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

A two-year (1990-1991) collection of sediment trap samples from three locat ions in the Bay of Bengal was analyzed for Al, Ca and U- and Th-series nucl ides to obtain their annual fluxes and pattern of deposition. The time-seri es traps were deployed at two depths: similar to (900-1100) and similar to (2100-3000) m at similar to 15 degrees N 89 degrees E (NBBT); similar to 13 degrees N 84 degrees E (CBBT) and similar to 5 degrees N 87 degrees E (SBB T). 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 settlin g particles for similar to 25 days. The mass fluxes in the NBBT shallow and deep traps were nearly the same, similar to 33 g m(-2) yr(-1). At the CBBT site the mass flux in the deeper trap was similar to 65 g m(-2) yr(-1), ab out 40% more than that in the shallow trap. The maximum particle flux of si milar to 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 la teral injection of particulate matter to the deep sea. Variations in Al dep osition flux in the traps is strongly coupled to that of mass flux, attribu table to the scavenging of aluminosilicates from the water column by sinkin g organic aggregates. The radionuclide fluxes (Th-230(a), Th-228 and Pb-210 ) in the traps provide insight into the role of their removal by vertical p article flux or lateral transport. At the NBBT site, the observations that (i) the authigenic flux of Th-230 (Th-230(a)) in the deep trap nearly balan ces its production in the water column above the trap depth, (ii) the diffe rences in the Pb-210 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 Th-228 fluxes are similar in the shallow and deep traps, all sugge st that at this site Th-230 and Pb-210 produced in the water column are rem oved predominantly by vertical particle flux. At the CBBT site, there is a consistent increase in the mass, Al and Th-228 fluxes in the deep trap rela tive to those in the shallow trap, and the deposition of Pb-210 and Th-230( a) are considerably in excess over their production. Enhanced particle aggr egation in the intermediate waters, resuspension of "nascent" sedimentary m aterial 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 th e SBBT site, farthest from the river mouths, based on nine months of data i s similar to 35 g m(-2) yr(-1) and is dominated by CaCO3. The authigenic Th -230 flux in the SBBT deep trap matches its overhead production, whereas fo r Pb-210 the measured nux is similar to 30% less than that expected from it s supply. This indicates that at this site the bulk of Th-230 produced is r emoved by vertical particle flux, whereas about one-third of the Pb-210 sup plied is carried off laterally and presumably removed at the ocean boundari es. Evidence for uranium removal from the thermocline by settling particles is seen in the higher (U/Th) ratio of several shallow trap samples relativ e to that in the deep traps. The Pb-210 and Th-228 fluxes in the shallow an d deep traps show strong co-variance with mass flux, which in turn is coupl ed to sediment discharge and productivity in the Bay through monsoon cycles . These observations suggest that nuclide scavenging professes in surface wat ers and interior of the Bay of Bengal respond quickly to particle flux vari ations. Thus, the study of mass, Al, Ca and radionuclide fluxes at these th ree sites provide not only data on spatial and temporal variability of thei r fluxes over a two-year period but also insight into the processes regulat ing their depositional fluxes. (C) 2000 Elsevier Science Ltd. All rights re served.