SIDEW studies the past, present and future effects of dense shelf water cascading (DSWC) on the oceans seafloor. The formation of dense water contributes to the deep ocean ventilation, plays a role in the global thermohaline circulation (and hence global climate), and involves the massive transfer of energy and matter (including sedimentary particles, organic carbon and pollutants) from shallow to deep waters. The impact of this process on seafloor and deep-sea ecosystems is unquestionable. But how does DSWC form? DSWC starts when surface waters over the relatively shallow continental shelves become denser than surrounding waters, sink and cascades downslope along the seabed usually at high speeds. DSWC is a widespread phenomenon that occurs intermittently in time and space in European Seas (Arctic, Atlantic and Mediterranean) and around the globe, especially in high latitudes. The main mechanisms leading to the formation of DSW are cooling, evaporation, sea-ice freezing with brine rejection and deep sub-ice shelf melting in Antarctica.
DSWC is highly sensitive to temperature change in both the lower atmosphere and the sea surface, so global warming will likely modify the frequency and intensity of DSWC in the coming decades, which will in turn alter the biogeochemical transfers from shallow to deep and the ventilation of the deep ocean. This could significantly affect the deep-sea ecosystems whose functioning is critically dependent on this process. Overall, dense water formation is expected to decline over both continental shelves and offshore, particularly in polar and subpolar latitudes where sea-ice production is declining.
DSWC has been extensively studied by the physical oceanography community in the last decades. However, the far-reaching effects of these flows on the seafloor relief, the deep-sea ecosystem, the demersal fisheries, and the transfer of the climate signal, organic carbon, chemical pollutants and litter to the deep have been largely unexplored. Although DSWC is frequently invoked as a possible mechanism controlling seafloor morphology on the outer shelf and continental slope, there is a lack of studies that integrate concepts from geomorphology, sedimentology and oceanography. This shortcoming is particularly evident in high-latitude margins, despite the fact that the largest number of DSWC occurrences has been reported there.
In this project we have studied the global seafloor imprint of DSWC, with special emphasis on the sensitivity of this widespread process to changes in climate and in ice-sheet and sea-ice extent. A main contribution of the project is a better understanding of the physical processes involved in the propagation of these dense flows and their capacity to erode, transport and deposit sediment on continental shelves and slopes.
