Spatiotemporal variations of the δ18O–salinity relation in the northern Indian Ocean

A new data set of oxygen isotopic composition (δ18O) and salinity (S) of surface and sub-surface waters of the northern Indian Ocean, collected during the period 1987–2009, is presented. While the results are consistent with positive P−E (excess of precipitation over evaporation) over the Bay of Bengal and negative P−E over the eastern Arabian Sea, a significant spatiotemporal variability in the slope (also intercept) of the δ18O–S relation is observed in the Bay; the temporal variability is difficult to discern in the Arabian Sea. The slope and intercept are positively and negatively correlated, respectively, with the annual rainfall over India, a rough measure of river runoff. Both the slope and intercept appear to be sensitive to rainfall; the slope (intercept) is higher (lower) during years of stronger monsoon. The observed variability in the δ18O–S relation implies that caution needs to be exercised in paleosalinity estimations, especially from the Bay of Bengal, based on δ18O of marine organisms.