PRESENT-DAY ORGANIC-MATTER SEDIMENTATION ON THE NW MEDITERRANEAN MARGIN - IMPORTANCE OF OFF-SHELF EXPORT

We present an overview of the organic matter distribution in surface s ediments sampled on the Gulf of Lions margin from 600 to 2,300 m depth . Surface deposits are characterized by a ''fluff'' layer a few millim eters to a few centimeters thick. In the underlying fine-grained sedim ents, the carbonate content is high (30% DW), and the organic carbon c ontent varies from 0.3 to 1% (DW). The highest organic carbon contents are along the axes and on the sides of the canyons (0.6-1%) and const itute midslope organic depocenters. The organic carbon content at 1,00 0 m depth depends on the width of the adjacent shelf. The lowest organ ic carbon value corresponds to the slightest declivity and, consequent ly, to a longer advective residence time. In the settling sides of the axial channel of the canyons, organic carbon concentrations decrease, although sedimentation rate is constant from the upper to the lower p art of the slope. This results from an increasing residence time in th e water column due to the advective transfer of organic material from epicontinental zone to upper and lower slope. In the lower axes and op en slopes, increasing sedimentation rates, due to sedimentary reworkin g and turbidity currents, dilute organic carbon with mineral grains.