemblage (dated at 10.5ka cal BP), including Textularia spp., Lachlanella compressiostomata, G. vivans, Bolivina spp., S. globosa, Heterolepa subhaidingeri, Cibicides refulgens, A. convexa, Asterorotalia spp. and Elphidium spp. The site was on the western edge of the gently sloping lake margin, closest to the Arafura Sill, in shallow brackish water (about a metre deep) at the time of transition, so there was little non-marine water to dilute the incoming oceanic water. Also in only around a metre of lake-water at the time of transition, core MD 31 was however, further from the Arafura Sill (although near a palaeochannel). Immediately above the typical lacustrine foraminifers (dated at 12.3ka cal BP) is one sample of choked lagoon assemblage (dominated by A. beccarii and Elphidium spp.), followed by a restricted to open lagoonal population consisting mainly of A. beccarii, E. reticulosum and Rosalina sp.1, with Textularia spp., Ammomassilina alveoliniformis, G. vivans, Bolivina spp., Loxostomina costatapertusa, Pararotalia spp. and Asterorotalia gaimardi. MD 30, in around 2m of brackish water at the time of marine influence, displays a facies change similar to MD 31. However, the choked lagoonal foraminiferal assemblage (of A. beccarii, Elphidium spp. and a few marine species) is younger than the rest of the Gulf of Carpentaria cores, dated at 12.4ka cal BP. Perhaps the position in proximity to a palaeochannel lead to the area around MD 30 pooling ocean water earlier than other areas. By 10.5ka cal BP the 199

211 Chapter 6 Discussion assemblage is more typical of restricted to open lagoonal conditions, with Gallitellia vivans, Bolivina spp., Rosalina sp.1, and A. convexa added to the assemblage, and other marine species also present. In a maximum of 5m water depth of brackish lake water when marine water infiltrated through the channels, MD 28 was alternately influenced by marine and brackish water conditions. The foraminiferal assemblages alternate between a choked lagoon assemblage, a restricted/open lagoonal assemblage and a closed lagoon assemblage, opening to marine influence toward the top of the facies. A choked lagoon assemblage (for the duration of one sample) of H. anderseni and Elphidium reticulosum (with rare Gallitellia vivans, Tenuitella sp.1, Rosalina sp.1 and Pararotalia sp.1) occurs immediately above a typical lacustrine assemblage of A. beccarii and H. anderseni, signifying the first sustained period of marine influence at 12ka cal BP. This is followed by an open lagoonal assemblage of diverse and abundant marine species, also with abundant A. beccarii and H. anderseni (for the duration of one sample). Again, for the duration of one sample, a choked lagoonal assemblage occurs, composed of dominant H. anderseni, A. beccarii and Elphidium spp., with rare marine species. Gradually, over the next six samples, more marine species are introduced to the assemblage (beginning after 10.7ka cal BP), including Quinqueloculina spp., Gallitellia vivans, Loxostomina costatapertusa, Murrayinella murrayi and Tenuitella sp.1. The foraminiferal assemblages of the deeper cores, MD 33 and MD 32 do not show a period of choked lagoon conditions. The assemblages change from lacustrine at 13.4ka cal BP (dominant H. anderseni, with A. beccarii) to more marine influenced, comparable to restricted/open lagoonal environments (abundant marine species, not as diverse as the true marine assemblage, and with a large percentage of reworked lacustrine species). It is apparent by this trend that the deeper waters of the gulf remained brackish even as sea-water mixed with the lake water on the western margin. In the large embayment of the Gulf of Carpentaria, brackish water environments and normal marine salinities would have coexisted. 200

