Pb-210 and Po-210: tracers of particle transfer on the Rhone continental margin (NW Mediterranean)

Pb-210 and Po-210 were measured in particulate matter collected by five sediment traps deployed in the Grand-Rhone canyon (Gulf of Lion, Mediterranean Sea) and on its adjacent open slope. The experiment, part of the JGOFS-France ECOMARGE program, lasted one year, from January 1988 to January 1989. Correlations of Pb-210 or Po-210 concentrations with major constituents of the flux were weak to nonexistent. The Pb-210 budget was evaluated for each sediment trap. The calculations indicated that there is a net lateral export of Pb-210 from the surface water layers of the canyon (80-200 m depth) and from the open slope (900 m depth). In the lower layers of the canyon (600-900 m depth), a net lateral input of Pb-210 is required. The two nuclides land especially the ratio Po-210/Pb-210) were useful in identifying the origin of the particles. A biological signature was seen clearly in the upper layers and on the open slope, where the Po-210/Pb-210 ratios were greater than unity. On the other hand, Po-210/Pb-210 ratios close to one at 900 m depth and (sometimes) at 600 m depth in the canyon denoted particles that originated from resuspension mainly on the open slope. A scenario of particle transfer process in this continental margin is proposed on the basis of these radionuclide data, the temporal variability of the particulate fluxes and the dynamical processes involved in shelf-slope exchanges