δ¹⁸O and salinity variability from the last glacial maximum to recent in the Bay of Bengal and Andaman Sea

Abstract

Oxygen isotopes of surface, thermocline and bottom dwelling foraminifera were analysed from two well-dated Andaman Sea cores and combined with nine previously published records from the Bay of Bengal (BoB) and Andaman Sea to create a transect spanning 20°N to 5°N. Combined with temperature estimates and the observed seawater μ¹⁸O-salinity relationship, these data are used to estimate past changes in BoB salinity structure. Compared to modern, mid-Holocene (9–6 cal ka BP) surface waters in the northern BoB were 2.5 psμ(8%) fresher, Andaman Sea were 3.8 psμ(12%) fresher, and southern BoB were 1.2 psμ(3.5%) fresher. Conversely, during the last glacial maximum (LGM), surface waters in the northern BoB were 2.9 psμ(9%) more saline while Andaman Sea were essentially unchanged and southern BoB were 1.7 psμ(4.9%) more saline compared to modern. The relative freshness of the Andaman during the last glacial maximum is likely the result of basin morphology during sea level low stand, resulting in reduced surface water mixing with the open BoB as well as shelf emergence, causing increased proximity of the core locations to river outflow. Sensitivity experiments using a regional ocean model indicate that the increased mid-Holocene north to south (20°N to 5°N) salinity gradient can be achieved with a μ50% increase in precipitation/runoff while the decreased glacial age gradient can be achieved with a μ50% reduction in precipitation/runoff. During the deglaciation, both surface and thermocline-dwelling species in the Andaman and northern BoB exhibit depleted ¹⁸O within the Younger Dryas (YD), indicating colder and/or more saline conditions. None of the records from the southern BoB site have clear YD structure, possibly due to the combined effects of bioturbation and low sedimentation rates.