Cascading of dense shelf waters is a global oceanographic phenomenon whose effects on sedimentation processes on continental margins have been poorly studied and largely underestimated. The north-western Mediterranean is one of the regions of the world where massive dense water formation occurs because of cooling and evaporation of surface waters during winter-time. Concurrent with the well known open-sea convection process on the MEDOC region, coastal surface waters over the wide shelf of the Gulf of Lion also become denser than the underlying waters and cascade downslope during sustained periods of time until reaching their equilibrium depth. Through this climate-driven phenomenon, dense shelf waters carrying large quantities of particles and associated elements in suspension and as bed load are rapidly advected hundreds of meters deep, mainly through submarine canyons, acting as an efficient cross-margin transport mechanism. Numerous mooring observations collected recently in the north-western Mediterranean in the context of successive research efforts have allowed characterizing the sediment fluxes associated to this phenomenon and identify several morphological evidences of sediment erosion and deposition attributed to dense shelf water cascading. Additionally, deep-sea hydrographic observations after major cascading events indicate a direct effect on the Western Mediterranean Deep Water (WMDW) thermohaline properties and a subsequent formation of a thick and persistent bottom nepheloid layer that can spread throughout the western Mediterranean basin. While this process starts to be relatively well understood in terms of water and sediment transport, many scientific questions arise on how the large fluxes associated with this phenomenon may determine the functioning of deep-sea ecosystems in the north-western Mediterranean. Recent findings suggest that dense shelf water cascades from the Gulf of Lion have a direct effect on the long-term fluctuations of deep-sea fisheries and on the distribution of cold-water coral communities. Because of the flushing and recurrent behavior of major deep-reaching dense shelf water cascading events (i.e., approximately once every 6-7 years), a continuous monitoring of this phenomenon should be necessary to assess in detail its effects and implications in the deep-sea ecosystem and living resources. Since cascading of dense shelf water from continental shelves is a global phenomenon whose effects may have been largely underestimated, it can be anticipated that, under present conditions, cascading sites identified worldwide could constitute preferential regions for active contemporary sediment transport, from the coastal ocean to the deep basins, along with the associated consequences. Overall, their influence on deep-sea ecosystems worldwide could be larger than previously thought, and in that sense such oceanographic phenomenon should be properly studied using a trans-disciplinary approach

CIESM International Conference on East - West Cooperation in Marine Science, 1-3 December 2014, Sochi

Peer Reviewed