Settling fluxes of U- and Th-series nuclides in the Bay of Bengal: results from time-series sediment trap studies

Abstract

A two-year (1990–1991) collection of sediment trap samples from three locations in the Bay of Bengal was analyzed for Al, Ca and U- and Th-series nuclides to obtain their annual fluxes and pattern of deposition. The time-series traps were deployed at two depths: ∼(900–1100) and ∼(2100–3000) m at ∼15°N 89°E (NBBT); ∼13°N 84°E (CBBT) and ∼5°N 87°E (SBBT). The deeper traps were several hundred meters above the sea floor. Each time-series trap had 13 cups and each cup was programmed to collect settling particles for ∼25 days. The mass fluxes in the NBBT shallow and deep traps were nearly the same, ∼33 g m −2 yr −1. At the CBBT site the mass flux in the deeper trap was ∼65 g m −2 yr −1, about 40% more than that in the shallow trap. The maximum particle flux of ∼300 mg m −2 d −1 was observed in the deep trap of CBBT site and is attributable to extreme floods in the Godavari and Krishna rivers and lateral injection of particulate matter to the deep sea. Variations in Al deposition flux in the traps is strongly coupled to that of mass flux, attributable to the scavenging of aluminosilicates from the water column by sinking organic aggregates. The radionuclide fluxes ( 230 Th a , 228 Th and 210 Pb ) in the traps provide insight into the role of their removal by vertical particle flux or lateral transport. At the NBBT site, the observations that (i) the authigenic flux of 230 Th ( 230 Th a ) in the deep trap nearly balances its production in the water column above the trap depth, (ii) the differences in the 210 Pb fluxes between the shallow and deep traps matches its in-situ production in the water column between the traps and (iii) the mass, Al and 228 Th fluxes are similar in the shallow and deep traps, all suggest that at this site 230 Th and 210 Pb produced in the water column are removed predominantly by vertical particle flux. At the CBBT site, there is a consistent increase in the mass, Al and 228 Th fluxes in the deep trap relative to those in the shallow trap, and the deposition of 210 Pb and 230 Th a are considerably in excess over their production. Enhanced particle aggregation in the intermediate waters, resuspension of “nascent” sedimentary material and focussing of particles in the region of the deep traps can all account for these observations. The particulate flux in the deep trap at the SBBT site, farthest from the river mouths, based on nine months of data is ∼38 g m −2 yr −1 and is dominated by CaCO 3. The authigenic 230 Th flux in the SBBT deep trap matches its overhead production, whereas for 210 Pb the measured flux is ∼30% less than that expected from its supply. This indicates that at this site the bulk of 230 Th produced is removed by vertical particle flux, whereas about one-third of the 210 Pb supplied is carried off laterally and presumably removed at the ocean boundaries. Evidence for uranium removal from the thermocline by settling particles is seen in the higher (U/Th) ratio of several shallow trap samples relative to that in the deep traps. The 210 Pb and 228 Th fluxes in the shallow and deep traps show strong co-variance with mass flux, which in turn is coupled to sediment discharge and productivity in the Bay through monsoon cycles. These observations suggest that nuclide scavenging processes in surface waters and interior of the Bay of Bengal respond quickly to particle flux variations. Thus, the study of mass, Al, Ca and radionuclide fluxes at these three sites provide not only data on spatial and temporal variability of their fluxes over a two-year period but also insight into the processes regulating their depositional fluxes.