Meso and small-scale variations of Pb-210 fluxes on the Northwestern Mediterranean continental margins
Résumé
Pb-210 was analysed in samples collected from (16) sediment traps, deployed during I year on the Northwestern Mediterranean continental margins, within the framework of the (MTP 1)-EUROMARGE-NB programme. The traps were moored within (5) submarine canyons and their adjacent open slope, corresponding to contrasting conditions of particle inputs (fluxes and constituents). The major meso-scale observation (at the margin) is the variation in Pb-210 fluxes along the slope. increasing by a factor of 2-3 between the entrance and the exit of the slope, in relation to the general (water) circulation. At a smaller scale (canyon), the Pb-210 fluxes showed trends which were common to the various sites. i.e. a seaward decrease at 500m depth and an increase, with depth, in the canyons. All these features are related to mass flux variations, except for periods with huge mass fluxes, when Pb-210 fluxes reached a constant value. Pb-210 activities decreased with increasing mass flux; then did not show clear relationship with the concentration of the major constituents of the flux. Pb-210 fluxes, obtained from the traps were mainly in excess of the theoretical Pb-210 flux, available in the overlying water column. Since Pb-210 inventories, measured on the basis of the shelf and slope bottom sediments. were also in excess in relation to the available flux. The margin, as a whole, appears as a sink for Pb-210. This boundary-scavenging process appears to be controlled completely by the mass flux of particles. Differences were observed between Pb-210 fluxes in the near-bottom traps and in the underlying sediments; there can be linked to mass flux and/or morphobathymetry (the trap flux is higher than the sediment flux in the canyon, but lower on the open slope). Overall, the differences were not in excess of 50%, confirming good representation of data collected by the sediment traps. However, this finding Must be taken into account when comparing organic carbon or other constituent fluxes, between the traps and the sediment.