Neogene oceanographic and climatic changes in the northern Indian Ocean: Evidence from Radiolaria

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1 Indian Journal of Marine Sciences Vol. 36(4), December 2007, pp Neogene oceanographic and climatic changes in the northern Indian Ocean: Evidence from Radiolaria *V. Sharma & L. Bhagyapati Devi Department of Geology, University of Delhi, Delhi , India *[ ] Neogene is one of the important periods in the earth s history in view of significant changes that took place during the period in climatic and oceanographic conditions. Radiolarians from well preserved cores, uplifted marine sequences, and surface sediments from the northern Indian Ocean provided opportunity to reconstruct the Neogene oceanographic and climatic events. Studies based on radiolarians from land-based sections revealed episodes of cooling and warming in the late Early Miocene to Middle Miocene. Investigations of radiolarian upwelling fauna in the cores from the western Arabian sea demonstrated strengthening and weakening of upwelling during late Middle Miocene to Recent. Examination of radiolarians from surface sediments suggested presence of Antarctic Bottom water in the Mozambique and Madagascar basins of the western Indian Ocean. Certain radiolarian species are found useful in understanding paleomonsoonal changes and have been used to interpret such changes. [Key words: Radiolaria, Indian Ocean, Neogene, paleoceanography, paleoclimate, climatic changes, oceanographic changes, biochronology, circulation, upwelling, paleotemperature, monsoonal changes] Introduction : The last about forty years have witnessed outburst of valuable information on Neogene oceanographic and climatic changes based on calcareous and siliceous microfossils like foraminifera, calcareous nannoplankton, radiolarians and diatoms. Usefulness of radiolarians, which were inadequately understood earlier, had been realized with the advent of Deep Sea Drilling Project (DSDP) in 1968, wherein study of radiolarians in drill cores from different ocean basins provided enormous information on morphology, evolutionary changes, stratigraphic ranges (and hence, in biostratigraphy) and oceanic and climatic history. Radiolarian studies were further augmented by the availability of deep-sea cores under the Ocean Drilling Programme (ODP) which started in Studies of land-based sequences also added useful information on the applied aspects of radiolarians. Neogene has been one of the most significant periods in the Earth s history. Besides major changes in flora and fauna, development of permanent icesheets on the Antarctica 1, 2, establishment of flow of oceanic current across the Drake passage 3 (in the earliest Neogene), final phases in the uplift of the *Corresponding author: Ph: Fax: Himalaya, closure of the Indonesian seaway (12-11 Ma) 4, 5 and onset of Asian monsoon 6 are important events. These events influenced climate and oceanic circulation whose signatures are preserved in the radiolarian faunal assemblages in the Neogene sediments recovered from deep sea cores and land based sections in the northern Indian Ocean. The Pacific Ocean accounts for the largest number of publications on ocean climate history based on radiolarians from surface sediments and deep sea cores. The vast geographic expanse of the Pacific, both longitudinally and latitudinally, gives rise to varied oceanographic and climatic conditions, fauna and flora, as a result of which it possibly attracted a large number of investigators. Contrary to this, the Indian Ocean is areally small and surrounded on three sides in the northern part by land masses. This configuration results in not so varied oceanographic conditions as are found in the Pacific or Atlantic. Radiolaria in ocean climate studies Radiolaria are, like many other microfossil groups, equally potential in oceanic and climatic studies through the Neogene. These studies include oceanic surface and deep circulation, upwelling, thermal and monsoonal changes. Radiolarians in sediments (surface sediments or in deep sea cores) can be used to unravel these phenomena. Being planktic,

