Fluxes and budget of Pb-210 on the continental margin of the Bay of Biscay(northeastern Atlantic)

As part of the ECOFER experiment (ECOMARGE program, JGOFS-France), concentr ations of the natural radionuclide Pb-210 were measured in particulate matt er collected by sediment traps deployed for a 14-month period (June 1990-Au gust 1991) on two mooring lines within the Cap-Ferret Canyon, on the French continental slope of the Bay of Biscay (north- eastern Atlantic). Pb-210 c oncentrations (range 63-218 dpm g(-1)) and fluxes (range 0.1-11.2 dpm cm(-2 ) yr(-1)) displayed typical gradients, increasing with depth at both moorin g sites. Also, at equivalent trap depths, Pb-210 activities were systematic ally higher on the deep slope mooring (3000 m) compared to the shallow one (2300 m). Fluxes of Pb-210 generally decreased seaward and exhibited tempor al variations, but with no marked seasonal trend? essentially matching vari ations in total mass flux. Various correlations between Pb-210 activities a nd major constituents of trapped particles were examined. Pb-210 activities were independent of the particle composition, indicating that in this area scavenging of this nuclide from the water column was not directly tied to the quality of settling particles. The Pb-210 budget, as determined by the balance of local supply flux and fluxes to sediment traps and sediments, im plies important lateral inputs to both mooring sites (65-89% of the flux me asured by traps), a feature typical of continental margin environments. On the basis of annual mean Pb-210 activity in traps of the two mooring sites, two particle sources are hypothesized. The first source, located in shallo w waters of the shelf and upper slope, supplies the entire slope region. Th e second source, probably composed of resuspended particles from upper slop e sediments, feeds essentially the upper and intermediate slope region. Max imum sedimentation rates, determined on seven box-cores, decreased with bot tom depth, from 0.3 to 0.4 cm yr(-1) at the canyon heads down to 0.06 cm yr (-1) at 3000 m. A good agreement was observed, at both mooring sites, betwe en Pb-210 fluxes registered by the near-bottom traps and excess Pb-210 inve ntories within the underlying sediments. suggesting that nuclide fluxes mea sured by traps during this experiment are representative of fluxes on a lon ger time scale (10-100 yr). Nevertheless, total mass fluxes measured by tra ps were 3-4 times lower than Pb-210-determined sediment accumulation rates, a discrepancy that may be more apparent than real, given the possible erro rs in the latter due to bioturbation. (C) 1999 Elsevier Science Ltd. AU rig hts reserved.