Hydrodynamics of the Mediterranean Cassidaigne submarine canyon from observations

The more than 6000 submarine canyons worldwide constitute key oceanic morphologies incising continental slopes. They favor exchanges of organic matter, water masses, carbon, heat or pollutants between shallow and deep waters, driving ecosystems. They interact with the shelf and slope circulation generating local dynamics that have rarely been surveyed and observed.

Observations of currents, temperature and turbidity along the Cassidaigne canyon, in the Gulf of Lions, northwestern Mediterranean Sea, were carried out to understand the specific circulation patterns in submarine canyons and their transitions. Two oceanographic cruises led in 2017 and 2019 gathered data from the outer shelf and canyon head at 100-400 m to the base of the continental slope at 1900 m depth.

The Cassidaigne canyon is characterized by a steep and narrow morphology and is located in an active circulation area. Near the canyon head and on the shelf, the current is modulated by the stratification, the bottom morphology, the general circulation and the wind. Intermittent upwellings and consecutive relaxations lead to strong transient dynamics characterized by near inertial oscillations inside the canyon. The succession of upwellings induce a quasi-permanent residual up-canyon flow as observed in the narrow gorge area at 1700 m depth. Along-slope currents crossing the canyon during Northern Current intrusion events in the Gulf of Lions favor a temporary down-canyon circulation. Finally, turbidity currents were observed for the first time in connection with upwelling events, suggesting the triggering role of canyons’ internal hydrodynamics on shelf sedimentary processes.