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Late Neogene deep-sea benthic foraminifera at ODP Site 762B, eastern Indian Ocean: diversity trends and palaeoceanography
Palaeogeography, Palaeoclimatology, Palaeoecology 173 (2001) 1±8
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Late Neogene deep-sea benthic foraminifera at ODP Site 762B, eastern Indian Ocean: diversity trends and palaeoceanography A.K. Rai*, V.B. Singh Department of Earth and Planetary Sciences, Allahabad University, Allahabad 211-002, India Received 13 June 2000; accepted for publication 1 May 2001
Abstract In the present investigation, the Late Neogene deep-sea benthic foraminiferal diversity at ODP Site 762B has been measured by using Hurlbert's diversity index (Sm). This index measures the expected number of species to a common sample size (m). Here, m is put equal to 100. This method is relatively more advantageous than other commonly used diversity indices because it eliminates differences in sample size. The Late Neogene section at Site 762B is characterised by marked variations in species diversity (S100) of the deep-sea benthic foraminiferal assemblages. Species diversity broadly exhibits an inverse relationship with the values of Uvigerina proboscidea abundance and percentage of infaunal taxa in the examined section of the site. This suggests that temporal variations in primary productivity of the overlying surface waters are re¯ected in the Late Neogene deepsea benthic foraminiferal diversity trends. The species diversity pattern explains that the trophic level of the bottom water changed at the beginning of the Late Pliocene (,3.5 Ma) due to an increased surface water productivity. The higher values of species diversity during the Early Pliocene was due to a stable deep-sea environment with low trophic resource levels. However, the Late Pliocene and Pleistocene interval is largely characterised by low diversity re¯ecting an unstable deep-sea environment with higher surface water productivity. A distinct increase in the values of species diversity along with low percentages of U. proboscidea and infaunal taxa during the latest Pliocene (,2.3±1.9 Ma) suggests low organic carbon in¯ux in the bottom waters due to a decrease in the surface water productivity. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Neogene; Benthic foraminifera; Indian Ocean; Diversity; Palaeoceanography
1. Introduction Species diversity is a valuable method to measure the complexity of faunal or ¯oral distribution in palaeontological samples. The main characteristic of the measure of species diversity is the reduction of information provided by the species distribution in a sample to a single member (Malmgren and Sigaroodi, 1985). There has been a growing awareness in
* Corresponding author. E-mail addresses: [email protected] (A.K. Rai).
understanding the ecological and evolutionary signi®cance of species diversity. The modern deepsea is characterised by higher species diversity of many groups of organisms (Gage and Tyler, 1991). The earlier assumption of low species diversity in various groups at greater depths was due to low density of the larger invertebrates and also their inadequate sampling. The benthic foraminifera with preservable tests in fossil records have the advantage of relatively high densities with a reasonable number of species and their proportions. Recent studies have suggested several intervals of marked ¯uctuations in the deep-sea benthic
0031-0182/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0031-018 2(01)00299-1
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A.K. Rai, V.B. Singh / Palaeogeography, Palaeoclimatology, Palaeoecology 173 (2001) 1±8
of western Australia (Fig. 1). The SEC and SIC ¯owing west and north, respectively, are the eastern wing of an anti-clockwise gyre in the south Indian Ocean (Tomczak and Godfrey, 1994). The water ¯ow of the SIC is called the Indian Central Water (ICW). A part of Indonesian through¯ow water (Tomczak and Godfrey, 1994) starts ¯owing southward along the western coast of Australia as the Leeuwin Current (Cresswell and Golding, 1980). Beneath the LC, high salinity waters, and South Indian Central waters, are carried northward by the undercurrent (Cresswell, 1991), acting as a part of the South Indian Current. This current in turn, in¯uences water masses as deep as 2000 m (Tchernia, 1980) and is part of a major Southern Hemisphere gyre, moving anticlockwise in the Indian Ocean (Fig. 1). 2. Material and methods
Fig. 1. Oceanographic setting and location of ODP Site 762B in the eastern Indian Ocean. Depth contours are in kilometres. (ITF: Indonesian Through¯ow; SEC: South Equatorial Current; LC: Leeuwin Current; SIC: South Indian Current; TRANS: Tropical±Subtropical Transition Zone.)
