ND-SR ISOTOPIC COMPOSITION OF PRESENT-DAY SEDIMENTS FROM THE GIRONDE ESTUARY, ITS DRAINING BASINS AND THE WESTGIRONDE MUD PATCH (SW FRANCE)

Abstract The isotopic composition of silty clay sediments in the Gironde Estuary, which were sampled between 1986 and 1995, shows homogeneity. The sediments from the Garonne and the Dordogne Rivers, including the tributary Isle River, are well mixed within the turbidity maximum zone. The composition of the clay solid load of the upstream Garonne River was also homogeneous between 1985 and 1990, even though there is a large diversity and temporal variabilities in the floods of the many tributaries of this river. Isotopic data were used to calculate the silty clay contribution of the two main rivers to the total estuarine stock: 70% of the total material is brought in by the Garonne River and 30% by the Dordogne River, including its tributary (the Isle River). The same isotopic signature can be observed in the present-day sediments and suspended materials of the WestGironde mud patch, on the adjacent Aquitaine continental plateau.