Species diversity of Miocene deep-sea benthic Foraminifera and watermass stratification in the Northeastern Indian Ocean

Abstract

The Miocene species diversity of deep-sea benthic foraminifera (expressed as alpha index, information function H[S], and Sanders' rarefaction values) at DSDP Sites 214 (1671 m) and 216 (2262 m), and ODP Site 758 (2923 m) in the northeastern Indian Ocean was overall higher than the modern diversity, but with major variations. The alpha and H(S) values were relatively low at the shallowest Site 214, medium at intermediate-depth Site 216, and highest at the deepest Site 758. Across the Oligocene/Miocene boundary, when the Antarctic Circumpolar Current (ACC) might have originated, diversity dropped at Sites 214 and 758. All diversity parameters decreased for a short time at about 17 Ma at all sites (more prominent at Site 216), at which time production of Northern Component Water (NCW) may have peaked. At shallow Site 214 and deep Site 758, the alpha and Sanders' values show an abrupt decrease at about 12.5 Ma, a time of major expansion of the East Antarctic Ice Sheet (EAIS) and increased production of Antarctic Bottom Water (AABW), Northern Component Water (NCW) and Indonesian Intermediate Water (IIW). These decreases in diversity are not seen in the records of Site 216. In the latest Miocene (about 7.15 to 6.5 Ma) the alpha and Sanders' values at Sites 214 and 758 decreased further, during the Chron-6 global carbon shift and the Indo-Pacific biogenic bloom in productivity. The H(S) values also show a decrease in this interval at Sites 214 and 758. We suggest that deep-ocean circulation played a significant role in shaping the long-term diversity trends in the northeastern Indian Ocean. Productivity, which might in turn have been influenced by the circulation changes, dominantly affected the diversity in the latest Miocene.