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

The dispersal of particulate Rhone 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 Pu-238 (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 lime scales of months to a few years. An evaluation of the Cs-137 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 Rhone 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 Rhone influence between the pro-delta and the middle of the continental shelf, whereas trace amounts of Pu-238 in the deep-sea sediments indicate that the recent Rhone sediment fraction is quite small. (C) 1997 Elsevier Science Ltd.