foraminiferal diversity throughout the Cenozoic (e.g. Thomas, 1985, 1986; Woodruff, 1985; Rai and Srinivasan, 1992; Smart and Murray, 1995; Bornmalm, 1997). Considering the vastness of size and varying water depths of the ocean, there is still very little information available on species diversity. In the present work, benthic foraminifera at ODP Site 762B in the eastern Indian Ocean have been selected for the study of their diversity pattern during Late Neogene. Site 762B is situated at 19853.24 0 S, 112815.24 0 E in 1360 m water depth near Exmouth Plateau in the eastern Indian Ocean, offshore western Australia (Fig. 1). This site is located near the deep oxygen minimum zone and also in the tropical to subtropical transition zone (,208S to 158S) in the eastern Indian Ocean. The high planktic foraminiferal concentration in this zone agrees with the high primary productivity of this area (Nomura, 1995). The South Equatorial Current (SEC), the Leeuwin Current (LC) and South Indian Current (SIC) are the parts of the current system off the coast
The present investigation is based on 76 subsamples from a 108.9 m long core. These samples are 10 cm 3 plugs of sediment commonly taken at about 1.5 m intervals. The Late Neogene section at this site comprises dominantly of foram±nanno ooze. The samples were treated with 5% hydrogen peroxide solution and water (1:3 ratio) and wet sieved through 63 mm and 149 mm Tyler sieves. Though the H2O2 is aggressive to carbonates, the concentration is too low (,2%) to affect the tests of benthic foraminifera. Therefore, H2O2 treatment may not matter much for the overall diversity structure. Benthic foraminifera are well preserved and generally do not constitute more than 5% of the total foraminiferal population. After drying, a microsplitter was used to separate a representative proportion of the .149 mm fraction estimated to contain more than 300 specimens of benthic foraminifera. The .149 mm fraction was selected for micropalaeontologic analyses because it provides most climatic information in short time (Imbrie and Kipp, 1971) and is the size fraction adopted by many major palaeoclimatic studies, particularly in the Indian Ocean (Corliss, 1979; Gupta and Srinivasan, 1992a,b; Wells et al., 1994; Nomura, 1995 and others). Thus, the results of the present study can be suitably compared with other Indian Ocean works. The benthic foraminiferal quantitative data have been used to calculate
A.K. Rai, V.B. Singh / Palaeogeography, Palaeoclimatology, Palaeoecology 173 (2001) 1±8 Table 1 Characteristic species of shallow and other infaunal taxa (after Wells et al., 1994) Shallow infaunal taxa
Other infaunal taxa
Bulimina spp., Pullenia bulloides, Uvigerina peregrina
Cassidulina laevigata, Ehrenbergina trigona, Melonis barleeanum, Pullenia spp., Uvigerina spp.
Hurlbert's diversity index, Sm (Hurlbert, 1971). This index, Sm, denotes the ªexpected species diversityº by Smith and Grassle (1977), permits standardisation of species-counts to a common sample size. Consequently, with this method it is possible to estimate the number of species (Sm) expected in a sub-sample of m specimens taken at random from a sample of N specimens m # N: This method has advantage over more commonly used diversity indices such as the Simpson index and Shannon Wiener information index in that it eliminates differences in sample size. The Hurlbert's diversity index is de®ned by the function: Sm
S X
1 2 C N 2 Ni; m=C N; m
i1
where Sm is the expected number of species in a random sub-sample of size m m # N: In the present study we set m at 100, well below the lowest number of specimens counted per sample. N is the number of specimens in the sample and S is the number of species in which N specimens are distributed. Ni is the number of individuals in the ith species, SNi N: C N 2 Ni; m N 2 Ni!=m! N 2 Ni 2 m! and C N; m N!=m! N 2 m! for N 2 Ni $ m and N $ m; respectively, and zero for N 2 Ni , m and N , m; respectively (Smith and Grassle, 1977). The percentages of shallow infaunal and other infaunal taxa have been calculated following Wells et al. (1994). In this study the term `shallow infaunal taxa' refers only to those taxa for which such a microhabitat preference is well known (see Wells et al., 1994). A list of characteristic species of shallow and other infaunal morphogroups is given in Table 1. The species diversity (S100) pattern is also compared with the percentage of Uvigerina proboscidea and total number of benthic foraminiferal specimens per gram dry weight of .149 mm size
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fraction (Fig. 2). The original data set is available from the ®rst author. 3. Results The sample size was standardised at 100 individuals (S100) using Hurlbert's method. Values of Hurlbert's diversity index (S100) and total number of benthic foraminiferal specimens per gram dry sediment (spec./g) are plotted at Site 762B (Fig. 2). Results are shown on newly integrated chronologies of Berggren et al. (1995). The species diversity index varies from 24 to 39 species per 100 specimens with an average value of 33 (Fig. 2). In general, the values of S100 remain relatively high during the latest Miocene and Early Pliocene period with a few intervals of minor ¯uctuations. At the beginning of the Late Pliocene (,3.5 Ma), species diversity shows a distinct decrease and remains relatively low with few intervals of marked ¯uctuations during most of the Late Pliocene and Pleistocene. The diversity also exhibits greater variations during the Late Pliocene and Pleistocene. Percentages of infaunal taxa and Uvigerina proboscidea also show a sudden increase at about 3.5 Ma and remain relatively high in the rest of the younger section (Fig. 2). U. proboscidea abundance, percentage of infaunal taxa and total benthic foraminiferal abundance show an almost opposite trend to the values of species diversity. The early part of Late Pliocene is broadly characterised by less diverse fauna along with higher percentages of infaunal taxa and U. proboscidea abundances. However, the latest Pliocene (,2.3±1.9 Ma) is marked by relatively higher species diversity and low percentages of infaunal taxa and U. proboscidea. The early and middle Pleistocene (.1.8±0.4 Ma) intervals show low species diversity with marked ¯uctuating trends. This period is also marked by several pulses of distinctly higher abundances of U. proboscidea (.30%) and dominance of infaunal taxa. The Late Pleistocene (,0.2 Ma to Holocene) is characterised by relatively higher values of species diversity with corresponding low abundances of U. proboscidea and moderately higher occurrences of infaunal taxa.
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Fig. 2. Plots of Hurlbert's diversity index (S100), percentage of Uvigerina proboscidea, percentage of total infaunal taxa (dotted line for % of shallow infaunal and solid line for other infaunal) and total number of benthic foraminiferal specimens per gram dry weight of .149 mm size fraction (spec./g) at ODP Site 762B. Ages are taken from Berggren et al. (1995).
