Dense-water cascading and the resulting sedimentation patterns in Lacaze-Duthiers and Cap Creus Canyons, Gulf of Lions

Modern sediment dispersal in the Gulf of Lions is controlled by the interaction of strong marine storms and dense-water formation on the shelf. Regional cyclonic circulation and the narrowing of the southwest shelf lead to off-shelf sediment export via Lacaze-Duthiers and Cap Creus Canyons. Canyon heads are areas of sediment bypassing due to intense current interaction and the frequent occurrence of dense-water cascading. In winter 2004-2005, off-shelf export was dominated by dense-water cascading with additional contributions from internal-wave resuspension, hemipelagic settling and sediment gravity flows. Sedimentation in the canyon heads is asymmetrical with erosion (and coarse sediment) dominating the western flank and main entrant, and pockets of fine-grained sediment on the eastern flank where the Northern Current drapes over the canyon edge. Sedimentation in the lower canyon depths (>400 m) shows evidence of both hemipelagic settling and intermittent sediment gravity flows. Modern fine-grained sediment accumulation in the southwest canyons accounts for <10% of the annual sediment discharge from the Rhône River. This is likely a conservative estimate as the area used to calculate the budget was based on channel-floor areas only. Areas of increased sediment focus may occur within deeper portions of canyons, which were not resolved in this study. Lacaze-Duthiers and Cap Creus Canyon are modern conduits for sediment transport from the shelf to the deep sea with increased deposition likely occurring in years when dense-water cascading occurs in conjunction with winter-storm resuspension events.