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

In February 2004, anthropogenic radionuclides (Cs-137, U-236, Pu-239 and Pu-240), 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-236 and Cs-137 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-236 and Cs-137, and between 1.0 and 1.5 cm for Pu isotopes. The excess of U-236 was observed more in the water column than in the sediment, whereas the Pu239+240 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-137) and 15% (Pu239+240) 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-137 and 34-45% for Pu239+240). Contributions to the total Cs-137 and Pu239+240 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.