4. Discussion Recent research has revealed that the deep-sea environment is not as stable as was previously realised. However, it is also in¯uenced by shortterm changes, such as benthic storms, which can substantially disturb the sediment surface (Kaminski, 1985). The deep-sea trophic level is highly in¯uenced by the seasonal input of phytodetritus, which increases the abundance of a few opportunistic species and generally promotes benthic activity (Gooday, 1994). The seasonal input of phytodetritus is an environmental disturbance to which certain opportunistic foraminifera respond very rapidly and their assemblages show low diversity (Gooday, 1988). This is because the limited numbers of species
colonise the detritus and the resulting increase in the abundance of these species causes an overall decline in diversity. In general, interpretations of fossil faunal diversity are not simple because many biotic and abiotic factors affect the composition of fossil assemblages. However, some important factors controlling species diversity include changes of trophic (resource) level and stability of the environment (Gage and Tyler, 1991; Sanders, 1969; Valentine, 1973). The magnitude of phytodetritus in¯ux largely controls the trophic resource or the food availability for benthic foraminifera (Gooday, 1988) which is related to the surface productivity (Smart et al., 1994; Thomas et al., 1994). Thus, the diversity pattern may be useful to understand the changes in the deep-sea environment
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Fig. 3. Comparison of the relative abundance of Uvigerina proboscidea and Hurlbert's diversity index (S100) at ODP Site 762B. Correlation coef®cient is computed using running mean data set.
which may be helpful in understanding global climatic and palaeoceanographic changes. The low diversity may represent a period of new environment or a severe environment for the pre-existing assemblage (Slobodkin and Sanders, 1969). Sudden environmental change may also be responsible for a less diverse assemblage. Stability of the environment through geologic time is suggested to be the major factor for higher diversity in the deep-sea (Gage and Tyler, 1991; Buzas and Gibson, 1969; Buzas, 1972). However, Dayton and Hessler (1972) suggested that biological disturbance such as cropping was possibly responsible to promote diversi®cation. Recent explanations of high diversity in the deep sea have concentrated on processes maintaining diversity, especially patch dynamics (Thistle, 1998). These have included the generation of patch mosaics by disturbance (e.g. Grassle and Maciolek, 1992; Lambshead, 1993) or by the activities of deep-sea organisms that modify the benthic environment (e.g. Jumars, 1976; Gage, 1996). Earlier studies have shown that the Cenozoic species diversity is in¯uenced by the changes in the nature of the deep-sea environment caused by tectonics (gateways) (Woodruff, 1985) and/or climatically induced changes in ocean circulation (Thomas, 1985, 1990; Rai and Srinivasan, 1992; Gupta and Srinivasan, 1992a) and magnitude of surface water productivity. Gibson and Buzas (1973) have discussed that species diversity may be de®ned by the carrying capacity of its environment, which is reached rather
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quickly on a geological time scale. Most of the Neogene deep-sea benthic foraminiferal species are geologically long ranging. The temporal changes in the species diversity are likely to be due to the shortterm disappearance and reappearance of long ranging taxa (Flessa and Jablonski, 1983). Recent studies such as those of Gooday (1988, 1993) and Smart et al. (1994) have documented benthic foraminiferal responses to sudden, sometimes seasonal in¯uxes of phytodetritus. It appears that some species are able to take advantage of this ¯uctuating input of organic matter. At Site 762B, species diversity shows an almost opposite trend with the relative abundance of U. proboscidea and percentage of total infaunal taxa (Fig. 3). The abundance of U. proboscidea and infaunal taxa thus largely de®nes the diversity patterns. U. proboscidea is one of the most dominant benthic foraminiferal species at various bathyal sites in the tropical Indian Ocean during the Neogene. Increased relative abundances of this species have been observed in intervals of high surface water productivity during the Neogene (Nomura, 1995; Gupta and Srinivasan, 1992b; Wells et al., 1994; Rai and Srinivasan, 1994, 1996). Various trophic requirements of species due to their own ecologic niche can be responsible for their infaunal, epifaunal or opportunistic behaviour (Altenbach et al., 1999). The abundance of infaunal vs. epifaunal taxa is considered to be related to the amount of organic matter that reached the ocean bottom and thus, ultimately to the primary productivity in the overlying surface waters (Corliss and Chen, 1988; Wells et al., 1994). Den Dulk et al. (1998) suggested that benthic epifaunal numbers start to drop signi®cantly under the oxygen de®ciency, which coincide with the periods of maximum organic matter supply. Increased burial ef®ciency of organic matter due to severe oxygen depletion is most likely responsible for the good correlation between high Corg concentrations and the proportion of deep infaunal species (Jorissen, 1999; Den Dulk et al., 2000). Recently, Den Dulk et al. (2000) have demonstrated that the deep infaunal species are most abundant under high productivity conditions, and vice versa. Thus, the variations in the deep-sea benthic foraminiferal diversity during the Late Neogene at this site appear to be primarily related to the changes in the trophic level of the
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Fig. 4. Plots of Shannon Wiener diversity index: H(S) and percentage of Uvigerina proboscidea at DSDP Site 214 along the Ninetyeast Ridge, Indian Ocean (from Gupta and Srinivasan, 1992a,b). Ages of the epoch boundaries are modi®ed after Berggren et al., 1995.
