234Th scavenging and particle export fluxes from the upper 100 m of the Arabian Sea

Abstract

We have determined the particle scavenging rates, export fluxes of @u234@@Th and settling particles from the upper 100 m of the Arabian Sea as a part of the JGOFS (India) Programme. The spatial and temporal measurements made in the open ocean profiles reveal close similarities in the dissolved @u234@@Th: @u238@@U disequilibria, suggesting that the rates of particle-associated scavenging processes are generally uniform in the central Arabian Sea. The observed disequilibrium integrated for the upper 100 m yields a mean scavenging residence time of ~k30 days and a removal rate of ~k 3400 dpm m@u-2@@ d@u-1@@ for @u234@@Th, from dissolved to particulate phases. The deficiency of total @u234@@Th (dissolved + particulate) relative to @u238@@U allows us to compute the vertical export flux of particulate @u234@@Th. The flux data for the upper 100 m show spatial variations with enhanced export fluxes centered around 22~'N 67~'E, a region characterized by higher rates of column primary productivity. Using the @u234@@Th export fluxes and the measured specific activity of @u234@@Th in the sediment traps, we have computed th eparticle and carbon fluxes at 100 m. These results reveal that the particle fluxes determined from sediment traps are systematically low and the estimated C export fluxes are grossly out of proportion with the column primary production.