Seasonal biogeochemical fluxes of234Th and210Po in the Upper Sargasso Sea: Influence from atmospheric iron deposition

The naturally occurring radionuclide tracers, 234Th-238U and 210Po-210Pb pairs, were measured bimonthly at the Bermuda Atlantic Time-Series Study (BATS) site in the Sargasso Sea from October 1996 to August 1997. Biological production and biogeochemical fluxes of thorium, polonium, and particulate organic carbon were highest in December 1996 and August 1997. Although the enhanced production and fluxes in December 1996 could be due to winter nitrate inputs from the subsurface ocean, those in August 1997 were not expected, on the basis of the nutrient mass balance and extreme stratification of the Sargasso Sea during this summer. The fixed-nutrient inputs from the atmosphere, eddy intrusion, or deepwater upwelling were unlikely sources of such elevated summer productivity. Alternatively, the unusually high atmospheric “wet” deposition of Fe, which occurred during this summer period, appears to have fueled high nitrogen fixation and thus greater carbon and reactive-element export. Also as important, ocean production did not respond to significant “dry” deposition of Fe. This seems to be due to the reported lower solubility of Fe dust in seawater versus rainwater.