Origin and variability of downward biogeochemical fluxes on the Rhone continental margin (NW Mediterranean)

A one year study of downward particle fluxes conducted in the northwestern Mediterranean Sea is presented. Two mooring lines equipped with sediment tr aps and current meters were deployed at around 1000 m depth on the northeas tern continental slope of the Gulf of Lions, one inside the Grand-Rhone can yon and the other outside on the adjacent open slope. Mean total mass fluxe s increased slightly with trap depth inside the canyon, a feature quite typ ical of fluxes in continental margin environments. The near-bottom trap ins ide the canyon collected more material than its counterpart deployed at equ ivalent depth on the open slope, indicating a preferential transport of mat erial within the canyon. Major biogeochemical constituents (organic and ino rganic carbon, opal, and siliciclastic residue) revealed a marked differenc e in particle composition between the sub-surface (80 m) and deeper traps, suggesting the existence of at least two sources of material. The two shall ower traps showed a clear biological signal: flux peaks were related to per iods of surface biological production, especially perceptible in summer and autumn. The particulate matter trapped at deeper levels in the canyon and on the open slope was characterized by a more stable composition with a maj or lithogenic contribution, originating from sedimentary material most prob ably resuspended on the upper- or mid-slope. The seasonal variability was d ominated by the summer/winter alternation; the latter period was characteri zed by a weak stratification of the water column and an enhanced current va riability favoring vertical exchanges. The present results are compared wit h those obtained previously in the Lacaze-Duthiers canyon on the southweste rn side of the Gulf of Lions. The comparison shows strong differences betwe en the NE entrance and the SW exit of the gulf, with respect to the general along-slope circulation of water masses, both in terms of intensity of par ticulate fluxes and transport processes. (C) 1999 Elsevier Science Ltd. All rights reserved.