Natural dissolution of modern shallow water benthic foraminifera: taphonomic effects on the palaeoecological record

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1 ELSEVIER Palaeogeography, Palaeoclimatology, Palaeoecology 146 (1999) Natural dissolution of modern shallow water benthic foraminifera: taphonomic effects on the palaeoecological record John W. Murray a,ł, Elisabeth Alve b a School of Ocean and Earth Science, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK b Department of Geology, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway Received 15 October 1997; revised version received 5 May 1998; accepted 29 May 1998 Abstract Comparison of the living and dead assemblages of benthic foraminifera from the coastal areas of the Skagerrak Kattegat reveals that, although there are considerable similarities in terms of the presence=absence of species, there are major differences in relative abundance at species level. Furthermore, the relative abundance of calcareous tests in the living assemblages is considerably higher than that of the dead assemblages (average values of 67 and 30%, respectively). Since the environments are microtidal, there is little transport of foraminiferal tests due to tidal currents so there are no introduced exotic species although there is local transport caused by waves and wave-induced currents. However, the major taphonomic process appears to be dissolution, of calcareous tests. This is manifested through etching and breakage of test walls, leading to complete decalcification which leaves a residue of organic linings of certain taxa (e.g., Ammonia beccarii), and a marked increase in abundance of agglutinated tests in the dead assemblages. Although dissolution is normal in marsh settings throughout the world, the Skagerrak Kattegat area is unusual in the intensity of dissolution in the subtidal zone. Whereas most of the calcareous species occur throughout the investigated depth range (except marsh), two of the subtidal agglutinate species (Eggerelloides scaber and Ammotium cassis) show upper depth limits. Consequently, the agglutinated foraminifera are better indicators than calcareous foraminifera of upper depth limits in these shallow subtidal environments Elsevier Science B.V. All rights reserved. Keywords: taphonomy; carbonate dissolution; shallow water foraminifera; marsh; sea level; Skagerrak Kattegat 1. Introduction Over a three year period, a baseline ecological study of shallow water (6 m) benthic foraminifera along the Skagerrak Kattegat coasts has been carried out. A principal aim has been to provide data necessary to enhance climatic and environmental interpretations of past (particularly Quaternary) marginal Ł Corresponding author. Tel.: C ; Fax: C ; jwm1@mail.soc.soton.ac.uk marine successions. In order to do this, it is essential to investigate the ecology and distribution of the living assemblages and to determine the taphonomic changes during fossilization. The results for the living assemblages are discussed in Alve and Murray (1999). In the present paper we present detailed results for the dead assemblages and focus particularly on taphonomy and sea level=water depth indicators. A more complete description of the environment is given in Alve and Murray (1999) and only a summary is given here. The Skagerrak Kattegat area /99/$ see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S (98)

2 196 J.W. Murray, E. Alve / Palaeogeography, Palaeoclimatology, Palaeoecology 146 (1999) is characterized by a very small tidal range (<40 cm, but occasionally higher due to variations in atmospheric pressure and temporal wind directions), and thermohaline water stratification, at least in the summer, with little diurnal variation in salinity or temperature. There is ice cover along the Norwegian and Swedish coasts, and sometimes along that of Denmark, during the winter, and a wide annual range of temperature from about 0 to >20ºC. The Swedish and southern coasts of the Kattegat are partly influenced by the brackish surface water outflow from the Baltic whereas the Skagerrak and western Kattegat shorelines are more influenced by water from the North Sea (salinity ). The transparency of the water, due to the low supply of sediment (at least during the summer months) and lack of tidal mixing, allows the development of seagrass meadows which stabilise the sediment surface and provide both food and protection for the benthos. The remains of these plants also contribute to the total organic carbon (TOC) content of the fine-grained sediments. Previous work on the living, dead and=or total foraminiferal assemblages from the shallow inshore has been confined to various parts of Oslo Fjord (Goës, 1894; Kiær, 1900; Christiansen, 1958; Risdal, 1964; Thiede et al., 1981; Alve and Nagy, 1986, 1990; Alve, 1990, 1995) and Swedish estuaries (Olsson, 1973, 1976; Nyholm et al., 1977). There have been no previous foraminiferal studies of the Skagerrak coast of Norway or the Danish Kattegat coast and Limfjord except for a study of the Øresund at the entrance to the Baltic (Hansen, 1965). Baltic shallow water faunas have been studied by Lutze (1965, 1968a,b, 1974), Grabert (1971), Wefer (1976) and Brönnimann et al. (1989). 2. Material and methods From 1994 to 1996, 171 samples were collected from 27 localities along the Skagerrak Kattegat coast from the intertidal zone to a water depth of 6 m (Fig. 1) but two (162, 163) were not used in this study. The majority of samples were collected in the month of July in successive years. The exceptions were samples (Aug.) and (Oct.). The depths recorded are those at the time of sampling. Because of the irregular tidal pattern, it was difficult to know the exact position of average sea level in the investigated areas. However, it was clear that the water level was never very high or very low because it was always within the range of intertidal algal cover on rocks or jetties, i.e., the maximum deviation from mean sea level was in the range of about cm at most stations. For a number of stations from water depths of less than around 0.5 m at the time of sampling, it has not been possible to be certain whether they are normally subtidal or intertidal. Further details of the sampling areas are given in Alve and Murray (1999, table 1). The subtidal foraminiferal samples were collected with a small grab, each with a volume of about 100 cm 3. The grab was carefully operated so that negligible loss of surface sediment took place. On removal from the water, it was gently opened in a bowl. The surface oxidized sediment layer was scooped off and placed in a container. In areas exposed at the time of collection, samples were scraped from the sediment surface (to a depth of 1 2 cm). All samples were preserved in 70% ethanol. In the laboratory, the foraminiferal samples were washed on a 63 µm sieve (large fragments of organic material were removed on a 1 mm sieve), stained for 1 h in rose Bengal (1 g=l), and washed again to remove the surplus stain. Those samples not rich in organic material were dried at 40ºC and the foraminifera were concentrated using tetrachloroethylene or carbon tetrachloride. Organic-rich samples were spread out over a large area in a pan and allowed to dry at 40ºC to avoid forming a hard block. The dry material was then gently brushed off into a container. It was not necessary to float such samples in a heavy liquid. For each sample separate assemblages of living and dead individuals were picked (ideally individuals in each, although some were too small to yield this number). Most Reophax moniliformis tests were fragmented so it was impossible to decide exactly how many individuals were present. In order to reduce over-representation, we decided to count only those specimens with three or more chambers. Another problem is that individuals of Goesella waddensis lacking their initial chambers are impossible to distinguish from R. moniliformis. Consequently, all G. waddensis have been counted together with R. monil-