212 Chapter 6 Discussion In comparison with Torgersen et al. (1988), the facies classified as Transitional in this thesis is similar to the top of Unit II, with the introduction of Heterolepa subhaidingeri and other species into the lacustrine Ammonia beccarii and Helenina anderseni assemblage. It is probable that the species identified as Pseudorotalia inflata by Torgersen et al. (1988) equates to Asterorotalia gaimardii, and that the Elphidium hispidulum of Torgersen et al. (1988) is identified as E. reticulosum in this study. Palynological analysis reveals charcoal concentration drops sharply at the base of the transitional facies (Chivas et al. 2001; van der Kaars, pers. comm., 2004). In all cores this occurs exactly at the point where foraminiferal analysis shows the first sustained marine influence (not necessarily corresponding to a parallel transition in sediments and pollen assemblages). In all cores (except MD 29), the transition is also marked by a decrease in algae (Chivas et al. 2001; van der Kaars, pers. comm., 2004). Algae in MD 29, although negligible at the transition, increase briefly later in the marine section. In addition, the first sustained presence of mangrove pollen occurs in all cores (except MD 33) at the transition (Chivas et al. 2001; van der Kaars, pers. comm., 2004). Situated on the western edge of the Gulf of Carpentaria, mangrove development around MD 33 was delayed by thousands of years, perhaps until inundation of Torres Strait Marine: ka cal BP Sea-level continued to rise, overtopping the height of the 53m bpsl Arafura Sill and inundating the lagoonal Lake Carpentaria (Fig. 6.7). The transition to marine conditions, dated to around 10.5ka cal BP, is immediately obvious sedimentologically, and in the palynological and micropalaeontological assemblage data. Wet and unconsolidated green-grey muddy sediments were deposited below the ocean waters, creating reducing conditions and maintaining iron in the green-grey ferrous state. The lack of coherence in 14 C ages in the cores with the most unconsolidated marine section (MD 30-33), and the inability to 201

213 Chapter 6 Discussion GULF Figure 6.7 Marine environment of the Gulf of Carpentaria ka cal BP. The yellow shaded area with associated yellow arrows represents the marine water. The facies diagrams for the relevant cores are shown and the section referred to is circled in red on the facies diagrams. 202

214 Chapter 6 Discussion discriminate deepening water within the marine facies of these cores suggests that the marine samples examined do not necessarily portray discrete time slices. The lacustrine sediments immediately below the marine succession are either of a semi-consolidated black sticky mud, or of a consolidated iron-stained mud (indicating sub-aerial exposure). All cores display a similar pollen composition which characterises the marine phase (Chivas et al., 2001; van der Kaars, pers. comm., 2004): transported woodland, lowland and montane taxa, and mangrove pollen are present, as well as the swamp and grassland taxa that dominate the non-marine phase. Abundant and diverse species of foraminifera characterise the marine conditions, as opposed to the lower diversity assemblages of the non-marine and transitional environments. Ostracods assemblages of most of the cores also show a noticeable change from lacustrine to marine species at this stage (Reeves, 2004). However, the transitional waters of the Gulf of Carpentaria may not have been fully marine until the sea-level rose over the Torres Strait Sill (presently 12m deep) and circulation between the open oceans to the east (Pacific Ocean) and west (Indian Ocean) could occur. Foraminiferal assemblage data show that in some cores (MD 28, 29 and 33) marine fauna took some time to colonise the waters after the return to marine conditions. In these cores the marine assemblage, although diverse, is dominated by one or two species (e.g. Elphidium spp., Ammonia beccarii, Helenina anderseni, Pararotalia spp.) at the base of the facies, becoming proportionally more evenly distributed later in the facies. The initial input of marine water possibly flowed towards the southeastern section, following gravity to pool at the deepest area, and setting an anti-clockwise circulation in motion. By at least 8.7ka cal BP, a recognisably homogeneous and distinct marine foraminiferal assemblage was established throughout the Gulf of Carpentaria. 203

215 Chapter 6 Discussion 6.4 Implications for climate ~35-23ka cal BP At the beginning of this facies Lake Carpentaria was at its maximum extent, at least 300km in width and 500km in length, giving a total surface area of 150,000km 2. A contraction of the brackish lake waters from 58m bpsl to 63m bpsl (from 12m to 7m water depth at the lake s deepest point), and associated increase in salinity, began 35±5ka cal BP and continued until around 23ka cal BP. Both cores MD 28 and 29 have a similar brackish lake facies (with foraminifers Ammonia beccarii and Helenina anderseni dominant) at the base of the studied section, with core MD 29 displaying a lake edge facies with a shallowing of waters to periods of sub-aerial exposure. High precipitation during MIS 3, around 40ka cal BP, is recorded from northern and inland Australia (Kershaw, 1992; Magee et al., 1995; devogel et al., 2004). The large size of Lake Carpentaria and its brackish composition support evidence of a climate high in precipitation. However, a reduction in summer precipitation is noted from 46ka cal BP by van der Kaars and De Deckker (2002) from the North West Cape of WA, with even drier conditions beginning around 35ka cal BP. In Wanda Swamp, Indonesia, a shift in climate possibly heralding cooler, drier and more seasonal conditions, is noted around 35ka cal BP (Hope, 2001). Other sites across Australia show a reduction in precipitation generally beginning around 30ka cal BP (Hesse et al., 2004). The Gulf of Carpentaria core GC 2 provides evidence of a wetter climate before 26ka BP (uncalibrated 14 C), with an increase in salinity at 23ka BP (uncalibrated 14 C, Torgersen et al., 1988). The present study from the Gulf of Carpentaria, although not very well constrained chronologically, supports this evidence of high precipitation before 35ka cal BP, with a climate drying towards the glacial period. 204

