Sediment budget and transfer of suspended sediment from the Gironde estuary to Cap Ferret Canyon

Abstract

A study of the particulate organic carbon content, the grain size of suspended sediment and turbidity in a transect between the Gironde estuary and Cap Ferret Canyon indicates that particles from the Gironde river are transferred to near the bottom of the canyon. The separation of turbid estuarine water into bottom and surface nepheloid layers takes place in the Gironde itself and in the nearshore zone. The development of these two separate turbid layers is due to vertical (a turbid surface plume) and horizontal (bottom layer) salinity gradients. The bottom nepheloid layer is caused primary by a horizontal supply of suspended sediment from the Gironde rather than from resuspension or vertical sedimentation as suggested by other workers. On the median and outer shelf the distribution of particles in the water column is influenced by thermal stratification. The thickness of the bottom nepheloid layer increases from the estuary towards Cap Ferret Canyon as a function of the thickness of the bottom waters. Detached nepheloid layers are observed at the shelf break coinciding with isopycnal surfaces due to the presence of cold waters in the Cap Ferret Canyon. Superimposed advective inputs from the northern part of the shelf and resuspension of particles from mud patches contribute to the supply of particles towards the canyon. Suspended sediment in transit over the shelf break represents 2–3% of the total discharge from the Gironde estuary. This is a considerable amount as 50% of the suspended sediment is trapped in the west Gironde mud patch and another undefined portion of the Gironde sediment load is transported along the coast by tidal currents.