3 J.W. Murray, E. Alve / Palaeogeography, Palaeoclimatology, Palaeoecology 146 (1999) Fig. 1. Map of the Skagerrak Kattegat with investigated areas shown by numbers from 1 to D Isefjærfjord; 2 D Kvastadkilen; 3 D Dype Holla (Lyngør); 4 D Tøkersfjord (Lyngør); 5 D Hasdalen; 6 D Kilsfjord; 7 D Tjøme; 8 D Borre; 9 D Horten; 10 D Sandebukta; 11 D Bunnefjord; 12 D Hunnebotn; 13 D Hålkedalskilen; 14 D Tjärnö; 15 D Finnsbobukten; 16 D Gullmarsvik; 17 D Hafstensfjord; 18 D Kungsbackafjord; 19 D Jonstorp; 20 D Kildehuse; 21 D Kalundborg; 22 D Vejle Fjord; 23 D Havhuse (Kalø Vig); 24 D Vosnæs Pynt (Kalø Vig); 25 D Løgstør; 26 D Valsted; 27 D Frederikshavn. Long arrows show general surface water circulation in the Skagerrak (after Svansson, 1975). JCW D Jutland Coastal Water; BW D Baltic Water; NCC D Norwegian Coastal Current. iformis and the few instances where we are confident that the former is present are noted below. Ammotium cassis and Ammobaculites balkwilli were also commonly fragmented. We therefore counted only those fragments which included the initial part. Species diversity was calculated for samples with ½100 individuals using the Fisher alpha index (Fisher et al., 1943) and the information function H.S/ (see Murray, 1991, for formula and application). Variants of agglutinated taxa are illustrated in Plate I. The full range of agglutinated taxa are illustrated in Murray and Alve (1999) and the calcareous forms in Alve and Murray (1999). At most stations, additional samples were taken for grain size and total organic carbon (TOC) analysis. Sediment size analysis was carried out using standard sieving techniques, sieved on 63, 125, 250, 500, and 1000 µmmeshes.toc analysesweremade using the Leco combustion method. Water temperature and salinity were measured using either a Salinity Temperature Bridge, type M.C.5, or a thermometer and Atago salinity refractometer.

4 198 J.W. Murray, E. Alve / Palaeogeography, Palaeoclimatology, Palaeoecology 146 (1999) PLATE I PLATE I SEM photographs of selected agglutinated taxa. # refers to the sample number (area number; see Fig. 1) Variants of Ammobaculites balkwilli Haynes. 1, 2. Typical forms, #98 (24) Compressed form, #122 (21) Variants of Reophax moniliformis Siddall Normal form, 6, 7, #64 (17), 8. #122 (21) Irregular form, #31 (1). 14. Deformed individual, #31 (1). 15, 16. Typical Goesella waddensis Van Voorthuysen, showing triserial initial portion, uniserial main part, and terminal circular aperture, #120 (21).

