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SYSTEMATIC DECLINE IN BENTHIC FORAMINIFERAL SPECIES DIVERSITY LINKED TO PRODUCTIVITY INCREASES OVER THE LAST 26 MA IN THE INDIAN OCEAN

Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur – 721 302, India

^* E-mail: anilg{at}gg.iitkgp.ernet.in

Deep-sea benthic foraminferal species diversity from the late Oligocene-Holocene sequence of Ocean Drilling Program (ODP) Site 757, Hole B, southeastern Indian Ocean (1652 m water depth), is examined in terms of the information function (H), equitability (E), number of species (S) and Sanders’ rarefied values. The values of these parameters reached their maximum about 18–15 Ma, and thereafter had a continuous stepwise decrease. Productivity increased significantly in the Indo-Pacific Ocean (the "biogenic bloom") and the oxygen minimum zone (OMZ) intensified over large parts of the Indian Ocean beginning about 15 Ma. The diversity values show more abrupt changes since 10–8 Ma, when the strength of the Indian summer monsoon substantially increased and there were significant changes in Southern Hemisphere (SH) and Northern Hemisphere (NH) ice volume. Diversity values decreased abruptly 8–6 Ma (the Chron-6 negative Carbon Shift), 3.2–2.3 Ma (major NH glaciation) and 1.6–0.9 Ma, coinciding with increased percentages of Uvigerina proboscidea, a species indicative of high surface productivity. Thus, we suggest that fluctuations in species diversity at ODP Site 757 were related to changes in productivity during the studied interval.