bottom waters mainly due to the variations in the rates of surface water productivity. The Early Pliocene at Site 762B is characterised by relatively warm and stable deep-sea environment with low trophic resource levels due to decrease in surface water productivity as revealed by higher values of species diversity, low U. proboscidea abundance and decrease in total benthic foraminiferal abundance. Gupta and Srinivasan (1992b) also marked a decrease in the abundance of U. proboscidea during Early Pliocene at DSDP Site 214 along the Ninetyeast Ridge in the Indian Ocean (see Fig. 4). The earth experienced an interval of global warming (Kennett and van der Borch, 1986; Hodell and Venz, 1992) and increased eustatic sea level (Haq et al., 1987) during the Early Pliocene. At Site 762B species diversity shows a marked decrease near the early/late Pliocene transition (,3.5 Ma) which corresponds with the increase in the number of total benthic foraminiferal specimens. This was also the time of sudden increase in the percentages of U. proboscidea and infaunal taxa. The above variations in the faunal parameters reveal a distinct change in the deep-sea environment, i.e.,
increased trophic level across the early/late Pliocene transition. The early part of the Late Pliocene (,3.5± 2.3 Ma) is characterised by low S100, increase in U. proboscidea percentages and moderately higher percentages of infaunal taxa suggesting an unstable deep-sea environment with relative increase in the ¯ux of the organic carbon to the bottom waters due to increased level of the surface water productivity. The latest Pliocene interval (,2.3±1.9 Ma) is marked by a low organic carbon deep-sea environment due to decrease in the primary productivity of overlying surface waters as evidenced by higher species diversity and very low abundances of U. proboscidea and infaunal taxa. Closely similar faunal changes during latest Pliocene have also been recorded by Gupta (1999) from DSDP Site 214 at the Ninetyeast Ridge, eastern Indian Ocean. Gupta (1999) explained that a weaker Indian monsoon during the latest Pliocene was probably responsible for low surface water productivity and oxygenated bottom waters. The large ¯uctuations in the values of species diversity along with changing percentages of U. proboscidea and infaunal taxa during most of the Pleistocene may possibly be due to rapid changes in the trophic level of the bottom waters in response to the changes in the surface water productivity. Gupta and Srinivasan (1992a,b) observed intervals of low diversity and higher abundances of U. proboscidea at the beginning of the Late Pliocene and the Early Pleistocene at DSDP Site 214 along the Ninetyeast Ridge re¯ecting increased surface water productivity (see Fig. 4). Nomura (1995) recognised the development of U. proboscidea assemblage re¯ecting higher surface water productivity during Pliocene±Pleistocene in the eastern Indian Ocean. The rapid faunal changes during Pleistocene appear to be in response to the glacial and interglacial cycles. A further study of the samples at closer intervals may be useful to identify glacial and interglacial cycles and their responses on the faunal assemblages during the Pleistocene. 5. Conclusions Late Neogene deep-sea benthic foraminiferal diversity (S100) at ODP Site 762B varies between 24 and 39. Species diversity is negatively correlated with the relative abundance of U. proboscidea. In general,
A.K. Rai, V.B. Singh / Palaeogeography, Palaeoclimatology, Palaeoecology 173 (2001) 1±8
intervals of low species diversity (S100) also correspond to the higher percentages of infaunal taxa. Thus, changes in the species diversity during the Late Neogene largely re¯ect variations in the food availability of bottom waters in response to the changing surface water productivity. Relatively higher diversities associated with low abundances of U. proboscidea and infaunal taxa during most of the early Pliocene interval re¯ect a stable deep-sea environment with low trophic levels. However, decrease in species diversity along with a sudden increase in the percentages of U. proboscidea and infaunal taxa occurred during the Late Pliocene and most of the Pleistocene (3.5 Ma onwards). This suggests increased food supply to the bottom waters due to higher rates of surface water productivity during the Late Pliocene onwards. A more diverse benthic fauna along with a marked decrease in the abundances of U. proboscidea and infaunal taxa during the latest Pliocene (,2.3±1.9 Ma) suggests that a distinct decline in the primary productivity of overlying surface waters has reduced the supply of organic carbon to the bottom waters.
Acknowledgements A.K. Rai wishes to thank ODP for providing samples. Thoughtful reviews by M.V.S. Guptha signi®cantly improved the quality of the manuscript. Also we thank the reviewers W.A. Berggren, J.W. Murray and Stefan W. Nees for their valuable comments. This research was supported by the grants of Council of Scienti®c and Industrial Research, Government of India to AKR (No. 24 (0231)/96/ EMR-II).
