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

As part of the ECOFER experiment (ECOMARGE program, JGOFS-France), concentrations of the natural radionuclide 210Pb were measured in particulate matter collected by sediment traps deployed for a 14-month period (June 1990–August 1991) on two mooring lines within the Cap-Ferret Canyon, on the French continental slope of the Bay of Biscay (northeastern Atlantic). 210Pb concentrations (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 mooring sites. Also, at equivalent trap depths, 210Pb activities were systematically higher on the deep slope mooring (3000 m) compared to the shallow one (2300 m). Fluxes of 210Pb generally decreased seaward and exhibited temporal variations, but with no marked seasonal trend, essentially matching variations in total mass flux. Various correlations between 210Pb activities and major constituents of trapped particles were examined. 210Pb 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 210Pb budget, as determined by the balance of local supply flux and fluxes to sediment traps and sediments, implies important lateral inputs to both mooring sites (65–89% of the flux measured by traps), a feature typical of continental margin environments. On the basis of annual mean 210Pb activity in traps of the two mooring sites, two particle sources are hypothesized. The first source, located in shallow waters of the shelf and upper slope, supplies the entire slope region. The second source, probably composed of resuspended particles from upper slope sediments, feeds essentially the upper and intermediate slope region. Maximum sedimentation rates, determined on seven box-cores, decreased with bottom 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, between 210Pb fluxes registered by the near-bottom traps and excess 210Pb inventories within the underlying sediments, suggesting that nuclide fluxes measured by traps during this experiment are representative of fluxes on a longer time scale (10–100 yr). Nevertheless, total mass fluxes measured by traps were 3–4 times lower than 210Pb -determined sediment accumulation rates, a discrepancy that may be more apparent than real, given the possible errors in the latter due to bioturbation.