5 J.W. Murray, E. Alve / Palaeogeography, Palaeoclimatology, Palaeoecology 146 (1999) Results The foraminiferal data are given in Appendix A, table A1 (this table has been placed on the WWW as an Online Background Dataset 1 ); fourteen samples yielded fewer than 50 dead individuals and are marked with a star. The living foraminifera were divided into 6 ecological categories by Alve and Murray (1999) and the same pattern is followed here (see Table 1 for a summary). Marshes are treated separately from other intertidal=subtidal areas because of their distinctive faunas Marsh environments Six marsh areas were sampled Borre (area 8), Horten (9), Bunnefjord (11), Hunnebotn (12), Hålkedalskilen (13), and Kalundborg (21) (Fig. 1). With the exception of Borre and Hålkedalskilen, samples were collected from dense Carex acuta marsh with some Salicornia, and thesediment betweenthe plants was covered with green filamentous algae. At Borre (8), there was some green algal cover between the sparse Carex acuta but no Salicornia was present. In addition, at Hunnebotn (12) and Horten (9), samples were also collected from the damp sediment surface beneath a litter layer (dry leaf debris several centimetres thick) among Phragmites australis landward of the Carex acuta marsh. At Hålkedalskilen (13) there were only scattered Salicornia plants with no Phragmites australis or Carex acuta. The marsh sediment median diameter shows a wide range (from phi 0.2 to C4.2; gravel=coarse sand to fine sandy mud). The TOC content of 6 of the 14 marsh samples was analysed and the values vary from 0.4 to 14.8%. The marsh assemblage agglutinated foraminifera are exclusively organo- and ferro-agglutinated (i.e., none with a calcareous cement). The most widely distributed and abundant species are Jadammina macrescens and Miliammina fusca (categories 2 and 3, respectively) and these are dominant in most of the samples (table A1 1 ). They are accompanied by Tiphotrocha comprimata (category 1) or, more rarely, by Trochammina inflata (category 2), and by calcareous 1 To be reached via: mirror site: Ammonia beccarii or Elphidium williamsoni (category 3). However, on the highest marsh at Hunnebotn (area 12, samples 159, 170), the dominant form is Balticammina pseudomacrescens (category 1) which was not found elsewhere. At Kalundborg (area 21) the dominant form was Haplophragmoides wilberti (category 2) and this was rare elsewhere. The unvegetated sediment areas bordering the marsh also sometimes have common marsh taxa. For instance, J. macrescens at Bunnefjord (11) and Hålkedalskilen (13) while at Kalundborg (21) there is H. wilberti, J. macrescens, and T. inflata. At Horten (9), two stations (44, 45) from 0.1 to 1 m have >50% J. macrescens even though there is no marsh immediately adjacent (table A1 1 ) Intertidal subtidal The majority of samples are dominated by agglutinated taxa. Most assemblages are M. fusca dominated (found in 19 of the 27 sample areas). Eggerelloides scaber assemblages (category 5) are restricted to the Skagerrak coast of Norway. Ammoscalaria runiana assemblages (category 4) are found mainly along the Kattegat coast of Denmark. Reophax moniliformis assemblages (category 4) are sparsely present throughout the region. Ammotium cassis assemblages (category 5) are confined to Horten (9). Calcareous assemblages include those of A. beccarii (category 3) which is widely distributed and E. williamsoni which is especially common along the Danish Kattegat coast. Assemblages rarely present are those of Elphidium excavatum (category 4, areas 4, 19, 21), Haynesina germanica (category 3, areas 16, 17), and Cibicides lobatulus (category 6, area 3, particularly associated with bioclastic sands). Most of the assemblages are found from the intertidal zone down to the limits of sampling. However, the A. cassis, A. runiana, E. scaber, E. excavatum, and H. germanica assemblages are all found only subtidally. The E. williamsoni assemblage is restricted to depths of 0 1 m with the exception of one occurrence at 4.5 m Species diversity The marsh Fisher alpha species diversity values are generally less than 1.2 except at Hålkedalskilen

6 200 J.W. Murray, E. Alve / Palaeogeography, Palaeoclimatology, Palaeoecology 146 (1999) Table 1 Comparison of living and dead depth distributions for principal species (data on living from Alve and Murray, 1999) Species Category Live, Depth range Makes up Makes up As dominant dead (m) ½5% of ass. ½10% of ass. species B. pseudomacrescens L, D IT a IT a IT a IT a T. comprimata 1 L IT a IT a IT a IT a D IT a 1 IT a IT a ND T. inflata L IT 0.5 IT a IT a ND D IT 3 IT a 1 IT a 0.5 ND J. macrescens L IT 1 IT 0.3 IT IT 2 D IT 6 IT 3 IT 1.5 IT a 1 H. wilberti L IT 4 IT 2 IT a IT a D IT 4 IT 0.5 IT a 0.5 IT a E. albiumbilicatum L IT 1 IT 1 IT ND D IT ND A. salsum L IT 4 IT 2 IT 0.5 IT D IT 5.5 IT 0.1 IT M. fusca L IT 6 IT 5 IT 5 IT 3.5 D IT 6 IT 6 IT 4.5 IT 5 3 E. williamsoni L IT 5 IT 5 IT 5 IT 5 D IT 6 IT 6 IT 4.5 IT 4.5 H. germanica L IT 6 IT 6 IT D IT A. beccarii L IT 6 IT 6 IT 6 IT 6 D IT 6 IT 6 IT 6 IT 5.5 E. pulchella L IT 4 IT 4 IT 4 IT 4 D R. moniliformis L IT 6 IT 5 IT D IT 6 IT 6 IT 6 IT 6 A. balkwilli L IT 5.5 IT 5 IT D IT 6 IT 5 IT 5 ND E. oceanensis 4 L IT D IT 2 1 ND E. excavatum L IT D IT 6 IT P. (L.) haynesi L IT ND D A. runiana L IT D IT 6 IT 6 IT O. kilianensis L ND D 3.5 ND E. scaber L ND 5 D IT 6 IT 6 IT A. cassis L ND D C. lobatulus L IT 4 IT D IT 5 IT 5 IT 5 IT 5 E. margaritaceum L IT 5 IT ND D IT 5 IT 5 5 ND 6 R. anomala L IT 5 2 ND D IT ND N. depressulus L D 0.5 5

