Changes in the Indian Ocean surface hydrography driven by the seaway closure and monsoonal circulation since the late Oligocene

Closure of the East Tethyan and Indonesian gateways and development of the Indian or South Asian monsoon (SAM) wind system had profound control on Indian Ocean circulation. However, the dynamics of the Indian Ocean surface hydrography since the late Oligocene hitherto remains poorly constrained. Planktic foraminiferal relative abundance and stable isotope data from ODP Site 758A, eastern equatorial Indian Ocean (EEIO) reveal a conspicuous thick mixed layer between-25.6 and 14.7 Ma, except for a short-lived thinning from 20.7 to 19.4 Ma. We suggest that existence of a thick mixed layer and perhaps higher sea surface temperatures in the EEIO increased convection and atmospheric moisture transport to the southern hemisphere, which contributed to the expansion of Antarctic ice volume. The East Tethyan Seaway closure and constriction of the Indonesian Gateways cooled the surface and deep waters at-14.7 Ma when the mixed layer thickness began to decrease and thermocline began to shoal. Seasonal fluctuations between mixed layer and thermocline were secular from 25.6 to 12.9 Ma which became more intense and frequent in the younger interval. A shift in the surface hydrography with high frequency changes at-12.9 Ma was driven by the onset of the SAM wind system, leading to the development of the modern summer monsoon surface circulation in the Indian Ocean.