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Contributions to XV Indian Colloquium on Micropaleontology and Stratigraphy. Allied Printers, Dehra Dun, India, pp. 267±273. Sanders, H.L., 1969. Benthic marine diversity and the Stability-time hypothesis. In: Woodwell, G.M., Smith, H.H. (Eds.), Diversity and stability in ecological systems, 22. Brookhaven National Laboratory, New York, pp. 71±81. Slobodkin, L.B., Sanders, H.L., 1969. On the contribution of environmental predictability to species diversity. In: Woodwell, G.M., Smith, H.H. (Eds.), Diversity and stability in ecological systems, 22. Brookhaven National Laboratory, New York, pp. 82±95. Smart, C.W., Murray, J.W., 1995. Miocene deep-sea benthic foraminifera from the Atlantic and Indian Oceans: diversity patterns and palaeoceanography. Rev. EspanÄola de Paleontologia, 59±68. Smart, C.W., King, S.C., Gooday, A.J., Murray, J.W., Thomas, E., 1994. A benthic foraminiferal proxy of pulsed organic matter paleo¯uxes. Mar. Micropaleontol. 23, 89±99. Smith, W., Grassle, J.F., 1977. Sampling properties of a family of diversity measures. Biometrics 33, 283±292. Tchernia, P., 1980. Descriptive Regional Oceanography. Pergamon, Oxford, p. 252. Thistle, D., 1998. Harpacticoid copepod diversity at two physically reworked sites in the deep sea. Deep-Sea Res. II. 45, 13±24. Thomas, E., 1985. Late Eocene to Recent deep-sea benthic foraminifera from the central equatorial Paci®c Ocean. Init. Rep. DSDP 94, 997±1031. Thomas, E., 1986. Changes in composition of Neogene benthic foraminiferal fauna in equatorial Paci®c and North Atlantic. Palaeogeogr., Palaeoclimatol., Palaeoecol. 53, 47±61. Thomas, E., 1990. Late Cretaceous through Neogene deep-sea benthic foraminifers (Maud Rise, Weddell Sea, Antarctica). Proc. ODP Sci. Results 113, 571±594. Thomas, E., Booth, L., Maslin, M., Shackleton, N.J., 1994. Northeastern Atlantic benthic foraminifera during the 45,000 years: changes in productivity seen from the bottom up. Paleoceanogr. 10, 545±562. Tomczak, M., Godfrey, J.S., 1994. Regional Oceanography: An Introduction. Pergamon, London. Valentine, J.W., 1973. Resources supply and species diversity patterns. Lethaia 4, 51±61. Wells, P., Wells, G., Cali, J., Chivas, A., 1994. Response of deepsea benthic foraminifera to late Quaternary climate changes, southeast Indian Ocean, offshore western Australia. Mar. Micropaleontol. 23, 185±229. Woodruff, F., 1985. Changes in Miocene deep-sea benthic foraminiferal distribution in the Paci®c Ocean: Relationship to paleoceanography. In: Kennett, J.P. (Ed.), The Miocene Ocean: paleoceanography and biogeography. Geol. Soc. Am. Mem. 163, pp. 131±175.
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Late Neogene deep-sea benthic foraminifera at ODP Site 762B, eastern Indian Ocean: diversity trends and palaeoceanography A.K. Rai*, V.B. Singh Department of Earth and Planetary Sciences, Allahabad University, Allahabad 211-002, India Received 13 June 2000; accepted for publication 1 May 2001
Abstract In the present investigation, the Late Neogene deep-sea benthic foraminiferal diversity at ODP Site 762B has been measured by using Hurlbert's diversity index (Sm). This index measures the expected number of species to a common sample size (m). Here, m is put equal to 100. This method is relatively more advantageous than other commonly used diversity indices because it eliminates differences in sample size. The Late Neogene section at Site 762B is characterised by marked variations in species diversity (S100) of the deep-sea benthic foraminiferal assemblages. Species diversity broadly exhibits an inverse relationship with the values of Uvigerina proboscidea abundance and percentage of infaunal taxa in the examined section of the site. This suggests that temporal variations in primary productivity of the overlying surface waters are re¯ected in the Late Neogene deepsea benthic foraminiferal diversity trends. The species diversity pattern explains that the trophic level of the bottom water changed at the beginning of the Late Pliocene (,3.5 Ma) due to an increased surface water productivity. The higher values of species diversity during the Early Pliocene was due to a stable deep-sea environment with low trophic resource levels. However, the Late Pliocene and Pleistocene interval is largely characterised by low diversity re¯ecting an unstable deep-sea environment with higher surface water productivity. A distinct increase in the values of species diversity along with low percentages of U. proboscidea and infaunal taxa during the latest Pliocene (,2.3±1.9 Ma) suggests low organic carbon in¯ux in the bottom waters due to a decrease in the surface water productivity. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Neogene; Benthic foraminifera; Indian Ocean; Diversity; Palaeoceanography
1. Introduction Species diversity is a valuable method to measure the complexity of faunal or ¯oral distribution in palaeontological samples. The main characteristic of the measure of species diversity is the reduction of information provided by the species distribution in a sample to a single member (Malmgren and Sigaroodi, 1985). There has been a growing awareness in
* Corresponding author. E-mail addresses: [email protected] (A.K. Rai).
understanding the ecological and evolutionary signi®cance of species diversity. The modern deepsea is characterised by higher species diversity of many groups of organisms (Gage and Tyler, 1991). The earlier assumption of low species diversity in various groups at greater depths was due to low density of the larger invertebrates and also their inadequate sampling. The benthic foraminifera with preservable tests in fossil records have the advantage of relatively high densities with a reasonable number of species and their proportions. Recent studies have suggested several intervals of marked ¯uctuations in the deep-sea benthic
0031-0182/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0031-018 2(01)00299-1
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of western Australia (Fig. 1). The SEC and SIC ¯owing west and north, respectively, are the eastern wing of an anti-clockwise gyre in the south Indian Ocean (Tomczak and Godfrey, 1994). The water ¯ow of the SIC is called the Indian Central Water (ICW). A part of Indonesian through¯ow water (Tomczak and Godfrey, 1994) starts ¯owing southward along the western coast of Australia as the Leeuwin Current (Cresswell and Golding, 1980). Beneath the LC, high salinity waters, and South Indian Central waters, are carried northward by the undercurrent (Cresswell, 1991), acting as a part of the South Indian Current. This current in turn, in¯uences water masses as deep as 2000 m (Tchernia, 1980) and is part of a major Southern Hemisphere gyre, moving anticlockwise in the Indian Ocean (Fig. 1). 2. Material and methods
Fig. 1. Oceanographic setting and location of ODP Site 762B in the eastern Indian Ocean. Depth contours are in kilometres. (ITF: Indonesian Through¯ow; SEC: South Equatorial Current; LC: Leeuwin Current; SIC: South Indian Current; TRANS: Tropical±Subtropical Transition Zone.)
