Input of artificial radionuclides to the Gulf of Lions and tracing the Rhoˆne influence in marine surface sediments

The dispersal of particulate Rhoˆne River inputs to the shallow and deep Mediterranean Sea is studied using the particulate flux of artificial radionuclides originating from industrial discharges and atmospheric fallout. Among them, the long-lived 238Pu (half-life 88 years) is the most promising tracer. Studies of radionuclide fixation and mass-balance indicate that the suspended matter transport to the sea is reduced by temporal storing in the lower river course on time scales of months to a few years. luation of the 137Cs and Pu isotopes inputs to the river system since 1956 defines time-integrated particulate Pu concentrations and activity ratios, which are used to partition a marine sediment into two fractions, one originating from the Rhoˆne River and the other from other sources, and an older fraction indicative of mixing processes and diffusion. A preliminary test with three sediment cores confirms the applicability of the method. It indicates a sharp decrease of the Rh^one influence between the pro-delta and the middle of the continental shelf, whereas trace amounts of238Pu in the deep-sea sediments indicate that the recent Rhoˆne sediment fraction is quite small.