Cascades in Mediterranean submarine grand canyons

Continuous monitoring of water-column and near-bottom hydrosedimentary processes in the Mediterranean Sea over the last 15 years has resulted in a novel view of the functioning of this land-locked sea. Destratification of the water column and fast, dense, organic-matter-rich, and sediment-laden near-bottom currents occurring in late winter to early spring efficiently transfer matter and energy from the continental shelf and the upper ocean layers to the deep basin. These currents, known as dense shelf water cascading (DSWC), have been repeatedly measured by moored instrumentation during concurrent field experiments in the Gulf of Lion (northwestern Mediterranean Sea) and the Adriatic Sea (central Mediterranean). Physical oceanography observations made in the eastern Mediterranean in the early 1990s, together with observations of large-scale bed forms on the shelf floor, indicate that this phenomenon also occurs in the Aegean Sea (eastern Mediterranean) where it impacts the neighboring deep basins. The source areas of DSWC are the northernmost shelves of the Mediterranean Sea. Due to their location and inland topography, they are more exposed to the cold, persistent, intense northerly winter winds that cool shelf (and offshore) waters enough to make them denser than underlying waters, thus triggering their sinking once a density threshold is reached. It has also been observed that low river discharge on these shelves favors late winter-early spring cascading as shelf waters become denser than they would be under high river discharge. While offshore convection cells bring only "blue water" to the deep basin, DSWC events carry huge amounts of organic and inorganic substances as they scour the shelf and slope seafloor while sinking. Cascades of DSW may last for several weeks, and cascading waters sink continually deeper until they find their density equilibrium level, which changes from year to year. It has been observed that particularly intense DSWC events that carry shelf waters to the deepest parts of the western Mediterranean basin occur at subdecadal frequency. The influence of seafloor topography on the path followed by DSWC is best illustrated by submarine canyons. At specific locations, canyons are the main conduits for the cascading shelf waters, and from this developed the concept of "flushing submarine canyons." If the volume of cascading waters in a given event is too large, the canyons may be unable to accommodate it, and, therefore, those waters may escape from the canyons-especially where they are less entrenched. It has been also observed that DSW may cascade as sheet flows, sweeping continental slopes along tens of kilometers or more before spreading over the deep basin. The findings reported in this paper are just the tip of the iceberg in terms of the consequences of DSWC on deep-water mass formation and on the deep ecosystem of the Mediterranean Sea. As cascades often occur simultaneously with spring phytoplankton blooms in the various Mediterranean regions, there is no doubt that their role as a natural mechanism for carbon sequestration from the shallow ocean layers will demand the attention of the scientific community in the coming years.