Lead-210 and polonium-210 in ocean water profiles of the continental shelf and slope south of New England

Abstract

Concentration profiles of 210Pb and 210Po were measured on a traverse of the continental shelf and slope during the first phase of the SEEP (Shelf Edge Exchange Processes) program. Results from the upper 100 m show low concentrations of dissolved 210Pb over the shelf (<2dpm/100 kg) and increasing concentrations progressing into the slope water, where concentrations 10dpm/100 kg are found. This trend reflects the more efficient removal of 210Pb from the water column over the shelf. The deep waters over the slope show the commonly observed 210Pb deficiency, relative to 226Ra, due to removal by chemical scavenging. Vertical profiles indicate somewhat intensified scavenging of 210Pb near the sediment-water interface. Flux calculations for the shelf show an imbalance such that the input of atmospheric 210Pb must be compensated by an export of 210Pb off the shelf. This provides an important constraint on models that would predict the fate of materials introduced to the continental shelf. Over the slope, on the other hand, a good vertical flux balance is maintained, indicating that inputs of 210Pb to the slope water by cross-frontal particle transport or by exchange across the Gulf Stream may be unimportant. Results for 210Po in the surface waters show strong depletion, relative to 210Pb, in the dissolved form and enrichment in the particulate form, indicative of particulate transport out of the surface layer. Evidence for release of 210Po is seen in a sub-surface layer, centered at 50–100 m, where 210Po is enriched in the dissolved form and depleted in the particles.