The aim of this study was to evaluate and understand particular transfers dynamics and balances of radionuclides in the Gulf of Lions, particularly at the Rhône River mouth. Due to its 30.2 year half-life and of its great affinity with silts and clays, 137Cs was used as a Rhône River inputs tracer. Rhône prodelta sediments recorded values of 137Cs activities originated by nuclear power plants releases, global fallout and Tchernobyl accident (peak at 600 Bq.kg-1). A sharp decrease in liquid radioactive effluents releases and the dismantlement of the Marcoule fuel reprocessing plant in 1997 induced 137Cs fluxes decrease to the Mediterranean Sea. At present time, mean concentrations are around 10 Bq.kg-1 in the prodelta sediments.
Sedimentary records of different oceanographic campaigns achieved between 2001 and 2008 enabled to map a 20 km2 137Cs accumulation area close to the Rhône River mouth and to estimate a store of 3.35 TBq, i.e. the eighth of the Gulf of Lions store, which area is about 15000 km2.
Other campaigns carried out in the framework of the CARMA and EXTREMA projects (2006-2008) allowed to observe surface and bottom nepheloïds behaviours and to link them to the prodelta sedimentation. Radiochronological analyses coupling 137Cs and 210Pb depth activity profiles allowed to estimate pluridecimetric accumulation rates next to the mouth. Short-live radionuclides like 7Be and 234Th were used to estimate sedimentary deposits thicknesses generated by some Rhône River floods.
These results were confirmed by an experiment which induced an instruments deployment at the Rhône River mouth during the winter 2006-2007. Altimeter data showed 8 cm thick sediment total accretion during two mean floods recorded by a current profiler. They also showed an important erosion phase linked to a south-east swell episode with a bottom shear stress reaching 5 Pa. An erodimeter enabled to evaluate the erosion shear stress threshold to 0.35 Pa next to the mouth. Results showed that the sediment remobilization was very important in this area but that the exports of suspended solid matters towards the Gulf of Lions were weak, or even deposited in fine on the prodelta.