Abstract:
In the present work, late Oligocene to latest Miocene ( approximately 28-6.5 Ma) deep-sea benthic foraminifera (over 149 mu m size fraction) were studied from ODP sites 760A and 761B on Wombat Plateau in the eastern Indian Ocean to understand the faunal diversity pattern. The samples were commonly examined at about 0.75m interval at both the sites. The benthic foraminiferal species diversity was measured in terms of Shannon-Weaver Index [H(S)], Hurlbert's Diversity Index (S (sub 100) ), Alpha Index (alpha ) and Equitability (E'), all of which show almost similar trends with few exceptions. The Miocene section at both the sites show marked fluctuations in the values of various diversity indices of deep-sea benthic foraminiferal assemblages. Distinctly low values of diversity indices across the Oligocene-Miocene transition correspond well with the higher values of benthic delta (super 18) O and delta (super 13) C reflecting relatively cold, nutrient depleted, young bottom waters which possibly indicate the influence of Antarctic Circumpolar Current (ACC) in this region. Faunal diversity reached to maximum values at both the sites near the end of Miocene climatic optimum at about 15 Ma reflecting warm bottom waters with low trophic levels. The low bottom water temperature, expansion of East Antarctic Ice Sheet (EAIS) and possibly higher trophic level appear to be responsible for less diverse fauna during the middle Miocene i.e. approximately 14Ma and younger. Further, decrease in the values of diversity during latest Miocene ( approximately 7.5-6.5 Ma) is suggested to be due to increased surface water productivity in response to the intensified monsoonal activity in the eastern Indian Ocean.