210Pb and Caesium Accumulation in the Rhône Delta Sediments

Abstract

210Pb,137Cs and134Cs profiles were determined on 13 box-cores from the Rhône submarine delta (north-western Mediterranean Sea). Maximum accumulation rates were estimated using the210Pb dating method or the evolution of the137Cs/134Cs activity ratio with depth in the sediment. Rates ranged from more than 20cmyear−1near the river mouth to 0·2cmyear−1on the shelf, decreasing rapidly seawards with a preferential S-SW direction from the mouth in relation to the spreading of surface and bottom nepheloid layers in this area. Mixing and diffusion processes were evidenced by137Cs that penetrated to greater depths than expected from the210Pb derived accumulation rates. The delta area is an important sink for the riverine inputs. The137Cs inventory over the studied area (480km2) was estimated to be 19·6TBq in 1990, of which more than 40% was found in the prodelta (30km2), in the vicinity of the river mouth. This inventory appears to be well in excess (50%) of both direct and indirect137Cs inputs arising from Chernobyl and weapons test fallout in this region. If it is assumed that this excess is due only to liquid discharges by nuclear installations on the Rhône river, it represents 20% of the137Cs cumulative discharges by the various nuclear power plants and the Marcoule reprocessing plant released between 1961 to 1990. The210Pb budget shows that the sediments are also efficient to scavenge elements transported by advection or diffusion on the shelf. The Rhône delta, and more particularly the prodelta area, is thus an effective sink for particle-reactive elements and pollutants.