216 Chapter 6 Discussion ka cal BP Cores MD 28 and MD 29 were sub-aerially exposed for part of this facies, indicating that at its lowest level the lake was around the -63m contour (maximum 7m deep and about 60,000km 2 in area), although it was not at this low level for the duration of the entire facies. Dominant Helenina anderseni in the foraminiferal assemblages indicates a higher salinity than the previous brackish lake. A drier environment persisted in the tropical north of Australia (Bowler and Wasson, 1984; Kershaw, 1978), and it is thought monsoonal rains were at their minima in Australia (e.g. Bowler, 1978). However, there is evidence that a monsoonal regime may have been still active (Nott and Price, 1999; English et al., 2001). Low water levels for Lake Carpentaria support the general view of a drier LGM. However, sustained rainfall in the catchment area was obviously necessary to maintain the 60,000km 2 lake. The peak in aeolian quartz grains in MD 33 most likely corresponds to the dust maxima at 21.5ka cal BP for the Gulf of Carpentaria core GC 2 reported by De Deckker (2001), indicating maximum aridity (and possibly also increased winds) at this time ka cal BP With lake-levels around the -62m contour (8m maximum water depth and ~70,000km 2 in surface area), an algal boom occurred, and monospecific populations of bivalves, foraminifer and ostracods dominated the lake, creating a shell layer in the sediments. Cores MD 28, 32 and 33 display this shell layer which has been dated to ka cal BP. The cores reveal a noticeable change in sediment characteristics from darker sediments to lighter, more oxidised sediments (shallower or experiencing greater circulation) during the deposition of the shell layer. Coupled with the evidence of greater fluvial input (e.g. the expansion of the lake), greater circulation is signified. During deposition of the shell layer, the saline Lake Carpentaria was freshening, and after the shell layer the waters had freshened and expanded to around the -59m contour. Cores MD 30 and MD 31 display a lake-edge facies, indicating the lake margin fluctuated around their position 59m bpsl. Ostracod and charophyte species indicative of fresher waters occur in cores MD (Reeves, 2004; 205

217 Chapter 6 Discussion Garcia, pers. comm., 2005). An increase in stream input is indicated by an increase in reworked microfossil forms in the deeper areas of the lake and an increase in well-preserved forms in the more marginal areas. The algal bloom (and associated foraminiferal, ostracod and bivalve population explosion) at 18.8ka cal BP in Lake Carpentaria also suggests a warming climate in the region, with a freshening of the lake waters due to increased precipitation. At this time, the climate was emerging out of the cooler and drier LGM (e.g. the termination of the LGM is dated at 19ka cal BP, Yokoyama et al., 2000). At 18.8ka cal BP Lake Carpentaria was beginning to fill after its lowest level in the studied period, although it took a few thousand years to noticeably expand in area. Coming out of the LGM, the Lake Lewis basin in central Australia records a marked increase in fluvial activity around 19ka cal BP (English et al., 2001); this is only obvious after 17ka cal BP in Lake Carpentaria. However, Hesse et al. (2004) state the post-glacial increase in precipitation was accompanied by a similar increase in evaporation, which may explain why Lake Carpentaria only expanded slightly. The dust peaks found in the Gulf of Carpentaria at 19.3ka cal BP, 15.8ka cal BP and 13.3ka cal BP by De Deckker (2001) are not discernable in this study. An indication of climatic fluctuations and increased precipitation is noted immediately after the shell layer, around 15±2ka cal BP. The lake-level increased to around the -59m contour (11m maximum water depth, and about 120,000km 2 in area). Seiches, indicating a possible increase in storminess occur within MD 29. An increase in Typha pollen in MD 32 and MD 33 signals an increase in wetland habitat, possibly due to annual flooding of low-lying areas. The alternate dominance of ostracod species Ilyocypris and Cyprideis also points to alternate wet/dry periods (Reeves, 2004). Lighter-coloured laminations in MD 33 may have been due to episodic flooding, transporting oxidised sediment to this section closest to the influence of present-day Australian streams. These signals of a changing climate may represent either the seasonal droughts and floods of a monsoonal regime (e.g. Nanson et al., 1991) or inter-decadal fluctuations such as ENSO, which are documented to have still occurred throughout the LGM and post-glacial period, although at 206

