Scavenging Processes and Export Fluxes Close to a Retreating Seasonal Ice Margin (Ross Sea, Antarctica)

The distribution of dissolved and particulate 234Th in the upper 200 m of the water column was obtained for three stations in the Ross Sea off Victoria Land and Terra Nova Bay. At site 24a, close to the retreating ice margin, all the sampled depths showed deficiencies in 234Th relative to the equilibrium with 238U. These are related to uptake of 234Th onto sinking particles. Residence times of 234Th in solution and of particulate 234Th were 130-247 days and 8.1-6.6 days, respectively. A high particle flux (1.23-5.03 g m-2 d-1) was calculated at this station. At the other two sites (11c and 15c), 234Th depth profiles are irregular, probably due to the release of dissolved 234Th by decomposing particles at certain depths, or to the contribution from lateral advection. Bulk mass fluxes measured by floating traps at stations 11c and 15c are very low (66-138 mg m-2 d-1). Also fluxes of organic carbon and nitrogen, and biogenic silica are reported for these two sites. The calculated fluxes are discussed with respect to methodologies and to the dynamics of the ice margin retreat.