Dense shelf water cascades and particle transport. A process-based numerical model approach

Downslope overflows of dense shelf-water, also known as dense shelf-water cascading (DSWC), are an important atmospheric-driven oceanographic process that occur in certain polar and temperate margins around the world. DSWC events are essential to the formation and ventilation of the deep ocean waters and provide an important link between shallow and deep waters, as they involve not just the massive transfer of water volumes but also sedimentary particles, organic carbon, pollutants and litter.

Field observations show that DSWC can rapidly reshape the seafloor, particularly in submarine canyons. It has been suggested that dense water fluxes could generate continental slope gullies in Polar Regions too. In situ near-bottom velocities up to 1.25 m·s^-1 have been measured for these currents, which are similar to those attained by turbidity currents, although suspended sediment concentrations tend to be very much lower in DSWC, with values of 0.002 to 0.005 g·l^-1. For this reason, these dilute flows have largely been considered as inefficient pumps for sediment transport. However, the water volumes transported by DSWC events are exceptionally large, as these flows can last for days to weeks, or even months in certain polar regions. Hence, we advocate that this fact is enough to reconsider the former assumption. We tackle this question using a process-based depth-integrated numerical model for gravity-driven density flows, which was initially developed for turbidity currents (Nixes-Tc model, developed at IFREMER). Our modelling analysis, based on Antarctica field observations, show the importance of confining morphological features (i.e. coast capes, cross-shelf troughs, canyons and gullies) to concentrate and guide dense shelf water flows and, ultimately, to renew the oceans deep water. We also study the capacity of individual DSWC events to transport sediment and provide insight into the cumulative effect of repeated DSWC events in shaping the seafloor.

Acknowledgments: This project has received funding from the Spanish Ministry of Science and Innovation and the Spanish State Research Agency (grants EIN2020-112179 and PID2020-114322RBI00), from the European Union's Horizon 2020 research and innovation programme (Marie Sklodowska-Curie grant 658358), and from a postdoctoral grant of the International Association of Sedimentologists (IAS).