218 Chapter 6 Discussion reduced levels (e.g. Tudhope et al., 2001). The onset of the Walker Circulation around 16ka cal BP (Haberle and Ledru, 2001), coupled with El Niño teleconnections, may have increased the amount and variability of precipitation in the region. Alluvial flood deposits in northern Australian rivers lead Wyrwoll and Miller (2001) and Hesse et al. (2004) to argue that the Australian summer monsoon was initiated 14ka cal BP. There is also evidence that palaeoflooding increased in tropical regions around 13ka BP (Thomas and Thorpe, 1995). Hesse et al. (2004) note that the onset of the monsoon in the entire Australian region can only be constrained to 15-13ka cal BP. The record from the Gulf of Carpentaria supports these findings of increased precipitation and variability ka cal BP At approximately 12.7ka cal BP Lake Carpentaria again reached its maximum extent within the studied period, expanding out to the -58m contour (12m maximum water depth and 150,000km 2 surface area) until the transition to marine conditions began 12.2ka cal BP. The Younger Dryas cold and dry period (13-11ka cal BP), although apparent in the Northern Hemisphere, is not clearly evident in Australia and the region (e.g. Gagan et al., 2004; De Deckker, 2001). Indication of an arid period around 13-12ka cal BP is lacking in Lake Carpentaria, and increased precipitation is noted around 12.7ka cal BP. Pollen studies of Indonesian and Australian lakes indicate regionally low levels from around 30ka to 13ka BP, with the establishment of fairly stable, higher, lake levels around 13-12ka BP (Kershaw, 1986; Dam et al., 2001; Hope, 2001). The inundation of the Sunda shelf to the north around 12ka BP (Gingele et al., 2002) provided a larger area for atmospheric convection which may be reflected in the increased precipitation over the catchment area of Lake Carpentaria. The increase in precipitation around Lake Carpentaria from 18.8ka cal BP was sustained until the record is lost with the transition to marine conditions. 207

219 Chapter 6 Discussion 6.5 Implications for oceans ~35ka cal BP Around 35±5ka cal BP an event is postulated which deposited well-preserved marine foraminifers (benthic and planktonic forms) into Lake Carpentaria. This event is found at 145cm in core MD 29, situated on the north-eastern margin of the lake. A locally catastrophic climatic event such as a large cyclone or storm surge may have dumped a large quantity of marine waters and associated fauna into the lake. Alternatively, a brief rise in sea-level may have lead to infiltration of marine water into the lake via a channel in the Arafura Sill. Jones and Torgersen (1988) document channels in the Arafura Sill cutting down to 62m bpsl during this time, while deeper channels at 75m bpsl are noted as probably older than the base of their core (34ka BP). The assemblage does not appear to be a transported thanatocoensis, indicating a saline body of water was at least briefly sustained, and favouring the hypothesis of a higher sealevel. A marine influence in the form of sea-level around the height of the channels in the Arafura Sill is indicated. This record corresponds with isotopic evidence of a brief incursion of marine waters into the Gulf prior to 35ka BP (uncalibrated 14 C, McCulloch et al., 1998). Present models of the highest possible sea-level around 40-30ka BP range from around -60m up to -100m and fluctuations of sea-level around the height of the channels in the Arafura Sill are likely to have occurred (e.g. Fig. 6.8). Relative sea-level is shown as the authors assume RSL and eustatic sea-level to be very similar in areas distal from the ice-loaded continental shelves. To account for the sustained marine influence in Lake Carpentaria the model calculating a sea-level of around 65-60m bpsl at 35±5ka cal BP is favoured. At this level, marine water would infiltrate into the lake through the 62m deep channels in the Arafura Sill during extreme climatic events and/or during fluctuations in sea-level of a few metres. However, deeper channels in the Arafura Sill cutting down to 75m bpsl did exist in the last glacial cycle (Jones and Torgersen, 1988), and may have still been deep enough to connect Lake Carpentaria to the ocean at this stage, so a lower sea-level around 75m bpsl cannot be discounted. 208

