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Marine biodiversity variation in the low-latitude region Possible Impact of Polar Climatic Perturbations A. D. Singh, Centre of Advanced Study in Geology Banaras Hindu University, Varanasi-221005 In order to understand response of marine biota to changes in ocean environment linked to global climatic fluctuations, it is essential to study how various biotic groups were evolved in the oceans, their diversifications, and selective extinctions during certain geological periods and its possible causes?. Marine sediment records have been the best storehouse of history of changes in oceans’s environments (physico-chemical) occurred in the past and its impact on the marine life. Valuable information about the past biological changes in oceans are extracted from the shells preserved in sediments (as fossils) that were secreted by microscopic fauna and flora remarkably sensitive to minor changes in oceanic environment. Studies have suggested that climatic perturbations triggered changes in ocean environment (sea surface temperature, sea level, ocean currents, surface and bottom circulations, nutrient condition and oxygen level etc.) causing major changes in marine biota. The changes in physicochemical condition of ocean influence profoundly marine biotic communities in respect of their biogeographic distribution, abundances, diversities and morphologies. We considered foraminfera for our study as they are very sensitive to changes in ocean environment. Our study reveals a general low diversity of benthic foraminifera during 15 to 30, 000 years ago, a period of glacial stage when northern Hemisphere glaciation enhanced, followed by a gradual increase in diversity reaching to the modern values in the Holocene period which is equivalent to the warm interglacial stage. Within the glacial stage, there have been periods of intensified cold events as recorded from the Greenland ice core, during which benthic diversity in the Arabian Sea increased. The diversity fluctuated in the warm Holocene period and that also probably to be linked with polar climate perturbations. 66 ❘ Science & Geopolitics of Arctic & Antarctic: SaGAA 2011 Session: 2 Introduction There is increasing concern regarding the impacts of climate change on marine biodiversity and its conservation and management. Although the oceans cover more than 70 % of the Earth’s surface, our knowledge on marine biodiversity patterns is limited as compared to that of the terrestrial biodiversities. In order to understand response of marine biota to changes in ocean environment linked to global climatic fluctuations, it is essential to study how various biotic groups were evolved in the oceans, their diversifications, and selective extinctions during certain geological periods and its possible causes? Recently, a group of scientists is of the opinion that climate change (what we are witnessing today?) can lead extinction of certain marine species (eg. Thomas et al., 2004). But, such statement should be taken into consideration with a caution, because influence of climatic perturbations on marine biota (animals and plants) is complex and ocean specific. In view of this, it has become necessary to study history of changing biotic patterns in oceans that took place in the geological past as a consequence of major changes in oceanographic and climatic conditions. Marine sediment records have been the best storehouse of history of changes in oceans’s environments (physicochemical) occurred in the past and its impact on the marine life. Valuable information about the past biological changes in oceans are extracted from the shells preserved in sediments (as fossils) that were secreted by microscopic fauna and flora remarkably sensitive to minor changes in oceanic environment. Recent decades have witnessed a rapid developments in various aspects of ocean research (both modern and past ocean environments), with the birth of a new discipline in Earth System Science i.e. Paleoceanography dealing with the evolution of the ocean basins, its circulation history and hydrographic, climatic and biotic patterns. As a result , several International ocean drilling programmes have been initiated that enabled to recover a large number of sediment cores from the world oceans, rich in microfossils of groups such as foraminifera, pteropods, ostracodes, nannoplanktons, radiolarian and diatoms. The oceanic microfossil records are not only important to portray the history of changes in biotic patterns, but also crucial to better understand how global climate change in the past has influenced marine biota. Studies have broadly revealed that periods of evolution and diversification of planktons in oceans were coincident with the major climatically induced oceanographic changes (Wei and Kennett, 1986). Evidences have suggested that climate played a major role in promoting unprecedented increase in marine biodiversity around 460 million years ago (Trotter et al., 2008). During this period the ocean surface temperature was estimated as similar to that of today. The physico-chemical and biological environments of modern oceans are closely coupled with characteristics (waxing and waning) of polar ice caps. Before around 60 million years ago, the global climate was warm and geographical thermal gradients were low (Douglas and Savin, 1975). Oceanic surface temperature of 100C was at high latitudes and 200C at low latitudes. Significant changes occurred during 45-60 million years ago, which was a transitional phase between earlier thermospheric circulation and later the psychrospheric and thermohaline circulation. The history of present day glacial mode of Earth was initiated during 25-30 million years ago with the beginning of ice build up on southern pole. Subsequently, major diversification in foraminiferal group took place during 22-23 million years ago (beginning of the Neogene period), when global climatic condition and ocean circulation had changed significantly resulting