Past 100 ky surface salinity-gradient response in the Eastern Arabian Sea to the summer monsoon variation recorded by delta super(18)O of G. sacculifer

• Published in: Global and planetary change 2005-07, Vol.47 (2-4), p.135-142
• Main Authors: Chodankar, Anjali R, Banakar, Virupaxa K, Oba, Tadamichi
• Format: Article
• Language: English
• Subjects: Marine
• ISSN: 0921-8181
• DOI: 10.1016/j.gloplacha.2004.10.008

Northward flowing coastal currents along the western margin of India during winter-spring advect low-salinity Bay of Bengal water in to the Eastern Arabian Sea producing a distinct low-salinity tongue, the strength of which is largely governed by the freshwater flux to the bay during summer monsoons. Utilizing the sedimentary records of delta super(18)O sub(G. sacculifer), we reconstructed the past salinity-gradient within that low-salinity tongue, which serves as a proxy for the variation in freshwater flux to the Bay of Bengal and hence summer monsoon intensity. The north-south contrast in the sea level corrected (residual)- delta super(18)O sub(G. sacculifer) can be interpreted as a measure of surface salinity-contrast between those two locations because the modern sea surface temperature and its past variation in the study region is nearly uniform. The core-top residual- delta super(18)O sub(G. sacculifer) contrast of 0.45ppt between the two cores is assumed to reflect the modern surface salinity difference of 1 psu and serves as a calibration for past variations. The residual- delta super(18)O sub(G. sacculifer) contrast varies between 0.2ppt at 75 ky B.P. (i.e., late-Marine Isotope Stage 5) and 0.7ppt at 20 ky B.P. (i.e., Last Glacial Maximum), suggesting that the overall salinity difference between the northern- and southern-end of the low-salinity tongue has varied between 0.6 and 1.6 psu. Considerably reduced difference during the former period than the modern suggests substantially intensified and northward-extended low-salinity tongue due to intense summer monsoons than today. On the other hand, larger difference (1.6 psu) during the latter period indicates that the low-salinity tongue was significantly weakened or withdrawn due to weaker summer monsoons. Thus, the salinity-gradient in the eastern Arabian Sea low-salinity tongue can be used to understand the past variations in the Indian summer monsoons.