Radionuclide fluxes in the Arabian Sea: The role of particle composition
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The influence of the composition of the vertical particle flux on the removal of particle reactive natural radionuclides (sup(230)Th and sup(231)Pa) from the water column to the sediments is investigated. Radionuclide concentrations determined in sediment traps moored in the western, central and eastern Arabian Sea were related to the major components (carbonate, particulate organic matter (POC), opal, lithogenic material) of the particle flux. These data were combined with sediment trap data previously published from the Southern Ocean, Equatorial Pacific and North Atlantic [Z. Chase, R.F. Anderson, M.Q. Fleisher, P.W. Kubik, The influence of particle composition and particle flux on scavenging of Th, Pa and Be in the ocean, Earth Planet. Sci. Lett. 204 (2002) 215-229; J.C. Scholten, F. Fietzke, S. Vogler, M. Rutgers van der Loeff, A. Mangini, W. Koeve, J. Waniek, P. Stoffers, A. Antia, J. Kuss, Trapping efficiencies of sediment traps from the deep eastern North Atlantic: The sup(230)Th calibration, Deep Sea Research II 48 (2001) 2383-2408]. The correlations observed between the particle-dissolved distribution coefficients (Kd)of sup(230)Th and sup(231) Pa and the concentrations of the particle types depend on the sediment trap data set used. This result suggests that scavenging affinities of the nuclides differ between oceanic regions. Several factors (Kd values, reactive surface areas of particles, inter-correlations in closed data set) can, however, influence the observed relationships and thus hamper the interpretation of these correlation coefficients as a measure of relative scavenging affinities of the nuclides to the particle types investigated. The mean fractionation factor (F(Pa/Th)=K sub(d)(Pa)/K sub(d)(Th)) from the Equatorial Pacific (F=0.11 plus or minus 0.03) is similar to that from the North Atlantic (F(Pa/Th)=0.077 plus or minus 0.026), and both are lower than the factors from the Arabian Sea (F(Pa/Th)=0.35 plus or minus 0.12) and from the Southern Ocean (F(Pa/Th)=0.87 plus or minus 0.4). For opal concentrations exceeding approx. 60%, an increase in the fractionation factors is observed causing a higher mean fractionation factor for the Southern Ocean trap data set. For the other areas investigated, differences in the mean fractionation factors cannot be related to the particles types considered. In the Arabian Sea, seasonally variable sup(231)Pa sub(ex)/ sup(230)Th sub(ex) ratios observed in the sediment traps as well as differences of the ratios between recently deposited phytodetritus (fluff) and normal surface sediments indicate seasonal changes in scavenging processes which the generally accepted reversible scavenging models do not envisage. It is assumed that variable sinking rates of particles, and/or particles not considered in this study (e.g. colloids, manganese oxides, transparent exopolymer particles) may play an important but as yet unexplored role in deep-water scavenging processes.
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