foraminiferal diversity throughout the Cenozoic (e.g. Thomas, 1985, 1986; Woodruff, 1985; Rai and Srinivasan, 1992; Smart and Murray, 1995; Bornmalm, 1997). Considering the vastness of size and varying water depths of the ocean, there is still very little information available on species diversity. In the present work, benthic foraminifera at ODP Site 762B in the eastern Indian Ocean have been selected for the study of their diversity pattern during Late Neogene. Site 762B is situated at 19853.24 0 S, 112815.24 0 E in 1360 m water depth near Exmouth Plateau in the eastern Indian Ocean, offshore western Australia (Fig. 1). This site is located near the deep oxygen minimum zone and also in the tropical to subtropical transition zone (,208S to 158S) in the eastern Indian Ocean. The high planktic foraminiferal concentration in this zone agrees with the high primary productivity of this area (Nomura, 1995). The South Equatorial Current (SEC), the Leeuwin Current (LC) and South Indian Current (SIC) are the parts of the current system off the coast
The present investigation is based on 76 subsamples from a 108.9 m long core. These samples are 10 cm 3 plugs of sediment commonly taken at about 1.5 m intervals. The Late Neogene section at this site comprises dominantly of foram±nanno ooze. The samples were treated with 5% hydrogen peroxide solution and water (1:3 ratio) and wet sieved through 63 mm and 149 mm Tyler sieves. Though the H2O2 is aggressive to carbonates, the concentration is too low (,2%) to affect the tests of benthic foraminifera. Therefore, H2O2 treatment may not matter much for the overall diversity structure. Benthic foraminifera are well preserved and generally do not constitute more than 5% of the total foraminiferal population. After drying, a microsplitter was used to separate a representative proportion of the .149 mm fraction estimated to contain more than 300 specimens of benthic foraminifera. The .149 mm fraction was selected for micropalaeontologic analyses because it provides most climatic information in short time (Imbrie and Kipp, 1971) and is the size fraction adopted by many major palaeoclimatic studies, particularly in the Indian Ocean (Corliss, 1979; Gupta and Srinivasan, 1992a,b; Wells et al., 1994; Nomura, 1995 and others). Thus, the results of the present study can be suitably compared with other Indian Ocean works. The benthic foraminiferal quantitative data have been used to calculate
A.K. Rai, V.B. Singh / Palaeogeography, Palaeoclimatology, Palaeoecology 173 (2001) 1±8 Table 1 Characteristic species of shallow and other infaunal taxa (after Wells et al., 1994) Shallow infaunal taxa
Other infaunal taxa
Bulimina spp., Pullenia bulloides, Uvigerina peregrina
Cassidulina laevigata, Ehrenbergina trigona, Melonis barleeanum, Pullenia spp., Uvigerina spp.
Hurlbert's diversity index, Sm (Hurlbert, 1971). This index, Sm, denotes the ªexpected species diversityº by Smith and Grassle (1977), permits standardisation of species-counts to a common sample size. Consequently, with this method it is possible to estimate the number of species (Sm) expected in a sub-sample of m specimens taken at random from a sample of N specimens m # N: This method has advantage over more commonly used diversity indices such as the Simpson index and Shannon Wiener information index in that it eliminates differences in sample size. The Hurlbert's diversity index is de®ned by the function: Sm
S X
1 2 C N 2 Ni; m=C N; m
i1
where Sm is the expected number of species in a random sub-sample of size m m # N: In the present study we set m at 100, well below the lowest number of specimens counted per sample. N is the number of specimens in the sample and S is the number of species in which N specimens are distributed. Ni is the number of individuals in the ith species, SNi N: C N 2 Ni; m N 2 Ni!=m! N 2 Ni 2 m! and C N; m N!=m! N 2 m! for N 2 Ni $ m and N $ m; respectively, and zero for N 2 Ni , m and N , m; respectively (Smith and Grassle, 1977). The percentages of shallow infaunal and other infaunal taxa have been calculated following Wells et al. (1994). In this study the term `shallow infaunal taxa' refers only to those taxa for which such a microhabitat preference is well known (see Wells et al., 1994). A list of characteristic species of shallow and other infaunal morphogroups is given in Table 1. The species diversity (S100) pattern is also compared with the percentage of Uvigerina proboscidea and total number of benthic foraminiferal specimens per gram dry weight of .149 mm size
3
fraction (Fig. 2). The original data set is available from the ®rst author. 3. Results The sample size was standardised at 100 individuals (S100) using Hurlbert's method. Values of Hurlbert's diversity index (S100) and total number of benthic foraminiferal specimens per gram dry sediment (spec./g) are plotted at Site 762B (Fig. 2). Results are shown on newly integrated chronologies of Berggren et al. (1995). The species diversity index varies from 24 to 39 species per 100 specimens with an average value of 33 (Fig. 2). In general, the values of S100 remain relatively high during the latest Miocene and Early Pliocene period with a few intervals of minor ¯uctuations. At the beginning of the Late Pliocene (,3.5 Ma), species diversity shows a distinct decrease and remains relatively low with few intervals of marked ¯uctuations during most of the Late Pliocene and Pleistocene. The diversity also exhibits greater variations during the Late Pliocene and Pleistocene. Percentages of infaunal taxa and Uvigerina proboscidea also show a sudden increase at about 3.5 Ma and remain relatively high in the rest of the younger section (Fig. 2). U. proboscidea abundance, percentage of infaunal taxa and total benthic foraminiferal abundance show an almost opposite trend to the values of species diversity. The early part of Late Pliocene is broadly characterised by less diverse fauna along with higher percentages of infaunal taxa and U. proboscidea abundances. However, the latest Pliocene (,2.3±1.9 Ma) is marked by relatively higher species diversity and low percentages of infaunal taxa and U. proboscidea. The early and middle Pleistocene (.1.8±0.4 Ma) intervals show low species diversity with marked ¯uctuating trends. This period is also marked by several pulses of distinctly higher abundances of U. proboscidea (.30%) and dominance of infaunal taxa. The Late Pleistocene (,0.2 Ma to Holocene) is characterised by relatively higher values of species diversity with corresponding low abundances of U. proboscidea and moderately higher occurrences of infaunal taxa.
