Early muddy deposits along the Gulf of Lions shoreline: A key for a better understanding of land-to-sea transfer of sediments and associated pollutant fluxes

Large quantities of sediment introduced by rivers especially during flood events lead to the formation of prodeltaic structures characterized by fine-grained deposits in the proximal area of the shelf. According to their mean grain size, it is largely admitted in the literature that these deposits mainly stand below the storm wave base. In the Gulf of Lions (NW Mediterranean), however, our data show that these muddy sediments also exist above the storm wave limit, in the uppermost unit of the Holocene prism. The purpose of this work is therefore to draw a detailed sedimentological and geochemical portrait of these units, and to discuss the processes involved in the sedimentation and remobilization of these sediments. Our results show that the fine particles accumulate earlier in the inner-shelf in front of the nearby rivers, in association with the chemical elements which are commonly associated with this grain size fraction (e.g. organic carbon, radionuclides, and heavy metals). This accumulation probably mainly occurs during storm and flood events. The early deposits are then exposed to resuspension events and can actively participate to the distribution of fine-grained sediment in the entire margin by advective transfers via the benthic nepheloid layer. These fine-grained nearshore prodeltas are a common feature of the Mediterranean deltaic systems and mark the frontal processes that occur at the continent/sea interface of non-tidal seas. Our results especially point out the potential use of heavy metals as markers of the tracking of riverine sediments, which is naturally an important condition when evaluating the ultimate fate of land-derived pollutants in the marine realm. But also more fundamental studies on the understanding of sedimentological processes and reconstruction of margin stratigraphies should consider these deposits as functional units for the distribution of platform sediments.