Stable oxygen isotopic composition (δ18O) of foraminifera from the sediments of the Northern Indian Ocean has been used to decipher past changes in the intensity of the South Asian Monsoon. The interpretation of planktonic foraminifera δ18O is mostly based on a combination of sea surface temperature, monsoon runoff and the global ice-volume effect.
During summer, the eastward flowing summer monsoon current carries the high salinity water from the Arabian Sea to the Bay of Bengal, whereas the Bay of Bengal receives fresh water both from direct rains and runoff from the Ganga-Brahmaputra rivers. This low salinity water is transported from the Bay of Bengal to the southeastern Arabian Sea during winter by the winter monsoon current (WMC) driven by the dry northeasterly winds. The West India Coastal Currents (WICC) carries this low saline water from southeastern Arabian Sea to the northern Arabian Sea (Fig. 1b). During the winter monsoon, high precipitation occurs over southeastern India (the southern states of Tamil Nadu and southeast coastal Andhra Pradesh), while rainfall at this time is quite low over Kerala compared to the summer monsoon.
Duplessy (1982), analyzing several sediment cores from the Bay of Bengal, showed that the summer monsoon rain was significantly reduced during the Last Glacial Maximum (LGM). Oxygen isotopes of LGM planktonic foraminifera were relatively higher, resulting from reduced freshwater discharge from the major monsoon fed Indian rivers. Likewise, Sarkar et. al. (1990) showed that the winter monsoon rains intensified during LGM by analyzing four different species of planktonic foraminifera from a sediment core from the Eastern Arabian Sea. Kudrass et. al. (2001) showed that the runoff into the Bay of Bengal had steadily increased from 21 ka (LGM) to ~4ka.
Using a fresh data set (Deshpande et. al., 2013) of seawater δ18O and salinity from the Eastern Arabian Sea (Fig. 1a), we investigated the seasonal variation in the salinity-δ18O relation in the Eastern Arabian Sea, dividing the latter into four major regions (Fig. 1a), three of which fall under the influence of the WICC, and one under the influence of the WMC. The seasonal variation in the salinity-δ18O relation for all four regions shows (except for region II that receives copious summer monsoon runoff from the Western Ghats), the summer monsoon runoff from the subcontinent to the Bay of Bengal, propelled by winter monsoon winds towards the Arabian Sea, appears to be the dominant control on the modern salinity-δ18O relation in the surface regions of the eastern Arabian Sea.
Deshpande, R. D., Muraleedharan, P.M., Raj Laxmi Singh., Bhishm Kumar., Someshwer Rao, M., Medha Dave., Sivakumar, K.U., Gupta, S.K., 2013. Spatio-temporal distributions of δ18O, δD and salinity in the Arabian Sea: Identifying processes and controls. Journal of Marine Chemistry 157, 144-161.
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Kumar, P.K., Ramesh, R. 2016. Revisiting reconstructed Indian monsoon rainfall variations during the last∼ 25 ka from planktonic foraminiferal δ18O from the Eastern Arabian Sea. Quaternary International. doi.org/10.1016/j.quaint.2016.07.012
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