Continental sources of particles escaping the Gulf of Lion evidenced by rare earth elements: Flood vs. normal conditions

We tested the ability of rare earth elements (REE) to trace the lithogenic origin of particles escaping the Gulf of Lion (NW Mediterranean) during the exceptional oceanic flood of December 2003. Suspended particulate matters were simultaneously collected at the entrance (river mouths) and the exit (canyon heads) of the hydrosystem for analysis. River-specific signatures could be roughly discriminated while shelf-exported particles depicted a rather similar signal resembling that for the Rhône particles. When normalizing data of shelf sediments and suspended particles in canyons to the Rhône signature, a river-to-sea continuity in REE patterns was shown. This suggests that (i) middle- and outer-shelf areas are mainly fed by the solid discharges of the Rhône River and (ii) particles leaving the shelf during the event mostly originate from this continental source (directly and/or indirectly via resuspension of shelf sediments). Upon closer examination, the influence of hydro-dynamical conditions on the composition of particles channeled to the open sea could be shown. During the flood event studied here, even though the influence of the Rhône River is dominant, most of the shelf-exported particles are also composed of materials originating from small rivers. Conversely, during “normal” conditions, particles escaping the shelf clearly exhibit the Rhône particle imprint, suggesting that inputs from small rivers are too low to contribute significantly to the export.