GRAIN-SIZE RELATIONSHIP BETWEEN SUSPENDED MATTER ORIGINATING IN THE GIRONDE ESTUARY AND SHELF MUD-PATCH DEPOSITS

The grain-size variations of the elementary fraction (without aggregates) of Gironde suspended matter during its transfer to the shelf were analyzed and compared with those of the surficial sediment of the west Gironde mud patch, located between water depths of 30 and 70 m, which is considered as a preferential deposition area for estuarine suspended matter (SM). The results made it possible to show that two processes complicate the relationships between the estuary and the shelf: a biogenic contribution identified as early as at the Gironde outlet, corresponding to a mode at 15-20-mu-m in the SM of the turbid plume, and an inorganic contribution of a fine-grained fraction coming from sandy shelf deposits corresponding to micas with a mode at 50-60-mu-m and settling on the inner part of the mud patch. The nourishment of the western and deeper part of this mud patch by estuarine suspended matter is derived from the benthic nepheloid layer; in terms of grain-size distribution this supply is characterized by the 7.5-mu-m mode, which is typical of particulate matter originating in the Gironde estuary.