Size sorting of fine-grained sediments during erosion: Results from the western Gulf of Lions

Sediment cores from the western Gulf of Lions France were subject to known bottom shear stresses with the goal of understanding size-specific sediment erodibility. On cruises in October 2004, February and April 2005, cores with an undisturbed sediment-water interface were collected along a transect extending seaward from the Tet river mouth. The cores were exposed to increasing shear stresses (0.01-0.4 Pa) onboard the vessel shortly after collection by using a Gust erosion chamber. Samples of the suspensate were collected during the erosion experiments and analyzed for disaggregated inorganic grain size (DIGS) using a Coulter Multisizer IIe. Size-specific mobility plots were generated by dividing the proportion of each grain size in suspension at each shear stress by its proportion in the sediment before erosion. If all grain sizes that make up the bottom sediment are eroded equally from the bed, then mobility equals one for all grain sizes. Values >1 indicate that the suspended sediment is enriched in the size class and values <1 indicate that the size class is enriched in the bed. Results show that in non-cohesive, sandy silts, fine grains (clays and fine silts) are eroded preferentially from the bed at low shear stresses. With increasing bottom stress progressively larger grains are eroded from the bed. In cohesive silts, preferential erosion of the finer sizes no longer occurs, with all sizes up to medium silts eroding at approximately the same rate. Effectively, a sandy silt can be winnowed of its fine grain fraction during erosion while cohesive silts cannot. This difference in the sortability of cohesive and non-cohesive sediment during erosion may control the position and maintenance of the sand-mud transition and the sequestration of surface-adsorbed contaminants.