²³⁴Th scavenging and particle export fluxes

from the upper 100 m of the Arabian Sea

M. M. Sarin, R. Rengarajan and V. Ramaswamy*

Physical Research Laboratory, Ahmedabad 380 009, India
*National Institute of Oceanography, Dona Paula 403 004, India

We have determined the particle scavenging rates, export fluxes of ²³⁴Th 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 ²³⁴Th : 238U 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 ~ 30 days and a removal rate of ~3400 m^-2 d^-l for ²³⁴ Th, from dissolved to particulate phases.

The deficiency of total ²³⁴Th (dissolved + particulate) ²³⁸U relative to allows us to compute the vertical export flux of particulate ²³⁴Th. The flux data for the upper 100 m show spatial variations with enhanced export fluxes centered arouiid 22^oN 67^oE, a region characterized by higher rates of column primary productivity. Using the  ²³⁴Th export fluxes and the measured specific activity of  ²³⁴Th in the sediment traps, we have computed the particle and carbon tluxes 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.