Monsoon-induced sea surface temperature changes recorded in Indian corals

Chakraborty, S. ; Ramesh, R. (1993) Monsoon-induced sea surface temperature changes recorded in Indian corals Terra Nova, 5 (6). pp. 545-551. ISSN 0954-4879

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The South Asian Summer Monsoon induces vertical mixing in the surface Arabian Sea, leading to a reduction in the Sea Surface Temperature (SST) of the order of 3-4°C. This reduction in temperature is recorded by modern corals (Porites) that grow in the Lakshadweep Islands (coralline islands located at about 350 km off the south-west coast of India) in their stable oxygen isotope ratios (denoted by δ18O). As large coral colonies of this genus are available, our results show that palaeomonsoon records for a few centuries back in time, a crucial input for climatic models aimed at predicting the Asian Monsoon, can be obtained from these corals. We also show that two corals separated by ~60 km show similar (δ18O variations as does a giant clam (Tridacna maximus) that grew near one end of the coral colonies. As this clam is known to precipitate CaCO3 in isotopic equilibrium with the ambient sea-water, it is possible to estimate the isotopic offset of coralline δ18O from that of the CaCO3 precipitated in isotopic equilibrium. This 'disequilibrium effect' appears to be constant around 4.5 ± 0.2%0. Our calculations show that SST (t, °C) is related to the coral δ18O (δc) and the sea-water δ18O (δw) by the equation t = 3.0-4.68 (δc −δw), which is in good agreement with such relationships for corals from the Pacific and Atlantic.

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