Cap de Creus canyon: a link between shelf and slope sediment dispersal systems in the western Gulf of Lions, France

Abstract

Previous work in the Gulf of Lions, France has suggested that significant amounts of sediment may be escaping through the western part of this tectonically passive margin, despite it being far-removed from the primary sediment source (the Rhone river, ~160 km to the NE). It is hypothesized that the westernmost Cap de Creus canyon is intercepting the regional sediment-transport pathway and directing it offshore, allowing significant sediment export through this area. The overall goal of this project is to determine pathways and causes of sediment movement into Cap de Creus canyon to determine its role in off-shelf sediment export within the Gulf of Lions. Box cores were collected within the canyon and on the adjacent shelf on five cruises (2003-2005). Geochronology (210Pb-derived accumulation rates), grain-size distributions, and sedimentary structures (x-radiography) were analyzed to assess sedimentation patterns. Results indicate two mid-depth shelf depocenters (30-90 m water depth) separated by a zone of bypassing created by current acceleration around a headland. Within the canyon, the northern flank and mid-depth thalweg are modern depocenters of fine-grained sediments. The canyon head and southern flank are considered non-depositional for fine grains, although the head may be accumulating sands. Material enters the canyon from the northern rim (via advection of shelf benthic nepheloid layers), the southern rim (via dense-water cascading off the shelf), and through the canyon head (primarily coarse-grains). Budget calculations indicate that 9-23% of the sediment input to the Gulf is sequestered on the shelf and 1-4% is accumulating in upper Cap de Creus canyon. An ephemeral mud layer within the canyon axis indicates rapid deposition and frequent flushing, suggesting that sediment is moving through the upper canyon. This is also supported by high suspended-sediment concentrations associated with off-shelf dense-water flows. This study suggests that Cap de Creus canyon is an important conduit of sediment past the shelf break and the western margin is a primary location of sediment export from the Gulf of Lions.