U- and Th-series nuclides in settling particles: implications to sediment transport through surface waters and interior ocean

The Bay of Bengal is a unique ocean basin receiving large quantities of fresh water and sediment supply from several rivers draining the Indian subcontinent. The annual flux of suspended sediments discharged into the Bay of Bengal is one billion tons, about one-tenth of the global sediment discharge into the ocean. The water and sediment discharge to the Bay, show significant seasonal variation, with maximum transport coinciding with the SW-monsoon (July-September). Earlier studies on the distribution of clay minerals in sediments have led to the suggestion that the sediments of the western Bengal Fan are mainly derived from the Peninsular rivers, whereas rest of the Fan sediments is influenced by the Himalayan rivers. Settling fluxes of particulate matter through the water column of the Bay of Bengal show seasonal trends resulting from monsoon enhanced sediment supply via rivers and biological processes in the water column. It is, thus, important to understand the influence of the seasonally varying particle fluxes on the solute-particle interactions and chemical scavenging processes in the surface and deep waters of the Bay of Bengal. In this context, measurements of U- and Th-series nuclides in the settling particles are most relevant. The radionuclide fluxes (230Th, 228Th and 210Pb) in the settling particles provide insight into the role of their removal by vertical particle flux and/or lateral transport (removal at the ocean boundaries). A study carried out in the Northern Bay of Bengal documents that the authigenic flux of 230Th, as measured in sediment trap samples from deep waters, is balanced by its production in the overhead water column. The sediment mass flux, Al and 228Th fluxes are similar in the settling particles through shallow and deep waters, suggesting predominant removal by vertical particle flux in the North Bay of Bengal. In the Central Bay, particulate mass, Al and 228Th fluxes are higher in the trap material from deep waters relative to that in the shallow water. The fluxes of 210Pb and 230Th are also in excess over their overhead production. The re-suspension of sedimentary material and lateral particle focusing are the likely processes operating in the Central Bay region. The particulate flux of 230Th in deep waters of the south Bay of Bengal is quite similar to its overhead production. These studies bring to focus the pronounced spatial and temporal variability in the radionuclide fluxes and processes regulating their deposition in the bottom sediments. (author)