Seasonality and composition of particulate fluxes during ECOMARGE—I, western Gulf of Lions

Abstract

As part of the ECOMARGE program (ECOsystèmes de MARGE continentale), sequential sediment traps were deployed on the continental margin of the Gulf of Lions, in the northwestern Mediterranean Sea. Two sites located in the southwestern part of this region were selected for the first phase of the experiment: inner shelf (bottom depth: 27 m) and continental slope (Lacaze-Duthiers Canyon, bottom depth: 650 m). The canyon was selected as representative of the continental slope, because canyons comprise more than 50% of the slope area. Total mass, organic matter, opal, carbonate and siliciclastic residue fluxes were measured biweekly at 50, 100, 300 and 600 m in the upper part of the Lacaze-Duthiers Canyon (from July 1985 to April 1986) and for longer periods (2 weeks to 2 months) at 10 and 25 m on the shelf (from May 1985 to June 1986). Mass fluxes increased generally with depth, reaching values as high as 20,000 mg m −2 d −1 in the 600 m slope (canyon) trap; this indicated lateral transport of biogenic and abiogenic particulate matter, from local (adjacent shelf and upper slope waters) and distant origin (Rhoˆne River). Shelf-slope particulate transfer attested by the paracontemporaneity of high flux events on the shelf and the slope and by the continuity of the shelf benthic nepheloid layer and intermediate nepheloid layers over the canyon: this rapid and took generally less than 16 days (trap sample resolution time). Particle fluxes and compositions defined a layered system, in which the surface layers< 100m) were essentially characterized by biogenic material (organic matter and opal), and deep layers chiefly characterized by the collection of elastic material (carbonate and siliciclastic material). Significant temporal increases in total mass and constituent fluxes showed two frequencies: a low seasonal frequency, which was related to seasonal variations of the Liguro-Provençal Current and to the winter increase of the Rhoˆne River and other coastal river discharges; variations at higher frequencies, which were related to various impulse events such as summer internal waves, autumn and winter storms and spring nutrient enrichment. The role of the Lacaze-Duthiers Canyon in the particulate transfer across the continental margin is discussed within the particular context of this advective system. Although the importance of local, intra-canyon resuspension of bottom sediments could not be estimated, it seems that the high suspended particulate matter concentrations and particle fluxes observed are essentially related to the fact that this canyon acts as a natural trap which collects particles from the entire Gulf of Lions. Comparison of the flux necessary to sustain the unsupported 210Pb inventory in the canyon sediments, with the mean 210Pb flux measured by traps and of 210Pb-determined mass accumulation rates with total mass fluxes indicate that the canyon partly acts as a modern sediment depocenter of the particles swept into it from the shelf and the slope; also partly as a modern conduit of particles to the deep basin. Several biological mechanisms are probably responsible for the rapid settling of particles onto the slope (within the canyon); these may, thereby, reduce lateral transfer from the shelf to the open ocean and constitute a kind of “biological barrier”.