The continental slope is a steep, narrow fringe separating the coastal zone from the deep ocean. During low sea-level stands, slides and dense, sediment-laden flows erode the outer continental shelf and the continental slope, leading to the formation of submarine canyons that funnel large volumes of sediment and organic matter from shallow regions to the deep ocean1. During high sea-level stands, such as at present, these canyons still experience occasional sediment gravity flows2–5, which are usually thought to be triggered by sediment failure or river flooding. Here we present observations from a submarine canyon on the Gulf of Lions margin, in the northwest Mediterranean Sea, that demonstrate that these flows can also be triggered by dense shelf water cascading (DSWC)—a type of current that is driven solely by seawater density contrast. Our results show that DSWC can transport large amounts of water and sediment, reshape submarine canyon floors and rapidly affect the deep-sea environment. This cascading is seasonal, resulting from the formation of dense water by cooling and/or evaporation, and occurs on both high- and low-latitude continental margins6–8. DSWC may therefore transport large amounts of sediment and organic matter to the deep ocean. Furthermore, changes in the frequency and intensity of DSWC driven by future climate change may have a significant impact on the supply of organic matter to deep-sea ecosystems and on the amount of carbon stored on continental margins and in ocean basins.
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This work was supported by the European Commission (EUROSTRATAFORM, EURODOM and HERMES projects), the Office of Naval Research, CNRS-INSU and the Catalan Government. Sediment cohesion data were provided by N. Sultan, N data by T. Tesi, and DOC data by M. Pujo-Pay. Multibeam bathymetry was collected in cooperation with Fugro Survey Ltd and AOA Geophysics. Contributions of the scientific and technical staff at the authors’ home institutions are warmly acknowledged. Author Contributions All authors contributed to the design and implementation of the experimental strategy. M.C. steered the integration and joint analysis of the data, interpreted side scan sonar data and wrote the final version of the paper in cooperation with S.H. P.P., X.D.d.M. and A.P. took the responsibility for time series, X.D.d.M. for hydrology data, and S.H. and J.F. for sediment trap data. All authors discussed the results and commented on the manuscript.