2 362 INDIAN J. MAR. SCI., VOL. 36, NO. 4, DECEMBER 2007 radiolarians can be found in sediments from shallow to greatest depths of the oceans. However, degree of dissolution of radiolarian skeletons may decide their abundance in the surface sediments. Normally, vertical profile of sea shows that silica content in sea water progressively increases with depth and it is only in the upper about 1000 m of seawater that maximum dissolution of radiolarian tests takes place and the rate of dissolution decreases with increasing depth 7. Dissolution in deeper parts may also take place due to presence of deep or bottom water silica-deficient currents, and thus, radiolarian preservation in surface sediments may have a direct relationship with bottom water characteristics. The bottom water currents may also transport radiolarian tests to far off sites and mix them with assemblages of different nature. In the northern Indian Ocean, studies related to radiolarian distribution in water column and surface sediments provide a fairly good understanding of their biogeography Such studies presented insight into the relationship between water mass and radiolarian distribution and are of immense value in understanding Neogene paleoceanography and paleoclimate of the Indian Ocean. Distribution of modern radiolarian species in the world oceans (including the Indian Ocean) based on radiolarian census data from core top samples is useful in understanding role of oceanic parameters in their distribution 13. Status of radiolarian studies in the Indian Ocean region For Neogene oceanographic and climatic studies in the northern Indian Ocean, besides investigations of radiolarians from surface sediments and sequences from drill sites, uplifted marine sequences exposed on various islands and on land areas bordering the ocean are equally important. Records of occurrences of radiolarians have been known from the Indian Ocean for over 150 years from the Neogene rocks (of Andaman-Nocobar) and surface sediments 14, 15. Radiolarian studies in the Indian Ocean gained impetus after the availability of material recovered from cores and surface sediments. These provided a better understanding of temporal and spatial distribution, morphology and evolution of radiolarian species. The outcome was availability of a more refined radiolarian biostratigraphic classification. In the Indian Ocean region, a number of publications on Neogene radiolarians are based on DSDP and ODP cores and land-based sections of Andaman Nicobar and Indonesia. A few of these are preliminary reports while others pertain to documentation of taxa, biostratigraphic, paleoclimatic and paleoceanographic aspects. Many of these studies involved other microfossil groups as well besides radiolarians. Studies related to oceanic and climatic aspects are discussed in the subsequent sections. Radiolarian biochronology and biostratigraphy Biostratigraphy is the fundamental requisite in any microfossil-based study so that studied samples/sequences could be precisely placed in geological time. Biochronological events further enhance the accuracy. Continuous refinements in Cenozoic zonation have taken place in radiolarian biostratigraphy. Updating of low latitude zonal scheme 16, 17, and particularly those applicable to the tropical Indian Ocean 18-20, resulted in better understanding of biostratigraphic zones, radiolarian events and ages. Recent advances 21 in radiolarian biostratigraphy integrating magnetostratigraphy have resulted in precise dating of geological events in the equatorial Pacific. Radiolaria and Paleoceanography Surface and deep water circulation Presence of Antarctic Bottom Water (AABW) currents is indicated by radiolarian assemblages from the surface sediments (depth range about m) of the tropical-subtropical western Indian Ocean. (Fig. 1). Extremely poor radiolarian content in the samples (depth range m) from the western part of the ocean (Mozambique and Madagascar basins) is ascribed to AABW 22. The circulation of bottom water, which is rich in oxygen and low in silica content with source in the Antarctic surface water, has been recognized in the Mozambique 23, 24 and Madagascar basins at depths greater than m. Being deficient in silica, AABW has corrosive effect on the siliceous radiolarian skeletons. Johnson & Nigrini 10 made similar observations wherein poor preservation of radiolarians in surface sediments east of Madagascar was ascribed to the corrosive effect of polar bottom water. In the Central Indian Basin, however, all studied samples are from depths greater than 5000 m and contain abundant radiolarians 22. In this basin Warren 23, 24 demonstrated a two layer structure. The upper deep water ( m) flows directly from

3 SHARMA & DEVI: NEOGENE CHANGES IN INDIAN OCEAN 363 Fig. 1 Location of surface sediment samples in the northern Indian Ocean used for radiolarian study (adopted from Sharma & Mahapatra 22 ). the Antarctic along the eastern flank of the Central Indian Ridge and its features diminish eastward across the basin 25. The lower deep water (>3800 m) has a different source. It is derived from the boundary current in the West Australian Basin and flows across deep saddles (particularly at 10 S latitude and possibly at 5 S latitude) on the Ninetyeast Ridge. It has relatively low silica content 26 but seems to have either very little or practically no effect in dissolving radiolarians in the Central Indian Basin 22, because the basin itself, particularly towards the north, forms a source of high silica in the deeper parts 25, 26. Existence of AABW current is suggested in the Central Indian Basin by determining various parameters in the surface samples from the basin, including organic carbon and number of radiolarian shells/gram dry sediments 27. Low organic carbon due to oxidation is attributed to the presence of oxygenrich AABW 27 which enters into the basin through the deep saddles of 90 E Ridge after flowing northward on the east side of the ridge 28. Nassellaria/Spumellaria (N/S) ratio, whose higher values coincide with the entrance of Antarctic Bottom Water current in the eastern side of the basin could be a good index for oxygen content of the water within the Central Indian Basin 27. Study of radiolarians in the early Neogene sequences from Andaman-Nicobar Islands spanning from Stichocorys wolffii Zone to Dorcadospyris alata Zone revealed oceanographic changes during late Early Miocene-Middle Miocene 29. A warmer surface water condition is interpreted during the interval represented by Stichocorys wolffii and early Calocycletta costata zones (late Early Miocene). Dominance of cold water immigrant fauna in the upper part of the Calocycletta costata and Dorcadospyris alata zones (Middle Miocene) indicates cooling of water (Fig. 2). Presence of cold water species in the assemblage is possibly due to their northward transport by surface currents as a result of expansion of temperate waters towards the tropics. This interval is also marked by increased

4 364 INDIAN J. MAR. SCI., VOL. 36, NO. 4, DECEMBER 2007 Fig. 2 Cumulative percentage of constituent species in the samples of each of the four radiolarian assemblages (1-4), percentage of cold water species and radiolarian and foraminiferal zones identified in the Early Neogene sequences of Andaman Nicobar. The evolutionary appearance of D. alata, which marks the lower boundary of D. alata Zone, is not recognized and hence the boundary is marked tentatively on morphotypic first appearance of D. alata. Age:*=Berggren et al. 50, ** = Nigrini et al., 21 (modified from Sharma & Daneshian 29 ).

