Surface paleoceanography of the eastern equatorial Indian Ocean since the latest Miocene: Foraminiferal census and isotope records from ODP Hole 758A

Abstract

The surface hydrography of the Indian Ocean (north of the equator) is influenced by the seasonal wind reversals, which affect the thermocline depth and mixed layer thickness as well as sea surface temperature (SST). We analyzed the surface-subsurface hydrographic changes in the Tropical Eastern Indian Ocean (TEIO) during the late Neogene and Quaternary, using planktic foraminiferal census data from ODP Hole 758A combined with surface-dwelling planktic foraminiferal stable oxygen and carbon isotope ratios. The occurrence of dominant thermocline-dwelling species and high surface productivity during the latest Miocene - Early Pliocene (between 6 and 3.4 Ma) reflects an intense Indian summer monsoon (ISM). Deepening of the thermocline and thickening of the mixed layer in the TEIO were driven by the weakening of the ISM after 3.4 Ma, causing fluctuations in surface productivity. Heat budget capacity increased after 3.4 Ma in the TEIO due to the thickening of the mixed layer, which may have also reduced the heat transport towards the South Pole. Further, the hydrography of the TEIO was affected by the advection of nutrient-rich saline water from the western Indian Ocean by the strong equatorial eastward flow between 2.7 and 1.2 Ma. We suggest that the development of the equatorial zonal Westerlies (Walker circulation) across the tropical Indian Ocean was a gradual process starting at ~3.4 Ma, which intensified at 2.7 Ma due to regional strengthening of the Indian winter monsoon.