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Abstract: In the framework of the European project Eurostrataform, an array of six near-bottom mooring lines was deployed in the heads of the major submarine canyons incising the continental slope of the Gulf of Lions (NW Mediterranean). All moorings were equipped with sediment traps, current meters and turbidity Optical Backscatters Sensors (OBS) situated at few meters above the bottom. Particulate mass fluxes were recorded weekly by the sediment traps between November 2003 and May 2004 and compared with horizontal export fluxes obtained from the turbidity and current meters. Results show that near-bottom vertical particulate mass fluxes generally increase along slope between the northeastern-most and the southwestern region of the Gulf of Lions. Horizontal export fluxes also show some increase southwestward with flux in the southwestern-most canyon up to two-orders of magnitude higher than in northeast canyons. Furthermore, most of the export flux is driven by energetic and short events such as marine storms and dense shelf water cascading. Although the consequence of storms and cascading was obvious from traps and OBS measurements, substantial differences between vertical and horizontal fluxes appeared. While sediment traps recorded the largest vertical flux during the autumn storm (4 December) with contributions from river discharge and resuspended material, horizontal flux as obtained from OBS data was highest in winter (21–24 February). We believe that such discrepancies are partly due to measurement artifacts originating from the particle size effects and/or the different height above the bottom occupied by the traps and OBS which complicate comparisons between vertical and horizontal fluxes. However, different water-column conditions (stratified versus unstratified) together with the hydrodynamic regime during the two periods appear as a likely explanation for the flux differences between fall and winter storms. [Copyright &y& Elsevier]

Abstract: To investigate the role of coastal canyons in the transfer of organic matter from the shelf to the slope and basin, we deployed sediment trap/current meter pairs at the head of five canyons in the Gulf of Lions (GoL) between November 2003 and May 2004. Analysis of organic carbon, biogenic silica, Corg isotopic composition, Corg/total nitrogen, chloropigments, and amino acids clearly shows the seasonal influence and effect of extreme meteorological events on the composition of collected particles. The sampling period was divided into three “scenarios”. The first corresponded to a large easterly storm and flood of the Rhone river during stratified water column conditions; the composition of material collected during this event was influenced by increased transfer of riverine and coastal particulate matter, with a lower Corg content. During the second “fall-winter” scenario, northern and northwestern winds blowing over the shelf caused cooling and homogenization of the shelf water column; particles collected at this time reflected the homogeneous source of particulate matter transported through canyons; particles sitting in the vicinity of canyon heads are most likely swept downslope by the general south-westward circulation. Organic tracers indicate a degraded origin for organic matter transported during this period. A third “spring” scenario corresponded to northern winds alternating with eastward windstorms that triggered and/or enhanced the cascading of dense waters accumulated on the bottom of the shelf due to previous cooling. These conditions occurred in conjunction with increased phytoplankton productivity in shelf surface waters. Organic matter advected mainly by dense shelf water cascading was fresher due to the transport of newly produced particles and a variable terrestrial fraction; this fraction depended on the proportion of resuspended material accumulated during previous high discharge periods that was involved in each transport pulse. The tight link shown between meteorological conditions and organic matter transport is important for continental margin geochemical studies as future changes in climatic conditions may lead to dramatic changes in carbon sequestration capability and in the ecosystems of deep margin environments. [Copyright &y& Elsevier]