Impact of Saharan dust events on radionuclides in the atmosphere, seawater, and sediments of the northwest Mediterranean Sea.

In February 2004, anthropogenic radionuclides (Cs, U, Pu and Pu), transported from the Sahara Desert, were observed in the Monaco air, and later in water and sediment samples collected at the DYFAMED site in the northwest (NW) Mediterranean Sea. While U and Cs in Saharan dust particles showed a high solubility in seawater, Pu isotopes were particle reactive in the water column and in the sediment. The impact of the Saharan deposition was found at 0-1.0 cm of the sediment core for U and Cs, and between 1.0 and 1.5 cm for Pu isotopes. The excess of U was observed more in the water column than in the sediment, whereas the Pu total inventories were comparable in the water column and the sediment. This single-day particle event represented 72% of annual atmospheric deposition in Monaco. At the DYFAMED site, it accounted for 10% (Cs) and 15% (Pu) activities of sinking particles during the period of the highest mass flux collected at the 200 and 1000 m water depths, and for a significant proportion of the total annual atmospheric input to the NW Mediterranean Sea (28-37% for Cs and 34-45% for Pu). Contributions to the total Cs and Pu sediment inventories were estimated to be 14% and 8%, respectively. The Saharan dust deposition phenomenon (atmospheric input, water column and sediment) offered a unique case to study origin and accumulation rates of radionuclides in the NW Mediterranean Sea.