Natural radionuclides in the Arabian sea and bay of Bengal: distribution and evaluation of particle scavenging processes

Abstract

Vertical and temporal variations in the activities of ²³⁴Th,²¹⁰Po and ²¹⁰Pb have been measured, in both dissolved and paniculate phases, at several stations in the eastern Arabian Sea and north-central Bay of Bengal. A comparative study allows us to make inferences about the particle associated scavenging processes in these two seas having distinct biogeochemical properties. A common feature of the ²³⁴Th profiles, in the Arabian Sea and Bay of Bengal, is that the dissolved as well as total (dissolved + particulate) activity of ²³⁴Th is deficient in the surface 200 m with respect to its parent, ²³⁸U. This gross deficiency is attributed to the preferential removal of ²³⁴Th by adsorption onto settling particles which account for its net loss from the surface waters. The scavenging rates of dissolved ²³⁴Th are comparable in these two basins. The temporal variations in the ²³⁴Th-²³⁸U disequilibrium are significantly pronounced both in the Arabian Sea and Bay of Bengal indicating that the scavenging rates are more influenced by the increased abundance of particles rather than their chemical make-up. In the mixed layer (0-50 m), the scavenging residence time of ²³⁴Th ranges from 30 to 100 days. The surface and deep waters of both the seas show an enhanced deficiency of dissolved ²¹⁰Po relative to ²¹⁰Pb and that of ²¹⁰Pb relative to ²²⁶Ra. The deficiencies of both ²¹⁰Po and ²¹⁰Pb in the dissolved phases are not balanced by their abundance in the particulate form indicating a net loss of both these nuclides from the water column. The scavenging rates of ²¹⁰Po and ²¹⁰Pb are significantly enhanced in the Bay of Bengal compared to those in the Arabian Sea. The mean dissolved ²¹⁰Po/²¹⁰Pb and ²¹⁰Pb/²²⁶Ra activity ratios in deep waters of the Bay of Bengal are ∼ 0.7 and 0.1, respectively, representing some of the most pronounced disequilibria observed to date in the deep sea. The Bay of Bengal and the Arabian Sea appear to be the regions of most intense particle moderated scavenging processes in the world oceans. This is evidenced by the gross disequilibria exhibited by the three isotope pairs used in this study.