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Fig. 2. Plots of Hurlbert's diversity index (S100), percentage of Uvigerina proboscidea, percentage of total infaunal taxa (dotted line for % of shallow infaunal and solid line for other infaunal) and total number of benthic foraminiferal specimens per gram dry weight of .149 mm size fraction (spec./g) at ODP Site 762B. Ages are taken from Berggren et al. (1995).
4. Discussion Recent research has revealed that the deep-sea environment is not as stable as was previously realised. However, it is also in¯uenced by shortterm changes, such as benthic storms, which can substantially disturb the sediment surface (Kaminski, 1985). The deep-sea trophic level is highly in¯uenced by the seasonal input of phytodetritus, which increases the abundance of a few opportunistic species and generally promotes benthic activity (Gooday, 1994). The seasonal input of phytodetritus is an environmental disturbance to which certain opportunistic foraminifera respond very rapidly and their assemblages show low diversity (Gooday, 1988). This is because the limited numbers of species
colonise the detritus and the resulting increase in the abundance of these species causes an overall decline in diversity. In general, interpretations of fossil faunal diversity are not simple because many biotic and abiotic factors affect the composition of fossil assemblages. However, some important factors controlling species diversity include changes of trophic (resource) level and stability of the environment (Gage and Tyler, 1991; Sanders, 1969; Valentine, 1973). The magnitude of phytodetritus in¯ux largely controls the trophic resource or the food availability for benthic foraminifera (Gooday, 1988) which is related to the surface productivity (Smart et al., 1994; Thomas et al., 1994). Thus, the diversity pattern may be useful to understand the changes in the deep-sea environment
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Fig. 3. Comparison of the relative abundance of Uvigerina proboscidea and Hurlbert's diversity index (S100) at ODP Site 762B. Correlation coef®cient is computed using running mean data set.
which may be helpful in understanding global climatic and palaeoceanographic changes. The low diversity may represent a period of new environment or a severe environment for the pre-existing assemblage (Slobodkin and Sanders, 1969). Sudden environmental change may also be responsible for a less diverse assemblage. Stability of the environment through geologic time is suggested to be the major factor for higher diversity in the deep-sea (Gage and Tyler, 1991; Buzas and Gibson, 1969; Buzas, 1972). However, Dayton and Hessler (1972) suggested that biological disturbance such as cropping was possibly responsible to promote diversi®cation. Recent explanations of high diversity in the deep sea have concentrated on processes maintaining diversity, especially patch dynamics (Thistle, 1998). These have included the generation of patch mosaics by disturbance (e.g. Grassle and Maciolek, 1992; Lambshead, 1993) or by the activities of deep-sea organisms that modify the benthic environment (e.g. Jumars, 1976; Gage, 1996). Earlier studies have shown that the Cenozoic species diversity is in¯uenced by the changes in the nature of the deep-sea environment caused by tectonics (gateways) (Woodruff, 1985) and/or climatically induced changes in ocean circulation (Thomas, 1985, 1990; Rai and Srinivasan, 1992; Gupta and Srinivasan, 1992a) and magnitude of surface water productivity. Gibson and Buzas (1973) have discussed that species diversity may be de®ned by the carrying capacity of its environment, which is reached rather
5
quickly on a geological time scale. Most of the Neogene deep-sea benthic foraminiferal species are geologically long ranging. The temporal changes in the species diversity are likely to be due to the shortterm disappearance and reappearance of long ranging taxa (Flessa and Jablonski, 1983). Recent studies such as those of Gooday (1988, 1993) and Smart et al. (1994) have documented benthic foraminiferal responses to sudden, sometimes seasonal in¯uxes of phytodetritus. It appears that some species are able to take advantage of this ¯uctuating input of organic matter. At Site 762B, species diversity shows an almost opposite trend with the relative abundance of U. proboscidea and percentage of total infaunal taxa (Fig. 3). The abundance of U. proboscidea and infaunal taxa thus largely de®nes the diversity patterns. U. proboscidea is one of the most dominant benthic foraminiferal species at various bathyal sites in the tropical Indian Ocean during the Neogene. Increased relative abundances of this species have been observed in intervals of high surface water productivity during the Neogene (Nomura, 1995; Gupta and Srinivasan, 1992b; Wells et al., 1994; Rai and Srinivasan, 1994, 1996). Various trophic requirements of species due to their own ecologic niche can be responsible for their infaunal, epifaunal or opportunistic behaviour (Altenbach et al., 1999). The abundance of infaunal vs. epifaunal taxa is considered to be related to the amount of organic matter that reached the ocean bottom and thus, ultimately to the primary productivity in the overlying surface waters (Corliss and Chen, 1988; Wells et al., 1994). Den Dulk et al. (1998) suggested that benthic epifaunal numbers start to drop signi®cantly under the oxygen de®ciency, which coincide with the periods of maximum organic matter supply. Increased burial ef®ciency of organic matter due to severe oxygen depletion is most likely responsible for the good correlation between high Corg concentrations and the proportion of deep infaunal species (Jorissen, 1999; Den Dulk et al., 2000). Recently, Den Dulk et al. (2000) have demonstrated that the deep infaunal species are most abundant under high productivity conditions, and vice versa. Thus, the variations in the deep-sea benthic foraminiferal diversity during the Late Neogene at this site appear to be primarily related to the changes in the trophic level of the
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Fig. 4. Plots of Shannon Wiener diversity index: H(S) and percentage of Uvigerina proboscidea at DSDP Site 214 along the Ninetyeast Ridge, Indian Ocean (from Gupta and Srinivasan, 1992a,b). Ages of the epoch boundaries are modi®ed after Berggren et al., 1995.