7 J.W. Murray, E. Alve / Palaeogeography, Palaeoclimatology, Palaeoecology 146 (1999) where they range up to 1.5. The intertidal subtidal values range from alpha with most between 1.0 and 2.9 and only 6 values ½3.0. With one exception of H(S) 2.11 (sample 13, area 3), all the H(S) values are less than Species distributions The majority of species are widely distributed. It was noted above that broken individuals of G. waddensis cannot be distinguished from R. moniliformis. However, complete individuals of G. waddensis (Plate I, 15, 16) were found in Oslo Fjord (areas 7 10), along the Swedish coast (areas 13, 16, 17) and in Denmark (21, 24) over a depth range from 0 5 m Destruction of tests Many dead assemblages contain the organic linings of calcareous foraminifera (principally of A. beccarii, Plate II, 3). Some individuals of A. beccarii, E. excavatum, E. williamsoni, andh. germanica show partial decalcification (in some instances, the organic lining is exposed, Plate II, 5, 6; or test wall layers peeling off, Plate II, 1, 2, 4, 8, 9, 12) or the test is so fragile that when touched with a moist paintbrush it disintegrates. In addition, the amount of dissolution can be gauged by comparing the living and dead assemblages from the same samples, accepting that there may be some differences caused by seasonal variability in the living assemblages. The percentages of calcareous tests are markedly lower in the dead (Figs. 2 and 3). Dissolution appears to be more severe along the Skagerrak coast of Norway, the Kattegat coasts of Sweden and southern Denmark (areas 1 21 but excluding the shell sands of area 3), and much less conspicuous along the Kattegat coast of Denmark and the Limfjord (areas 22 27). Some agglutinated tests are very fragile and, although they have survived sample preparation, they collapse when wetted with a moistened paintbrush. In particular, this applies to some juvenile individuals of M. fusca and occasionally to A. runiana. 4. Discussion A comparison of live and dead distributions based solely on presence=absence shows good overall agreement (Table 2) but there are major differences in abundance, as discussed below Taphonomy The relationships between living assemblages and the dead ones drawn from them have been discussed by several authors and most recently by Murray (1991) and Martin et al. (1995). The dead assemblages represent the time-averaged input from the death of living individuals and the effects of postmortem modifying processes, namely, transport (loss or gain of tests) and destruction of tests (dissolution of calcareous and disaggregation of fragile agglutinated forms; see also De Rijk and Troelstra, 1999). It must be stressed that only by making comparative studies of the living (ideally studied over a period of more than a year) and dead assemblages is it possible to identify the taphonomic processes operating and to properly determine the pathways to fossilization (see Murray and Alve, 1999). As an example, Fig. 2 illustrates the differences between the living and dead assemblages in area 9 (Horten, Norway). It clearly shows that calcareous taxa are much more common in the living assemblages and agglutinated taxa dominate the dead assemblages. This pattern would not have been so obvious if the comparison had been made between live and total (live plus dead) assemblages. This applies even though there is seasonality of the living assemblage composition Transport of tests Transport due to tidal action is not considered to be a significant process in introducing exotic IT D intertidal (including marsh); ND D not dominant. Categories: 1 D associated only with marsh plants; 2 D basically, but not entirely, associated with marsh plants; 3 D basically, but not entirely, restricted to non-marsh areas, 4 D solely recorded in non-marsh intertidal to subtidal environments; 5 D restricted to subtidal areas; 6 D basically living in the most open marine areas. a Instances where the species is associated with marsh plants.

8 202 J.W. Murray, E. Alve / Palaeogeography, Palaeoclimatology, Palaeoecology 146 (1999) PLATE II

