Sedimentation rates and fluxes in the continental shelf mud fields in the Bay of Biscay (France)

Sedimentation rates in shelf mud fields of the Bay of Biscay (the Gironde shelf mud patches and “La Grande Vasière”) were investigated for a 100-yr time scale, using 210Pb geochronology of nine cores. The distribution of apparent sedimentation rates reveals a general trend along the terrigenous dispersal system from the Gironde outlet of: no or negligible (<0.1 cm yr−1) cumulative sedimentation in the inner zone of the mud fields; 0.1–0.2 cm yr−1 in the outer zone where the supply is reduced; and a maximum value of about 0.5 cm yr−1 in the depocentre of the main mud field. Except for the latter value, the 210Pb sedimentation rates are of the same magnitude as longer-term rates based on 14C, and although slightly higher, are in good agreement with an earlier estimation determined for the rate of accumulation of surficial sediments, using pollen analyses. These differences are interpreted as being due to an increase in the supply of fine-grained sediment over the last century (from 0.2 to 0.5 cm yr−1), perhaps as a result of an intensification of estuarine suspended matter supply to the shelf. Compilation of a sediment budget shows that about one third of the annual Gironde solid discharge is trapped in the Gironde nearshore shelf mud fields. The mid-shelf muddy belt (i.e. “La Grande Vasière”) is the site of accumulation of fine-grained material from various terrigenous sources (mainly from the Gironde and Loire rivers). Two cores from the latter area yielded a maximum rate of sedimentation of 0.2 cm yr−1. Mixing processes dominate in this open middle shelf, due to the periodic sediment removal by long-period swells and bioturbation.