Caesium distribution in northwest Mediterranean seawater, suspended particles and sediments

Abstract

On account of 134Cs and 137Cs isotope content, Chernobyl fallout has been considered a useful single pulse input to study transport processes in the northwestern Mediterranean marine environment. Although the highest caesium values in seawater surface layers were observed in the Rhoˆne estuary, across the whole Gulf the long distance impact of the Rhoˆne releases (increased by continental runoff associated with the autumnal rains) has been demonstrated; this has used the existing discrepancy in the 137Cs/ 134Cs 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 137Cs fraction is contained mainly in the upper layer with slow downward vertical mixing. In the estuary, 137Cs 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 continuous records. The 137Cs linked to suspended matter in the Rhoˆne discharge is deposited rapidly on the continental shelf. There is no evidence that the Minor Rhoˆne Canyon might be an active pathway for exported Rhoˆne terrigenous sediments to the deep-sea domain. In areas of intense sediment deposition, such as the Rhoˆne estuary, the 137Cs/ 134Cs 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.