Controls on δ18O, δD and δ18O-salinity relationship in the northern Indian Ocean

Abstract

Oxygen isotopic composition (δ18O) and salinity (S) of the surface ocean are useful tracers for studying imprints of physical processes (such as runoff, sea ice melting, brine release caused by sea ice formation and large-scale ocean mixing) in the modern ocean, while δ18O-S relation is important to estimate paleo-salinity. Effects of Precipitation (P), Runoff (R) and Evaporation (E) (i.e., P+R – E budget) on δ18O, δD (hydrogen isotopic composition), S and their relations (δ18O-δD and δ18O-S) in the surface ocean are relatively well understood but understanding of such relations (e.g., δ18O-S) in subsurface ocean waters remains elusive. Based on a new dataset of 352 water samples, which includes vertical profiles of δ18O estimates, we have attempted to understand the effect of surface boundary currents and lateral mixing of different water masses on the δ18O-S and δD-δ18O relation in the northern Indian Ocean. Significantly stronger δ18O-S and δD-δ18O relations were observed during the winter compared to that during the spring with seasonal variability in the δ18O-S relation in surface water (δ18O = 0.14 (±0.01) × S – 4.58 (±0.48), R2 = 0.43, spring; δ18O = 0.28 (±0.01) × S – 9.10 (±0.30), R2 = 0.83, winter; δ18O = 0.26 (±0.01) × S – 8.55 (±0.22), R2 = 0.83, combined spring and winter). The higher slope and lower intercept of the δ18O-S relation during winter with respect to the spring implied that the runoff affects the δ18O-S relation during winter. We observed a significant correlation between d-excess and δ18O in the surface waters during the winter, while there was no correlation in this relationship during the spring. Slope and intercept of the δ18O-S relation [δ18O = 0.13 (±0.01) × S – 4.21 (±0.28), R2 = 0.84] in the subsurface waters off the Andaman Islands and in the surface Bay of Bengal were similar during spring. The δ18O-S relation for deeper water samples with lower slope and higher intercept compared to the mixed zone characterizes the presence of Persian Gulf Water mass at 200–500 m depth in the Andaman Sea.