major shifts in marine trophic conditions (Kennett and Srinivasan, 1983). In the last 60 million years of the Earth, s history, this was the period of an overall increase in marine biodiversity (Fig. 1). The latest major climatic-oceanographic event was the development of an ice sheet in the northern Hemisphere (Arctic region) during 2.5-3.0 million years ago. Thus, prior to 3 million years, polar ice sheets were restricted to Antarctica, and a bipolar symmetry in global climate was developed only after development of permanent ice sheets in the arctic Science & Geopolitics of Arctic & Antarctic: SaGAA 2011 ❘ 67 region (Srinivasan, 2003). The climatic studies based on the ice cores (polar ice sheets and high altitude regions), marine and terrestrial archives have revealed that the Earth has experienced cycles of cold (glacial: ice ages) and warm (interglacial) phases during the last 900, 000 years. The glacial phases are characterized by the expanded polar ice sheets, low atmospheric CO2 and low surface temperature. Super imposed on the glacial-interglacial cycles, there has been climate variability on shorter time scales (decadal, centennial and millennial) [eg. Dansgaard et al., 1993]. Studies further suggested that climatic perturbations triggered changes in ocean environment (sea surface temperature, sea level, ocean currents, surface and bottom circulations, nutrient condition and oxygen level etc.) causing major changes in marine biota. The changes in physicochemical condition of ocean influence profoundly marine biotic communities in respect of their 68 ❘ Science & Geopolitics of Arctic & Antarctic: SaGAA 2011 biogeographic distribution, abundances, diversities and morphologies. The nature of biotic patterns depends on sensitivity of biota and how do they respond to changes in various ecological parameters. In view of present scenario of changing climate, studies were undertaken on microfossil records of calcareous foraminifera from the Arabian Sea region generated from ocean bottom sediment archives to evaluate , if there has been any change in the diversity patterns in the past and if yes, was it linked with the climatic perturbations at high latitudes. Foraminfera are very sensitive to changes in ocean environment and this group comprises two subgroups one that lives in surface waters and other on ocean bottom, thus providing information about biotic responses to both the ocean surface and bottom conditions. On longer time scale, it has been observed that diversities of both the planktic and benthic were generally high with a gradual minor decline since 2.5 Session: 2 Science & Geopolitics of Arctic & Antarctic: SaGAA 2011 ❘ 69 million years ago coincident with the establishment of northern Hemisphere glaciations (Fig. 2). A high resolution time series of foraminiferal diversity record for the last 30, 000 years was also generated based on the Arabian Sea sediment core (Fig. 3). The study reveals a general low diversity of benthic foraminifera during 15 to 30, 000 years ago, a period of glacial stage when northern Hemisphere glaciation enhanced, followed by a gradual increase in diversity reaching to the modern values in the Holocene epoch which is equivalent to the warm interglacial stage. Within the glacial stage, there have been periods of intensified cold events as recorded from the Greenland ice core, during which benthic diversity in the Arabian Sea increased. The diversity fluctuated in the warm Holocene period and that also probably to be linked with polar climate perturbations. It is intriguing to record that diversity of planktic foraminifera did not change significantly in the last 30, 000 years as was seen in the benthic foraminifera. To evaluate the probable reasons for diversity variations in this region, the faunal diversity records were compared with the records of primary productivity and sea surface temperature that are known to be governed by the monsoon circulation pattern and greatly influenced by the global climatic perturbations (Singh et al., 2006, Anand et al., 2008). Data suggest that primary productivity and sea surface temperature in the Arabian Sea have significantly varied in the past in concert with the high latitude climatic perturbations. Variation in primary productivity also results changes in bottom oxygen condition with oxic environment during low productivity period and oxygen poor environment during high productivity period. Combined effects of these factors (primary productivity and sea bottom oxygen environment) seem to have influenced the benthic foraminiferal species diversity in recent past. On the other hand changes in hydrographic conditions do not appear to significantly influence the diversity pattern of planktic foraminifera. Important Issues to be addressed: (1) Has marine diversity changed through time and 70 ❘ Science & Geopolitics of Arctic & Antarctic: SaGAA 2011 space since pre-industrial era to post-industrial period ?. (2) How did different marine faunal and floral species particularly in coastal ecosystems respond to environment changes related to industrial developments? (3) Will it be possible to separate natural changes in marine biotic communities from anthropogenic changes?. (4) Are ocean waters in the Arabian Sea and Bay of Bengal acidifying in recent years ? if yes, how does it affect calcifying biota in the two basins ?. References: Thomas, C.D. et al., 2004. Nature, 427, 145-148. Wei, K-Y. and Kennett, J.P., 1986. Paleoceanography, 1, 67-84. Trotter, J.A. et al., 2008. Science, 321, 550-554. Douglas, R.G. and Savin, S.M., 1975. Initial Reports DSDP, 32, 509-520. Kennett, J.P. and Srinivasan M.S., 1983. Neogene planktonic foraminifera: A phylogenetic Atlas, Hutchinson Ross Publication, USA, 265p. Dansgaard, W. et al., 1993. Nature, 364, 218-220. Singh, A.D. et al., 2006. Geological Society of India, 68, 369-377. Anand, P et al., 2008. Paleoceanography, 23, PA4207, doi:10.1029/2007PA001564. Srinavasn, M.S., 2003. 32nd Prof. Birbal Sahni Memorial Lecture, Lucknow