9 J.W. Murray, E. Alve / Palaeogeography, Palaeoclimatology, Palaeoecology 146 (1999) species in the microtidal areas of the Skagerrak and Kattegat. This is evident from the fact that nearly all dead species are represented by living individuals; thus, there is no gain of exotic forms. This is in marked contrast with more tidally influenced areas such as the southern North Sea (e.g., Haake, 1962). However, there is some local transport of tests due to waves and wave induced currents. The depth ranges of some species, for example, J. macrescens and T. inflata, which live solely in association with marsh plants (Alve and Murray, 1999) have their dead ranges extended into the subtidal zone but their presence still indicates close proximity to a marsh. Also, the depth ranges of certain subtidal species are extended both shallower (E. scaber) and deeper (E. albiumbilicatum, A. salsum). However, the majority of the non-marsh species show a living depth distribution covering most of the investigated depth interval Abundance of agglutinated tests and syndepositional carbonate dissolution Based on the relative abundance of calcareous tests in the live and dead assemblages, it seems that little postmortem change is taking place in the intertidal shallow subtidal areas of Dype Holla (area 3), Tøkersfjord (4), Borre (8), Vejle Fjord (22), Kalø Vig (23, 24), Løgstør (25), and Valsted (26). However, overall there are pronounced differences between the living and dead assemblages (Fig. 3). Possible explanations for this include patchiness and seasonal changes in living assemblage composition as well as postmortem changes such as transport and carbonate dissolution. Of these, dissolution of calcareous tests is clearly active because in each sample the dead assemblage is generally enriched in agglutinated tests compared to the living and there are severely etched calcareous tests present. There are even live individuals with etched tests (illustrated in Alve and Murray, 1999), for example at Kvastadkilen and Kilsfjord (areas 2 and 6). The pathways from live calcareous-dominated to completely dissolved (non-calcareous) dead agglutinated assemblages are described in Murray and Alve (1999). Both living and dead assemblages (of >100 individuals) show a wide range of abundance of agglutinated tests (0 100%) with a mean value of 30% for the living and 67% for the dead. This indicates that overall syndepositional dissolution is very active. Carbonate dissolution is complex and may be caused by several processes including corrosive bottom or sediment pore waters (which may be brought about by the metabolization of organic matter, Reaves, 1986) and bacterial destruction (Freiwald, 1995). Following the postglacial rise of sea level, seawater entered the Baltic Sea via the Kattegat around 8000 years ago. The foraminiferal record in the straits between the two seas is calcareous throughout the past 8000 years (Lukashina, 1995). The development of corrosive bottom waters in the Baltic Sea is a relatively recent phenomenon. The deposits formed between 8000 and 4500 yr B.P. were calcareous but after that carbonate preservation was much re- PLATE II Dissolution features of calcareous tests as observed under the scanning electron microscope. # refers to the sample number (area number; see Fig. 1) Ammonia beccarii (Linné), #4 (3). 1. Detail of third chamber from most recent: loss of chamber walls and microcrystalline unit outlines emphasised through etching. 2. Wall layers peeling away. 3. Organic lining. 4. Detail of NE quadrant of fig Organic lining exposed in the lower part and wall layering in the centre of the image. 6. Loss of wall layers and exposure of organic lining in the final chamber Elphidium excavatum (Terquem), #120 (21). 8. Loss of wall layers especially in the final chamber (fig. 9). 10. Etched surface showing enlarged pores and rough crystal units. 11. Severely corroded specimen showing chamber damage. 12. Haynesina germanica (Ehrenberg) showing etched surface, #4 (3).

10 204 J.W. Murray, E. Alve / Palaeogeography, Palaeoclimatology, Palaeoecology 146 (1999) Table 2 Comparison of the biogeographic distributions of living and dead representatives (data on living from Alve and Murray, 1999) Area Norway Sweden Denmark No. areas A. balkwilli L ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 16 A. balkwilli D ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 22 A. runiana L ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 18 A. runiana D ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 22 A. cassis L ð ð ð 3 A. cassis D ð ð ð ð 4 A. salsum L ð ð ð ð ð ð ð ð ð ð ð ð ð ð 14 A. salsum D ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 16 B. pseudomacr. L ð 1 B. pseudomacr. D ð 1 E. scaber L ð ð ð ð ð ð 6 E. scaber D ð ð ð ð ð ð ð ð ð ð ð ð ð 13 H. wilberti L ð ð ð ð ð ð ð 7 H. wilberti D ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 15 J. macrescens L ð ð ð ð ð ð ð ð ð ð ð 11 J. macrescens D ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 23 M. fusca L ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 25 M. fusca D ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 25 P. (L) haynesi L ð ð ð ð ð ð ð 7 P. (L) haynesi D ð ð ð ð ð ð 6 R. moniliformis L ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 19 R. moniliformis D ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 22 T. comprimata L ð ð ð 3 T. comprimata D ð ð ð 3 T. inflata L ð ð ð ð ð 5 T. inflata D ð ð ð ð ð ð ð ð ð 9 A. beccarii L ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 24 A. beccarii D ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 23 C. lobatulus L ð ð 2 C. lobatulus D ð ð ð ð 4 E. albiumb. L ð ð ð ð 4 E. albiumb. D ð ð ð ð ð 5 E. excavatum L ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 19 E. excavatum D ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 19 E. macellum L ð 1 E. macellum D ð 1 E. margarit. L ð ð 2 E. margarit. D ð ð ð ð ð ð 6 E. oceanensis L ð ð ð ð ð ð ð 7 E. oceanensis D ð ð ð ð ð ð ð ð ð 9 E. williamsoni L ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 27 E. williamsoni D ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 25 E. pulchella L ð ð ð ð 4 E. pulchella D ð ð ð 3 H. germanica L ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 24 H. germanica D ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð ð 18 N. depressulus L ð ð 2 N. depressulus D ð ð ð 3 O. kilianensis L ð ð ð ð ð ð ð ð ð 9 O. kilianensis D ð ð ð ð 4 R. anomala L ð 1 R. anomala D ð ð 2

