Late Neogene deep-sea benthic foraminifera at ODP Site 762B, eastern Indian Ocean: diversity trends and palaeoceanography

In the present investigation, the Late Neogene deep-sea benthic foraminiferal diversity at ODP Site 762B has been measured by using Hurlbertʹs 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 reflected in the Late Neogene deep-sea 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 reflecting 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 influx in the bottom waters due to a decrease in the surface water productivity.