Evidence of a South Asian Proto?Monsoon During the Oligocene?Miocene Transition

The geological history of the South Asian monsoon (SAM) before Pleistocene is not well-constrained, primarily due to a lack available continuous sediment archives. Previous studies have noted an intensification SAM precipitation and atmospheric circulation during middle Miocene (?14 Ma), but no records are test how changed prior this. In order improve our understanding monsoonal evolution, geochemical sedimentological data were generated for Oligocene-early (30–20 Ma) from Indian National Gas Hydrate Expedition 01 Site NGHP-01-01A in eastern Arabian Sea, at 2,674 m water depth. We find initial glaciation phase (23.7–23.0 Oligocene-Miocene transition (OMT) be associated with increase column ventilation mass mixing, suggesting winter type circulation, possibly driven by relative southward shift intertropical convergence zone. During latter part OMT, or “deglaciation” (23.0–22.7 long-term decrease Mn (suggestive deoxygenation), Ti/Ca dissolution biogenic carbonate fraction suggest proto-summer system, characterized formation oxygen minimum zone Sea terrigenous material delivered runoff site. With evidence this site active OMT we that changes orbital parameters, as well changing Tethyan/Himalayan tectonics, caused step change proto-monsoon system intermediate-depth