220 Chapter 6 Discussion Figure 6.8 U-Th-dated relative sea-level estimates (from Waelbroeck et al., 2002). The red line indicates the height of the channels in the Arafura Sill during the last glacial cycle ka cal BP Marine waters do not influence Lake Carpentaria after the initial occurrence around 35ka cal BP until 12.7ka cal BP, indicating sea-level throughout the period below the -62m (or deeper) channels in the Arafura Sill. The Gulf of Carpentaria data are consistent with accepted sea-level models at this time in that no major fluctuations are noted. However, there is some disagreement with previous studies of the gulf, with microfaunal and isotopic evidence of the transition to marine conditions beginning around 14ka cal BP (Torgersen et al., 1988; De Deckker et al., 1988; McCulloch et al., 1989). More accurate modern dating techniques may produce a large enough difference to sufficiently explain the discrepancy in timing. 209

221 Chapter 6 Discussion ka cal BP At 12.7ka cal BP the cores closest to the Arafura Sill (MD 29, 30 and 31) record the presence of rare, fairly well-preserved marine species (including planktonic forms in MD 31). A sea-level rise to around 60m bpsl is indicated at this time, permitting occasional influx of marine waters into the brackish lake through channels in the Arafura Sill (such as associated with extreme storm events and/or king tides). A relative sea-level of 60m bpsl at 12.7ka cal BP is within the range of current models. The sea-level curve of Yokoyama et al. (2001a) is shown for comparison (Fig. 6.9) as it provides more detail for the relevant time period than Waelbroeck et al. (2002) who also utilise the data of Yokoyama et al. (2001a). Figure 6.9 Estimated eustatic sea-level, modified from Yokoyama et al. (2001a). The red lines indicate the height of the Arafura Sill and associated channels. 210

222 Chapter 6 Discussion A shell hash layer in these western-most cores (MD 29, 30 and 31), dated at 12.4ka cal BP, and associated with ooids, glaucony and echinoid spines, indicates sea-level at around lake levels (58m bpsl), allowing tidal exchange between Lake Carpentaria and the ocean. Therefore, sea-level rose at a rate of about 7mm yr -1 between ka cal BP. The transition to marine environments began around 12.2ka cal BP in the Gulf of Carpentaria, seen in the choked and closed lagoonal foraminiferal assemblages overlying the lacustrine facies, and the death of the fresh-tobrackish water algal population. At this stage sea-level was higher than lakelevel, and a continued influx of marine waters entered the lake. Sea-level was 57-56m bpsl. In the period from 12.4ka cal BP to 12.2ka cal BP sea-level rose a metre or two, at a rate of around 5-10mm yr -1. The rise in sea-level from 60m bpsl at 12.7ka cal BP, to 58m bpsl at 12.4ka cal BP and around 57-56m bpsl at 12.2ka cal BP is in accord with currently accepted sea-level data (Fig. 6.9). At 10.5ka cal BP sea-level rose over the -53m Arafura Sill, and as it flooded into the Carpentaria Basin it transported some underlying lacustrine sediments with it (a few centimetres of sediment), and eroded some areas (most notably around core MD 28, closest to the Arafura Sill). From 12.2ka cal BP to 10.5ka cal BP sea-level appears to have risen only 3-5 metres, a rate of only 2-3 millimetres a year. Various authors have noted a plateau in the rate of sea-level rise around the Younger Dryas (e.g ka cal BP, Yokoyama et al., 2001b; ka cal BP, Lambeck et al., 2002). No rapid rise in sea-level such as might be expected from meltwater pulse 1B (Fairbanks, 1989) can be inferred from this study. The transition to marine conditions in the Gulf of Carpentaria limits the sea-level to 53-52m bpsl around 10.5ka cal BP. This is slightly later than determined by Yokoyama et al. (2001a). 211