bottom waters mainly due to the variations in the rates of surface water productivity. The Early Pliocene at Site 762B is characterised by relatively warm and stable deep-sea environment with low trophic resource levels due to decrease in surface water productivity as revealed by higher values of species diversity, low U. proboscidea abundance and decrease in total benthic foraminiferal abundance. Gupta and Srinivasan (1992b) also marked a decrease in the abundance of U. proboscidea during Early Pliocene at DSDP Site 214 along the Ninetyeast Ridge in the Indian Ocean (see Fig. 4). The earth experienced an interval of global warming (Kennett and van der Borch, 1986; Hodell and Venz, 1992) and increased eustatic sea level (Haq et al., 1987) during the Early Pliocene. At Site 762B species diversity shows a marked decrease near the early/late Pliocene transition (,3.5 Ma) which corresponds with the increase in the number of total benthic foraminiferal specimens. This was also the time of sudden increase in the percentages of U. proboscidea and infaunal taxa. The above variations in the faunal parameters reveal a distinct change in the deep-sea environment, i.e.,
increased trophic level across the early/late Pliocene transition. The early part of the Late Pliocene (,3.5± 2.3 Ma) is characterised by low S100, increase in U. proboscidea percentages and moderately higher percentages of infaunal taxa suggesting an unstable deep-sea environment with relative increase in the ¯ux of the organic carbon to the bottom waters due to increased level of the surface water productivity. The latest Pliocene interval (,2.3±1.9 Ma) is marked by a low organic carbon deep-sea environment due to decrease in the primary productivity of overlying surface waters as evidenced by higher species diversity and very low abundances of U. proboscidea and infaunal taxa. Closely similar faunal changes during latest Pliocene have also been recorded by Gupta (1999) from DSDP Site 214 at the Ninetyeast Ridge, eastern Indian Ocean. Gupta (1999) explained that a weaker Indian monsoon during the latest Pliocene was probably responsible for low surface water productivity and oxygenated bottom waters. The large ¯uctuations in the values of species diversity along with changing percentages of U. proboscidea and infaunal taxa during most of the Pleistocene may possibly be due to rapid changes in the trophic level of the bottom waters in response to the changes in the surface water productivity. Gupta and Srinivasan (1992a,b) observed intervals of low diversity and higher abundances of U. proboscidea at the beginning of the Late Pliocene and the Early Pleistocene at DSDP Site 214 along the Ninetyeast Ridge re¯ecting increased surface water productivity (see Fig. 4). Nomura (1995) recognised the development of U. proboscidea assemblage re¯ecting higher surface water productivity during Pliocene±Pleistocene in the eastern Indian Ocean. The rapid faunal changes during Pleistocene appear to be in response to the glacial and interglacial cycles. A further study of the samples at closer intervals may be useful to identify glacial and interglacial cycles and their responses on the faunal assemblages during the Pleistocene. 5. Conclusions Late Neogene deep-sea benthic foraminiferal diversity (S100) at ODP Site 762B varies between 24 and 39. Species diversity is negatively correlated with the relative abundance of U. proboscidea. In general,
A.K. Rai, V.B. Singh / Palaeogeography, Palaeoclimatology, Palaeoecology 173 (2001) 1±8
intervals of low species diversity (S100) also correspond to the higher percentages of infaunal taxa. Thus, changes in the species diversity during the Late Neogene largely re¯ect variations in the food availability of bottom waters in response to the changing surface water productivity. Relatively higher diversities associated with low abundances of U. proboscidea and infaunal taxa during most of the early Pliocene interval re¯ect a stable deep-sea environment with low trophic levels. However, decrease in species diversity along with a sudden increase in the percentages of U. proboscidea and infaunal taxa occurred during the Late Pliocene and most of the Pleistocene (3.5 Ma onwards). This suggests increased food supply to the bottom waters due to higher rates of surface water productivity during the Late Pliocene onwards. A more diverse benthic fauna along with a marked decrease in the abundances of U. proboscidea and infaunal taxa during the latest Pliocene (,2.3±1.9 Ma) suggests that a distinct decline in the primary productivity of overlying surface waters has reduced the supply of organic carbon to the bottom waters.
Acknowledgements A.K. Rai wishes to thank ODP for providing samples. Thoughtful reviews by M.V.S. Guptha signi®cantly improved the quality of the manuscript. Also we thank the reviewers W.A. Berggren, J.W. Murray and Stefan W. Nees for their valuable comments. This research was supported by the grants of Council of Scienti®c and Industrial Research, Government of India to AKR (No. 24 (0231)/96/ EMR-II).
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