CESIUM DISTRIBUTION IN NORTHWEST MEDITERRANEAN SEAWATER, SUSPENDED PARTICLES AND SEDIMENTS

On account of Cs-134 and Cs-137 isotope content, Chernobyl fallout has been considered a useful single pulse input to stydy transport processes in the northwestern Mediterranean marine environment. Although the highest caesium values in seawater surface layers were observed in the Rhone estuary, across the whole Gulf the long distance impact of the Rhone releases (increased by continental runoff associated with the autumnal rains) has been demonstrated; this has used the existing discrepancy in the Cs-137/Cs-134 ratio for Chernobyl fallout, as opposed to the controlled authorized industrial liquid releases. The inventory in the water column after the Chernobyl accident, shows that the dissolved Cs-137 fraction is contained mainly in the upper layer with slow downward vertical mixing. In the estuary, Cs-137 in seawater is subject to high temporal, as well as vertical and horizontal spatial heterogeneity, with accumulation in the surface microlayer. This makes the calculation of caesium fluxes difficult without continous records. The Cs-137 linked to suspended matter in the Rhone discharge is deposited rapidly on the continental shelf. There is no evidence that the Minor Rhone Canyon might be an active pathway for exported Rhone terrigenous sediments to the deep-sea domain. In areas of intense sediment deposition, such as the Rhone estuary, the Cs-137/Cs-134 ratio can be used to estimate the present sedimentation rate; this can be up to 35 cm y-1, in the area of most abundant deposition.