5 SHARMA & DEVI: NEOGENE CHANGES IN INDIAN OCEAN 365 abundance of forms belonging to spongodiscids which are deep living and their increased abundance indicates cold water or upwelling 30. The warmer episode corresponds to that observed at tropical Indian ocean DSDP sites 216, 237 and 238 where a decreasing trend in δ 18 O values was recognized in both deep and shallow water planktic foraminifera 31 during the interval represented by the Stichocorys wolffii Zone. Cooling in the Middle Miocene is revealed by oxygen isotopic data by a positive shift in planktic foraminifera δ 18 O values 31 in the interval spanning from foraminiferal zones N7 to N13 in the Indian Ocean equatorial site 216. These zones correspond to radiolarian Calocycletta costata and Dorcadospyris alata zones (Fig. 2). Planktic foraminiferal assemblages from site 216 too indicate incursion of cool surface water in the northern Indian Ocean during the Middle Miocene 32. Oxygen isotopic records of both shallow water planktic foraminifera and benthic foraminifera at other equatorial sites, like ODP sites 709 and 758 show cooling in the Middle Miocene. At site 709, the cooling is recorded 33 in the interval from upper part of planktic foraminiferal zone N7 to within undifferentiated zones N8 to N12 which corresponds to the interval from the upper Calocycletta costata to Dorcadospyris alata zones. Initiation of a warmer condition in the upper Dorcadospyris alata Zone is recognized by the dominance of warm water assemblages and a declining trend upwards in the percentage of coldwater species 29. Warming of surface water is also indicated by a decreasing δ 18 O values in shallow water planktic foraminifera at site 758 from Zone N12 upwards 33. Upwelling In the western Indian Ocean, upwelling has been reported off Oman (Arabian Peninsula) and Somalia and along the west coast of India The upwelling phenomenon off Oman and Somalia is the result of southwesterly monsoon winds and is seen during the summer monsoon. The radiolarian assemblages of upwelling zones are distinct from those in the surrounding areas. Recognition of upwelling assemblage in the recent surface sediments is key to decipher upwelling in the region as well as its intensity in the past. The first report on radiolarian assemblages related to upwelling seems to be by Johnson & Nigrini 10, wherein they identified upwelling fauna in the surface sediments east of Arabian Peninsula. Three categories of upwelling species are recognized in cores (late Middle Miocene to Recent) off Oman and in a core (late Pleistocene to Recent) off Somalia These included (a) endemic upwelling species, (b) displaced middle latitude species and (c) enhanced tropical species. Presence of many of the forms similar to the upwelling assemblages belonging to the three categories in the northwestern Arabian Sea and in the upwelling area off Peru margin indicates that the assemblage may be globally diagnostic of upwelling conditions and that by its unique nature of assemblage, upwelling can be recognized 38, 40. Further, species belonging to these categories may be recognized in all upwelling areas though all the elements of assemblage need not be present and percent occurrence of the constituent species may vary 40. Clearly, recognition of such assemblages in the ancient sediments is suggestive of history of upwelling in the past. Fluctuation in the intensity of upwelling mechanism, like strengthening and weakening, can be interpreted by the change in the characteristic upwelling faunal assemblage. The study of upwelling fauna in drill cores off Oman revealed strengthening of the upwelling mechanism in the early Pliocene, at about 4.7 Ma and to a lesser degree near Pliocene-Pleistocene boundary at about 1.5 Ma, and end of an upwelling pulse in the late Miocene at about 9.6 Ma which probably started at about 11.9 Ma 38. Evidence of upwelling and cooling are recognized in the radiolarian assemblage of late Middle Miocene diatomites of Nias Island located west of northern Sumatra by the occurrence of radiolarian species which are generally associated with cold water habitat 41. Radiolaria and Paleoclimate Paleotemperature Most paleoclimatic studies involve determination of paleotemperature and paleomonsoonal changes. Microfossil - based transfer functions are good tools to estimate past temperatures. Radiolarian based transfer function for the southeast Indian Ocean (about 40 S to 65 S latitude) 42 has been successfully used on radiolarian samples from deep-sea surface sediments in the Indian Ocean between about 22 S and 59 S latitudes to estimate sea surface temperatures which compared well with the observed summer and winter sea-surface temperatures of the present day 12.

