Effect of Miocene paleoccanographic changes on the benthic foraminiferal diversity at ODP site 754A (southeastern Indian Ocean)

摘要

Miocene deep sea benthic foraminiferal diversity at ODP site 754A, in the southeastern Indian Ocean was examined in the present study. Paleoccanographic and paleoclimatic changes were inferred based on the above study. Species diversity was measured in terms of Shannon-Weaver Index [H(S)], Hurlbert's Diversity Index (S_(100)), Alpha Index (α) and Equitability (E'). Values of S_(100) show more positive correlation with H(S) and a value. All diversity parameters show almost similar pattern during the study interval. Present study infers that the changing trophic level has much influence on the faunal diversity. More diverse fauna of late early Miocene to early middle Miocene along with low δ~(18)O and high δ~(13)C values reflect warm and stable bottom waters with low food supply. This corresponds to the Miocene climatic optimum. This was also an interval of global wanning and reduced Antarctic ice-sheet volume. Significant decline in diversity and δ~(13)C values along with abrupt increase in δ~(18)O values at ～14 Ma indicate cold, relatively less oxygenated and unstable bottom waters with increased food supply due to higher surface productivity. Development of thermal gradient between low and high latitudes in response to the expansion of Antarctic ice-sheet during 15-12.5 Ma was responsible for increased upwelling and high surface productivity. Decrease in faunal diversity at ～7 Ma onwards along with low δ~(13)C and high δ~(18)O values corresponds with the Chron-6 negative Carbon shift, expansion of west Antarctic ice-sheets and lowering of sea level.