11 J.W. Murray, E. Alve / Palaeogeography, Palaeoclimatology, Palaeoecology 146 (1999) Fig. 3. Comparison of the % calcareous tests between live and dead assemblages averaged for 1 m intervals and plotted at the 0.5 m depth points. Fig. 2. Distribution of living (as % living assemblage) and dead (as % dead assemblage) foraminifera in relation to environmental parameters at Horten, Norway. Abbreviations: A. ba D A. balkwilli; A. ru D A. runiana; A. ca D A. cassis; M. fus D M. fusca; R. mo D R. moniliformis; T. co D T. comprimata; A. becc D A. beccarii; E. ex D E. excavatum; H. ge D H. germanica. Bar widths: widest to narrowest >50%, 30 50%, 10 30%, <10%, respectively. duced. At present, dissolution is particularly severe at depths >20 m in Flensburg Fjord (Exon, 1972). The outflowing Baltic surface water moves along the Swedish coast towards the Skagerrak. In addition, the inflowing freshwater from Sweden and Norway is from a region of mainly acid igneous and metamorphic rocks overlain by peaty soils. Along this coast and into Oslo Fjord, Alexandersson (1972, 1974, 1978, 1979) observed that bioclastic calcareous grains from sublittoral sands show signs of dissolution: dull etched surfaces, and increased fragility leading ultimately to disintegration (which he termed maceration). He considered that the seawater was undersaturated with respect to CaCO 3 for long periods of time and probably constantly. Furthermore, he believed that much of the dissolution took place in seawater rather than in the sediment. This may be true for the macrofauna but is perhaps less likely for the foraminifera which mainly live below the sediment water interface. Reaves (1986) noted that carbonate dissolution in muds may be controlled by the consumption of organic matter. In the summer, microbial activity leads to destruction of organic matter causing anoxic-sulphidic conditions at the sediment water interface and the formation of ferrous sulphide minerals with carbonate saturation of the pore waters. In the winter, the cool temperatures slow down microbial activity, the ferrous sulphide minerals become oxidized, and the ph is lowered causing carbonate dissolution. The process takes place in the top few centimetres of the sediment (Walter and Burton, 1990). In the study area, although the presence of decaying seagrass might be a possible cause of increased dissolution, there is no obvious link to this because both high and low degrees of dissolution are present within sediments with a dense cover of Zostera. Elsewhere, dissolution is most intense in highly bioturbated ar-

12 206 J.W. Murray, E. Alve / Palaeogeography, Palaeoclimatology, Palaeoecology 146 (1999) eas (often of slow sediment accumulation, Martin et al., 1995) where the build-up of alkalinity is inhibited by water-flushing in burrows and oxidation of solid phase sulphides during particle reworking. Thus, the best mollusc shell preservation occurs in those regions least affected by biological physical disturbance (Aller, 1982). It has commonly been observed that calcareous foraminifera are poorly preserved in marsh sediments (for example, Jonasson and Patterson, 1992; De Rijk and Troelstra, 1999). Severe dissolution of calcareous foraminifera has been reported from subtidal sediments in Long Island Sound, USA, where the irrigated burrows and tubes of the macrofauna prevented the build-up of carbonate alkalinity (Green et al., 1993). The foraminifera disappeared within the course of a year. Even in areas of tropical carbonate sediment accumulation carbonate dissolution takes place. In Florida Bay most pore fluids have excess [Ca 2C ] relative to the overlying seawater and this is a measure of active dissolution. Unlike terrigenous sediments, carbonate dissolution may extend to a depth of a metre below the sediment surface (Walter and Burton, 1990). The dissolution of foraminiferal carbonate by bacteria leads to pitting and then disintegration of chamber walls (Freiwald, 1995). This type of destruction is not commonly observed in our material. As noted above, in the Skagerrak Kattegat foraminifera the calcareous walls become so fragile that they are chalky and disintegrate en masse. In conclusion, carbonate dissolution is known to be a widespread phenomenon affecting both terrigenous and carbonate sedimentary environments. However, not all environments with terrigenous sediments experience severe dissolution (for example those of the southern North Sea coast, Haake, 1962) otherwise there would be a poor fossil record of calcareous foraminifera. The Norwegian and Swedish coastal areas of the Skagerrak and Kattegat are unusual in having such severe dissolution Destruction of agglutinated tests Some agglutinated tests are poorly held together with cement and may be fragile (e.g., Schröder, 1988; Murray, 1991; De Rijk and Troelstra, 1999). Douglas et al. (1980) suggested that, in the deep sea, the organic cement may be oxidized within the surface layer of sediment, whereas loss of agglutinated tests may be reduced in coastal areas with higher rates of sedimentation (Alve, 1996). Culver et al. (1996) recorded loss of M. fusca in the subsurface coastal barrier deposits of Virginia, USA. As already noted, some M. fusca disaggregated when being picked although this was relatively rare. It does mean that there is potentially some loss of agglutinated taxa from assemblages but it has not been possible to quantify this Indicators of sea level and water depth The living assemblages from this area were divided into six categories based on presence or absence of marsh plants, whether they were intertidal or subtidal, and salinity. Categories 1 5 comprise euryhaline and category 6 are essentially stenohaline species (Alve and Murray, 1999). Category 1 includes taxa found living only in association with marsh plants and, with the exception of some rare occurrences down to 1 m, this is true of the dead too (Table 1). Category 2 includes forms normally found living in association with marsh plants but also found in unvegetated areas. Local transport has been responsible for extending the depth ranges of some of the dead occurrences. Categories 3 and 4 include living species which are intertidal to subtidal, those in 3 sometimes occurring in marshes whereas those in 4 do not. Category 5 includes living euryhaline, subtidal taxa while category 6 includes essentially stenohaline forms found in both intertidal and subtidal settings. In general, there is some extension of the overall depth range for the dead occurrences. Some minor calcareous species which were locally abundant living are much less abundant dead due to the taphonomic processes discussed above (e.g., E. pulchella, E. oceanensis). On the other hand, some minor agglutinated live taxa become dominant dead species (e.g., E. scaber, A. cassis). There is considerable interest in defining past sea levels using microfossils. Marsh taxa have been especially used (e.g., D.B. Scott and Medioli, 1978, California and Nova Scotia; D.K. Scott and Leckie, 1990, Massachusetts) and specific assemblages have been correlated with elevation above mean sea level. However, De Rijk (1995) and De Rijk and Troelstra (1997) have suggested that in Massachusetts marshes salinity is more important than elevation in controlling distri-