6 366 INDIAN J. MAR. SCI., VOL. 36, NO. 4, DECEMBER 2007 Estimation of changes in sea surface temperatures (SSTs) over a long duration has been attempted for the Central Indian Ocean 43 which is a warm pool charecterized by SSTs >27 C for the last about hundred years. Employing transfer function, radiolarians from a deep sea sediment core from the central tropical Indian Ocean, corresponding to Brunhes-Matuyama were analysed which suggested that summer sea surface temperatures (SSTs) of the tropical Indian Ocean warm pool were most of the time >27 C for the last 1.4 Ma and fluctuated between C C, implying presence of the warm pool throughout the Quaternary 43. This study documented for the first time the tropical radiolarian based SST cycles at 400,000, 126,000 and 95,000 year due to the triplet periodicities in the earth s orbital eccentricity 43. Monsoonal changes A number of studies exist on interrelationship among monsoonal changes, Earth s orbital eccentricity, radiolarian assemblages and oceanic parameters in the northern Indian Ocean 43, Suprageneric radiolarian transfer function was used to decipher late Miocene paleomonsoonal changes in the northeastern tropical Indian Ocean 44. The result showed that SW monsoon was stronger during warmer periods between Ma and identified four warmer humid peaks associated with salinity minima during this period 44. Pyloniids/Spongodiscids ratio in the radiolarian assemblage in surface sediments from the tropical Indian Ocean is found useful in understanding monsoonal fluctuations. Surface distribution of the radiolarian factors indicates presence of three well defined water masses overlying the Central Indian Basin during southwest monsoon 46. The first factor (Pyloniid assemblage) is characterized by high salinity, cooler temperature and high potential primary productivity, while the second factor (Spongodiscids assemblage) is characterized by lower salinity, higher temperature and low potential primary productivity. The ratio Pyloniids/Spongodiscids is a salinity index reflecting monsoonal changes and hence monsoonal variation can be understood by finding variations in Pyloniids and Spongodiscids across the sequence 46. In another study, Pyloniid group is found to respond to astronomical forcing like solar radiation, earth s orbital eccentricity, axial obliquity and precessional perturbations and may be a good index to reconstruct paleomonsoonal changes 47. A close link is found between radiolarian fluxes in the Bay of Bengal and monsoonal precipitation induced variations in SST and salinity 49. Radiolarian fluxes showed a positive correlation with SST and a negative correlation with surface salinity, implying that similar radiolarian assemblages in cores may reveal variation in SST and salinity and hence in revealing monsoonal history in the geological past 49. Future directions In the Indian Ocean both drill core samples and land-based sequences have not been extensively studied from the view point of understanding paleoclimatic and paleoceanographic changes based on radiolarians. A more comprehensive study is required which should incorporate not only microfossil data but data from isotopic and paleomagnetic analyses, sedimentology and tectonic history of the region for precise dating and knowledge regarding initiation and past fluctuations in the monsoon system and for prediction of monsoonal changes in the future and climatic and oceanographic history in the geological past. Acknowledgement The authors are thankful to Dr. M.S Srinivasan, Professor Emeritus, Department of Geology, Banaras Hindu University, Varanasi for useful suggestions. Ms Shilpa Bajpai helped in various ways during the preparation of the manuscript. References 1 Shackleton N J & Kennett J P, Paleotemperature history of the Cenozoic and the initiation of Antarctic glaciation: oxygen and carbon isotope analyses in DSDP sites 277, 279 and 281. Init. Repts. DSDP., 29, (U.S. Govt. Printing Office, Washington, D.C.) 1975, pp Kennett J P, Cenozoic evolution of Antarctic glaciation, the Circum-Antarctic ocean, and their impact on global paleoceanography. J. Geophys. Res., 82 (1977) Kennett J P, Neogene paleoceanography and plankton evolution, Suid-Afrikaanse Tydskrif vir Wetenskap., 81 (1983) Kennett J P, Keller G & Srinivasan M S, Miocene planktonic foraminiferal biogeography and paleoceanographic development of the Indo-Pacific region, Mem. Geol. Soc. Amer., 263 (1985) Srinivasan, M. S. & Sinha, D. K., Early Pliocene closing of the Indonesian seaway: Evidence from the northeast Indian Ocean and Southweast Pacific deep sea cores. J. Southeast Asian Earth Sci., 16 (1998) Srinivasan M S & Sinha D K, Late Neogene ocean circulation changes in the tropical Indo-Pacific and evolution of the Asian Monsoon. In: Proc. 8 th International Congress on Pacific Neogene stratigraphy: Pacific paleoenvironments

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