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 sediments sampled on the Gulf of Lions margin from 600 to 2,300 m depth. Surface deposits are characterized by a ''fluff'' layer a few millimeters to a few centimeters thick. In the underlying fine-grained sediments, the carbonate content is high (30% DW), and the organic carbon content 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 constitute midslope organic depocenters. The organic carbon content at 1,000 m depth depends on the width of the adjacent shelf. The lowest organic carbon value corresponds to the slightest declivity and, consequently, 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 part of the slope. This results from an increasing residence time in the water column due to the advective transfer of organic material from epicontinental zone to upper and lower slope. In the lower axes and open slopes, increasing sedimentation rates, due to sedimentary reworking and turbidity currents, dilute organic carbon with mineral grains.