MONSOON-INDUCED SEA-SURFACE TEMPERATURE-CHANGES RECORDED IN INDIAN CORALS

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-degrees-C. This reduction in temperature is recorded by modem 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 deltaO-18). 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 approximately 60 km show similar deltaO-18 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 deltaO-18 from that of the CaCO3 precipitated in isotopic equilibrium. This 'disequilibrium effect' appears to be constant around 4.5+/-0.2 parts per thousand. Our calculations show that SST (t, degrees-C) is related to the coral deltaO-18 (delta(c)) and the sea-water deltaO-18 (delta(w)) by the equation t = 3.0 - 4.68 (delta(c) - delta(w)), which is in good agreement with such relationships for corals from the Pacific and Atlantic.