Convective mixing and transport of the Bay of Bengal water stir the δ18O-salinity relation in the Arabian Sea

Stable isotopes are widely used to understand contemporary ocean processes and reconstruct palaeoclimates. Here, we present the distribution of oxygen and hydrogen stable isotopes (delta 18O and delta D) in the Arabian Sea to understand their vertical profiles and establish a regional delta 18O-salinity (delta 18O-S) relationship. A large number (n = 102) of depth-specific water samples were collected along the latitudinal transect (65 degrees E) in April 2017. Our results show significant surface variability in the isotopic composition of seawater with relatively 18O-depleted water over the southern and 18O-enriched water over the northern Arabian Sea. Surprisingly, there was no relationship between delta 18O and S in the surface waters. This no-relationship has implications in palaeoclimate reconstruction wherein we assume a linear delta 18O-S relation. However, data from the entire water column sampled (0-1000 m) showed a strong relationship with delta 18O = 0.35(+/- 0.03) S - 11.84(+/- 1.12) (r = 0.79, p < 0.05) and delta D = (5.71 +/- 0.80) delta 18O - (0.66 +/- 0.61) (r = 0.64, p < 0.05). We propose that the surface delta 18O-S relationship is disturbed by the winter monsoon driven convective mixing and low salinity (and low delta 18O) water intrusion from the Bay of Bengal to the southern Arabian Sea. Strong delta 18O-S relations were observed for distinct water masses, however, the water masses could not be distinguished based on their isotopic signatures alone. The slope of the delta 18O-S relation was lower in the Arabian Sea High Saline Water compared to that in the Persian Gulf Water. A weak negative delta 18O-S relation is observed for the Red Sea Water mass. The water masses were further verified using a temperature-salinity (T-S) diagram. At each station, the delta 18O-S relationships of the vertical profiles were distinct, with increasing slope and decreasing intercept from the south to north direction. Overall, surface water evaporation and vertical mixing over the northern Arabian Sea and freshwater intrusion from the Bay of Bengal in the southern Arabian Sea control the delta 18O-S relationship in this region.