Pb-210 and caesium accumulation in the Rhone delta sediments
Résumé
Pb-210, Cs-137 and Cs-134 profiles were determined on 13 box-cores from the Rhone submarine delta (north-western Mediterranean Sea). Maximum accumulation rates were estimated using the Pb-210 dating method or the evolution of the Cs-137/Cs-134 activity ratio with depth in the sediment. Rates ranged from more than 20 cm year(-1) near the river mouth to 0.2 cm year(-1) on 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 by Cs-137 that penetrated to greater depths than expected from the Pb-210 derived accumulation rates. The delta area is an important sink for the riverine inputs. The Cs-137 inventory over the studied area (480 km(2)) was estimated to be 19.6 TBq in 1990, of which more than 40% was found in the prodelta (30 km(2)), in the vicinity of the river mouth. This inventory appears to be well in excess (50%) of both direct and indirect Cs-137 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 Rhone fiver, it represents 20% of the Cs-137 cumulative discharges by the various nuclear power plants and the Marcoule reprocessing giant released between 1961 to 1990. The Pb-210 budget shows that the sediments are also efficient to scavenge elements transported by advection or diffusion on the shelf. The Rhone delta, and more particularly the prodelta area, is thus an effective sink for particle-reactive elements and pollutants.