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

DRS at CSIR-National Institute of Oceanography

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Title Settling fluxes of U- and Th-series nuclides in the Bay of Bengal: Results from time-series sediment trap studies
 
Creator Sarin, M.M.
Krishnaswami, S.
Dalai, T.K.
Ramaswamy, V.
Ittekkot, V.
 
Subject sedimentation
sediment chemistry
sediment traps
suspended particulatic matter
aluminium isotopes
thorium isotopes
uranium isotopes
time series analysis
 
Description 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: approx. (900-1100) and approx. (2000-3000) m at approx. 15 degrees N 89 degrees E (NBBT); approx. 13 degrees N 84 degrees E (CBBT) and approx. 5 degrees N 87 degrees 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 approx. 25 days. 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 ( sup(230)Th sub(a), sup(228)Th and sup(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 (1) the authigenic flux of sup(230)Th ( sup(230)Th sub(a)) in the deep trap nearly balances its production in the water column above the trap depth, (2) the differences in the sup(210)Pb fluxes between the shallow and deep traps matches its in-situ production in the water column between the traps and (3) the mass, Al and sup(228)Th fluxes are similar in the shallow and deep traps, all suggest that the at this site sup(230)Th and sup(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 sup(228)Th fluxes in the deep trap relative to those in the shallow trap, and the deposition of sup(210)Pb and sup(230)Th sub(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 approx. 38 g m sup(-2) yr sup(-1) and is dominated by CaCO sub(3). The authigenic sup(230)Th flux in the SBBT deep trap matches its overhead production, whereas for sup(210)Pb the measured flux is approx. 30% less than that expected from its supply. This indicates that at this site the bulk of sup(230)Th produced is removed by vertical particle flux, whereas about one-third of the sup(210)Pb supplied is carred off laterally and presumably removed at the ocean boundaries. Evidence for uranium removal from the thermocline by setting particles is seen in the higher (U/Th) ratio of several shallow trap samples relative to that in the deep traps. The sup(210)Pb and sup(228)Th fluxes in the shallow and deep traps. The sup(210)Pb and sup(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
 
Date 2009-01-09T08:08:17Z
2009-01-09T08:08:17Z
2000
 
Type Journal Article
 
Identifier Deep-Sea Research (I: Oceanographic Research Papers), Vol.47; 1961-1985p.
http://drs.nio.org/drs/handle/2264/1667
 
Language en
 
Rights Copyright [2000]. All efforts have been made to respect the copyright to the best of our knowledge. Inadvertent omissions, if brought to our notice, stand for correction and withdrawal of document from this repository.
 
Publisher Pergamon