13 J.W. Murray, E. Alve / Palaeogeography, Palaeoclimatology, Palaeoecology 146 (1999) butions. Patterson (1990) invoked a combination of both elevation and salinity to explain the patterns observed on high marshes in British Columbia. The Skagerrak Kattegat area is not well suited to test these rival hypotheses because the tides are very small or non-existent and water level changes due to barometric pressure and wind stress are more significant. Also, the difference in elevation between the marsh front and the landward limit is often only a few tens of cm at the most. The typical marsh taxa found here (J. macrescens, T. comprimata, and T. inflata) are normally associated with high marshes elsewhere (for instance cm above mean sea level in California and Nova Scotia, Scott and Medioli, 1978). De Rijk (1995) and De Rijk and Troelstra (1997) found a positive correlation between the abundance of J. macrescens and T. comprimata and mean salinity. Likewise, they found a negative correlation between the abundance of Haplophragmoides manilaensis and salinity. Our records of salinity were taken only on the day of sampling. Nevertheless, we see the same general relationships. Balticammina pseudomacrescens was found only on the most landward and higher parts of the marsh and the cover of dry leaf litter indicates that this part of the marsh is not often flooded. In conclusion, it can be said that the marsh assemblages of the Skagerrak Kattegat are very reliable indicators of the upper limit of sea level and probably also of salinity (Alve and Murray, 1999). As regards indicators of water depth, the calcareous species do not show any pronounced upper or lower depth limits although ½10% living or dead E. williamsoni are confined to depths of <5 min this area. On the other hand, the agglutinated taxa A. cassis and E. scaber are found in abundances >10% only deeper than 3 m. Thus the agglutinated foraminifera may be the better indicators than calcareous foraminifera of upper depth limits in these shallow sublittoral environments as well as better indicators of sea level in marsh deposits Application to palaeoenvironmental interpretation Data on modern ecology form the basis for interpreting fossil assemblages to reconstruct past environments. The living assemblages record the picture at the time of sampling. In the absence of postmortem change, the dead assemblages give the time-averaged record of successive living assemblages (the ideal dead assemblages of Murray, 1991). Severe dissolution has affected the preservation of many of the dead assemblages in this area but they nevertheless contain useful ecological data which can be used to interpret fossil agglutinated assemblages. Similar dissolution loss of calcareous forms was reported by Culver et al. (1996) from coastal barrier deposits of Virginia, USA, and they urged caution in interpreting fossil assemblages. In the present study area, the living assemblages and those dead assemblages which have not undergone significant postmortem dissolution provide good analogues for the interpretation of fossil calcareous assemblages such as those found in the Quaternary (Knudsen and Nordberg, 1987; Knudsen, 1994). 5. Conclusions The dead assemblages of the Skagerrak Kattegat area show a good overall agreement in terms of presence=absence of species but often major differences of abundance from the living; the latter is primarily attributed to postmortem modification. The most significant taphonomic process in the Skagerrak Kattegat area is dissolution of calcareous tests. Many of the living assemblages are dominated by calcareous taxa but, for those affected by dissolution, the dead assemblages are dominated by non-calcareous agglutinated forms. The depth distribution patterns of living and dead taxa are similar. Marsh dead assemblages extend down to about half a metre water depth and individual taxa occur as dead tests down to 4 6 m due to local transport. Nevertheless, abundant marsh species are reliable indicators of local sea level. The composition of the assemblages seems to vary with salinity. In this region, agglutinated foraminifera are better indicators of water depth than calcareous forms. Ammotium cassis and Eggerelloides scaber are reliable indicators of water depths greater than about 3 and 2 m, respectively. Marsh agglutinated taxa are the best indicators of sea level.

