Late Oligocene–Miocene paleoceanographic evolution of the southeastern Indian Ocean: evidence from deep-sea benthic foraminifera (ODP Site 757)

Abstract

We quantitatively analyzed deep-sea benthic foraminifera from >125 μm size fraction from Ocean Drilling Program (ODP) Site 757 to understand deep-sea paleoceanographic changes in the southeastern Indian Ocean during the late Oligocene-Miocene (26.5-5.35 Ma). We used the knowledge of the ecology of Recent deep-sea benthic foraminifera for environmental interpretations. Factor and cluster analyses were run using percentages of 46 highest-ranked species that helped identify six biofacies defining six clusters of samples. The faunal data document a major shift in deep-sea ventilation, organic carbon flux and productivity at ~8.3 Ma, coinciding with the major intensification of the Indian Ocean monsoon system. This marks a change from cold and well-oxygenated deep waters with low and pulsed organic carbon flux during 26.5 to ~8.3 Ma to oxygen-poor deep waters with sustained flux of organic matter. From 15.0 to 7.6 Ma, the deep-sea currents were stronger in the southeastern Indian Ocean probably due to major expansion of the East Antarctic ice sheet and increased production of deep waters and in turn increased deep-sea circulation. The productivity increased in the Indo-Pacific region in the late Miocene and the modern monsoon regime probably established at 8.3 Ma.