Fractionation of 210Po and 210Pb in coastal waters of the NW Mediterranean continental margin

The natural radionuclides 210Po and 210Pb were analyzed in samples of surface sea water, rain and dry fallout, and river water collected along the NW Mediterranean coast as well as from a sediment trap moored 3 km south of Monaco. Using a box model calculation, the balances and fluxes of 210Po and 210Pb in the coastal waters of the NW Mediterranean were estimated. Atmospheric inputs of 210Po and 210Pb to Monaco coastal waters varied seasonally and were maximal in winter when storms and strong winds transfered continental 222Rn-rich air and aerosols of top soil particles to coastal surface waters, which in turn enhanced the 210Po and 210Pb input to these waters. The balance estimation using all fluxes in surface waters indicated that 210Pb was removed from surface water with residence times of 2.1, 0.40, and 2.7 years for dissolved, particulate, and total 210Pb, respectively. In the bottom water and surface sediments, additional excess 210Pb sinking and sedimentation fluxes were observed, suggesting a substantial down slope transport of sediment particles. Similarly, the residence times of 210Po in surface water were 1.2, 0.38, and 0.77 years for dissolved, particulate, and total 210Po, respectively; however, a deficit in the 210Po sinking flux in the bottom layer, compared to removal flux from the surface waters, suggested rapid degradation of 210Po-bearing biogenic particles during sinking following periods of low biological productivity.