14 208 J.W. Murray, E. Alve / Palaeogeography, Palaeoclimatology, Palaeoecology 146 (1999) Like the living assemblages, calcareous dead assemblages which are not much altered through dissolution serve as good analogues for the interpretation of Quaternary assemblages. Polysaccammina hyperhalina Medioli, Scott and Petrucci (in Petrucci et al., 1983) is the same as R. moniliformis (D.B. Scott, pers. commun., 1997) and their illustrations show a similar range of variability. They did not record G. waddensis van Voorthuysen in their samples. Acknowledgements We are pleased to acknowledge the Natural Environment Research Council for grant GR9=1591 A and the Industrial Liaison fund (Oslo University) for financial support. We are grateful to Pål Glad for help with sampling in Norway, to Eva Larsen, Siggurd Gausdal, Arne M. Svendsen, Bent Åsnes, Trond Smith, John Ingebrigtsen, and Tjärnö marinbiologiska laboratorium for kindly letting us use their boats, to Inger-Ann Hansen for performing the TOC analyses, and to Daphne Woods for carrying out the grain size analyses. Karen Luise Knudsen, Marit- Solveig Seidenkrantz, Sacha De Rijk, Dave Scott, and John E. Whittaker are thanked for their discussions on taxonomy. The manuscript was reviewed by Karen Luise Knudsen and Sacha De Rijk and by referees Bill Berggren, Jean-Pierre Debenay and Ron Martin; we thank them for their constructive comments. Appendix A. Comments on morphological variation in selected agglutinated species The following species show variability in their form: Ammobaculites balkwilli Haynes occurs as two variants (Plate I, 1 5). The typical form has an inflated test and when fully developed a length of around 670 µm. The second is somewhat smaller and is compressed in the plane of the planispiral (maximum length around 360 µm). Typical forms occur throughout the study area. Compressed forms occur in Oslo Fjord (area 8), along the Swedish (area 18) and Denmark (area 21) coasts from intertidal to 2 m. Haplophragmoides wilberti Anderson is very variable and includes forms regarded by D.B. Scott as H. manilaensis (pers. commun., 1997). Reophax moniliformis Siddall in its typical development has a near-cylindrical test with parallel sides and almost flush sutures (Plate I, 6 8). Some tests are more irregular, with recessed chambers and depressed sutures in parts (Plate I, 9 13). Whereas the normal variant occurs throughout the study area, the irregular form is present mainly around the Skagerrak (areas 1, 2, 4, 6, 8, 14 16) with only 2 occurrences in the Kattegat (21, 22); depth range 0 5 m. References Alexandersson, E.T., Shallow-marine carbonate diagenesis as related to the carbonate saturation level of seawater. Palaeontol. Inst. Univ. Uppsala Publ. 125, Alexandersson, E.T., Carbonate cementation in coralline algal nodules in the Skagerrak, North Sea: biochemical precipitation in undersaturated waters. J. Sediment. Petrol. 44, Alexandersson, E.T., Destructive diagenesis of carbonate sediments in the eastern Skagerrak, North Sea. Geology 6, Alexandersson, E.T., Marine maceration of skeletal carbonates in the Skagerrak, North Sea. Sedimentology 26, Aller, R.C., Carbonate dissolution in nearshore terrigenous muds: the role of physical and biological reworking. J. Geol. 90, Alve, E., Variation is estuarine foraminiferal biofacies with diminishing oxygen conditions in Drammensfjord, SE Norway. In: Hemleben, C., Kaminski, M.A., Kuhnt, W., Scott, D.B. (Eds.), Paleoecology, Biostratigraphy, Paleoceanography and Taxonomy of Agglutinated Foraminifera. Kluwer, Dordrecht, pp Alve, E., Benthic foraminiferal distribution and recolonization of formerly anoxic environments in Drammesfjord, southern Norway. Mar. Micropaleontol. 25, Alve, E., Benthic foraminiferal evidence of environmental change in the Skagerrak over the past six decades. Nor. Geol. Unders. Bull. 430, Alve, E., Murray, J.W., Marginal marine environments of the Skagerrak and Kattegat: a baseline study of living (stained) benthic foraminiferal ecology. Palaeogeogr., Palaeoclimatol., Palaeoecol. 146, Alve, E., Nagy, J., Estuarine foraminiferal distribution in Sandebukta, a branch of the Oslo Fjord. J. Foraminiferal Res. 16, Alve, E., Nagy, J., Main features of foraminiferal distribution reflecting estuarine hydrography in Oslo Fjord. Mar. Micropaleontol. 16, Brönnimann, P., Lutze, G.F., Whittaker, J.E., Balticammina pseudomacrescens, a new brackish water trochamminid from the western Baltic Sea, with remarks on the wall structure. Meyniana 41, Christiansen, B., The foraminifer fauna in the Dröbak Sound in the Oslo Fjord (Norway). Nytt Mag. Zool. 6, Culver, S.J., Woo, H.J., Oertel, G.F., Buzas, M.A., Foraminifera of coastal depositional environments, Virginia, U.S.A.: distribution and taphonomy. Palaios 11, De Rijk, S., Salinity control on the distribution of

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