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 sed iment traps deployed in the Grand-Rhone canyon (Gulf of Lion, Mediterranean Sea) and on its adjacent open slope. The experiment, part of the JGOFS-Fra nce ECOMARGE program, lasted one year, from January 1988 to January 1989. C orrelations of Pb-210 or Po-210 concentrations with major constituents of t he flux were weak to nonexistent. The Pb-210 budget was evaluated for each sediment trap. The calculations indicated that there is a net lateral expor t of Pb-210 from the surface water layers of the canyon (80-200 m depth) an d 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 ori gin 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 t han unity. On the other hand, Po-210/Pb-210 ratios close to one at 900 m de pth and (sometimes) at 600 m depth in the canyon denoted particles that ori ginated from resuspension mainly on the open slope. A scenario of particle transfer process in this continental margin is proposed on the basis of the se radionuclide data, the temporal variability of the particulate fluxes an d the dynamical processes involved in shelf-slope exchanges. (C) 1999 Elsev ier Science Ltd. All rights reserved.