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 s helf mud patches and "La Grande Vasiere") were investigated for a 100-yr ti me scale, using Pb-210 geochronology of nine cores. The distribution of app arent sedimentation rates reveals a general trend along the terrigenous dis persal 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 y r(-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 th e latter value, the Pb-210 sedimentation rates are of the same magnitude as longer-term rates based on C-14, and although slightly higher, are in good agreement with an earlier estimation determined for the rate of accumulati on of surficial sediments, using pollen analyses. These differences are int erpreted 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. Compi lation of a sediment budget shows that about one third of the annual Girond e solid discharge is trapped in the Gironde nearshore shelf mud fields. The mid-shelf muddy belt (i.e. "La,Grande Vasiere") is the site of accumulatio n of fine-grained material from various terrigenous sources (mainly from th e Gironde and Loire rivers). Two cores from the latter area yielded a maxim um rate of sedimentation of 0.2 cm yr(-1). Mixing processes dominate in thi s open middle shelf, due to the periodic sediment removal by long-period sw ells and bioturbation. (C) 2001 Elsevier Science Ltd. All rights reserved.