223 Chapter 6 Discussion 6.6 Summary The palaeoenvironmental history of the Gulf of Carpentaria documents the presence of a lake (from the beginning of the studied section 35±5ka cal BP) until the transition to marine conditions at 10.5ka cal BP (Fig. 6.10). Initially the lake may have been brackish and quite large, at least around the -58m contour (i.e. 12m deep at its deepest section, and over 150,000km 2 in surface area). Saline waters are noted around the time of the Last Glacial Maximum as the lower levels of precipitation caused the lake to contract (to the -63m contour) and concentrate solutes, increasing the salinity. Abundant monospecific populations of bivalves, foraminifera, and ostracods are noted for the interval dated ka cal BP (with lake levels still low, around the -62m contour), associated with increased precipitation after the LGM. The lake continued to freshen and expand to the -59m contour by around 15ka cal BP. By 12.7ka cal BP Lake Carpentaria was at its maximum extent (around the -58m contour, 12m maximum water depth and 150,000km 2 in surface area). Figure 6.10 Estimation of the varying level of Lake Carpentaria throughout its existence in the studied period. This estimation is only possible in the lacustrine phase (L) before 12.2.ka cal BP. The record is lost with the transition (T) to lagoonal conditions at 12.2ka cal BP, and no record is possible during the marine phase (M) after 10.5ka cal BP. 212

224 Chapter 6 Discussion Estimates of sea-level during MIS 3 are indefinite, ranging from -60 to -100m (e.g. Fig. 6.9). The Gulf of Carpentaria record is similarly uncertain, however, a sea-level in the upper range of estimates is indicated (65-60m bpsl, perhaps as low as 75m bpsl) between 40-30ka cal BP (Fig. 6.11). No modelled sea-level for 40-30ka cal BP is shown in Figure 6.11 for comparison, as the fluctuations and error margins would obscure the Gulf of Carpentaria data. A gradual increase in marine influence is noted in the lake from 12.7ka cal BP, constraining sea-level models to a height of -60m at 12.7ka cal BP, -58m at 12.4ka cal BP, and 57-56m bpsl leading to a transitional estuarine facies at 12.2ka cal BP (Fig. 6.11). These data support existing models. The overtopping of the Arafura Sill (i.e. sea-level 53-52m bpsl) is dated at 10.5ka cal BP, which appears a little late according to Yokoyama et al. (2001a). A slower rate of rise, perhaps related to the Younger Dryas, may be indicated. Figure 6.11 Estimation of relative sea-level from the Gulf of Carpentaria cores, overlain on the eustatic sea-level curve of Yokoyama et al. (2001a). The black bars represent the depth and age range of a very approximate record, while the circles indicate better constrained estimates. 213

225 Chapter 6 Discussion True marine foraminiferal assemblages in the north and eastern Gulf were established by 8.7ka cal BP, possibly only after Torres Strait was also submerged and circulation through the Gulf of Carpentaria could occur. The study of the palaeoenvironments of the Gulf of Carpentaria adds to the existing record, and opens to question some present models of climate and sea-level change from the Last Glacial Maximum to the present. The existence of the large (up to 12m deep) Lake Carpentaria before the LGM supports the existence of a strong Walker Circulation in the region enhancing precipitation. Evidence of sea-level around 65-60m bpsl during MIS 3 is on the high end of levels estimated by other authors. The contraction of Lake Carpentaria to around the -63m contour beginning after 35ka cal BP, and at its lowest levels between 23-18ka cal BP, adds to the evidence of a drier LGM around northern Australia, although with sufficient precipitation in the area to support the existence of a lake 60,000km 2. The freshening of Lake Carpentaria at 18.8ka cal BP coincides with many previous studies showing a general increase in precipitation for the region associated with the termination of the LGM. The fact that Lake Carpentaria did not noticeably expand until around 15ka cal BP suggests that effective precipitation stayed low between ka cal BP. A variability in rainfall is also noted in the climatic regime during this change to a higher effective precipitation ~15ka cal BP. The lake was near its maximum extent from around 15ka cal BP until the transition to marine conditions around 10.5ka cal BP, adding to the evidence of increased precipitation in the region, perhaps from a return of the Australian summer monsoon. Sea-level data from 12.7ka cal BP (-60m), 12.4ka cal BP (-58m) and 12.2 ka cal BP (~57m) strongly supports the model of Yokoyama et al. (2001a). The elusive southern hemisphere record of the Younger Dryas may be seen in the slow rate of sealevel rise from ka cal BP. In addition, the establishment of true marine foraminiferal assemblages in the north and eastern sites a few thousand years after the marine transgression over the Arafura Sill (only when Torres Strait was also flooded) may reflect the importance of water circulation patterns to foraminiferal habitats. 214

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