NATURAL RADIONUCLIDES IN THE ARABIAN SEA AND BAY OF BENGAL - DISTRIBUTION AND EVALUATION OF PARTICLE SCAVENGING PROCESSES

Vertical and temporal variations in the activities of Th-234, Po-210 a nd Pb-210 have been measured, in both dissolved and particulate 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 dis tinct biogeochemical properties. A common feature of the Th-234 profil es, in the Arabian Sea and Bay of Bengal, is that the dissolved as wel l as total (dissolved + particulate) activity of Th-234 is deficient i n the surface 200 m with respect to its parent, U-238. This gross defi ciency is attributed to the preferential removal of Th-234 by adsorpti on onto settling particles which account for its net loss from the sur face waters. The scavenging rates of dissolved Th-234 are comparable i n these two basins. The temporal variations in the Th-234-U-238 disequ ilibrium 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-u p. In the mixed layer (0-50 m), the scavenging residence time of Th-23 4 ranges from 30 to 100 days. The surface and deep waters of both the seas show an enhanced deficiency of dissolved Po-210 relative to Pb-21 0 and that of Pb-210 relative to Ra-226. The deficiencies of both Po-2 10 and Pb-210 in the dissolved phases are not balanced by their abunda nce in the particulate form indicating a net loss of both these nuclid es from the water column. The scavenging rates of Po-210 and Pb-210 ar e significantly enhanced in the Bay of Bengal compared to those in the Arabian Sea. The mean dissolved Po-210/Pb-210 and Pb-210/Ra-226 activ ity ratios in deep waters of the Bay of Bengal are approximately 0.7 a nd 0.1, respectively, representing some of the most pronounced disequi libria observed to date in the deep sea. The Bay of Bengal and the Ara bian Sea appear to be the regions of most intense particle moderated s cavenging processes in the world oceans. This is evidenced by the gros s disequilibria exhibited by the three isotope pairs used in this stud y.