Sediment transport along the Cap de Creus Canyon flank during a mild, wet winter

Abstract

Cap de Creus Canyon (CCC) is known as a preferential conduit for particulate matter leaving the Gulf of Lion continental shelf towards the slope and the basin, particularly in winter when storms and dense shelf water cascading coalesce to enhance the seaward export of shelf waters. During the CASCADE (CAscading, Storm, Convection, Advection and Downwelling Events) cruise in March 2011, deployments of recording instruments within the canyon and vertical profiling of the water column properties were conducted to study with high spatial-temporal resolution the impact of such processes on particulate matter fluxes. In the context of the mild and wet 2010–2011 winter, no remarkable dense shelf water formation was observed. On the other hand, the experimental setup allowed for the study of the impact of E-SE storms on the hydrographical structure and the particulate matter fluxes in the CCC. The most remarkable feature in terms of sediment transport was a period of dominant E-SE winds from 12 to 16 March, including two moderate storms (maximum significant wave heights = 4.1–4.6 m). During this period, a plume of freshened, relatively cold and turbid water flowed at high speeds along the southern flank of the CCC in an approximate depth range of 150–350 m. The density of this water mass was lighter than the ambient water in the canyon, indicating that it did not cascade off-shelf and that it merely downwelled into the canyon forced by the strong cyclonic circulation induced over the shelf during the storms and by the subsequent accumulation of seawater along the coast. Suspended sediment load in this turbid intrusion recorded along the southern canyon flank oscillated between 10 and 50 mg L⁻¹, and maximum currents speeds reached values up to 90 cm s⁻¹. A rough estimation of 10⁵ tons of sediment was transported through the canyon along its southern wall during a 3-day-long period of storm-induced downwelling. Following the veering of the wind direction (from SE to NW) on 16 March, downwelling ceased, currents inside the canyon reversed from down- to up-canyon, and the turbid shelf plume was evacuated from the canyon, most probably flowing along the southern canyon flank and being entrained by the general SW circulation after leaving the canyon confinement. This study highlights that remarkable sediment transport occurs in the CCC, and particularly along its southern flank, even during mild and wet winters, in absence of cascading and under limited external forcing. The sediment transport associated with eastern storms like the ones described in this paper tends to enter the canyon by its downstream flank, partially affecting the canyon head region. Sediment transport during these events is not constrained near the seafloor but distributed in a depth range of 200–300 m above the bottom. Our paper broadens the understanding of the complex set of atmosphere-driven sediment transport processes acting in this highly dynamic area of the northwestern Mediterranean Sea

External forcings, oceanographic processes and particle flux dynamics in Cap de Creus submarine canyon, NW Mediterranean Sea

“…Increased current speeds (up to 0.68 m s −1 ) and SSC (up to 7.37 mg L −1 ) were recorded. The fact that flooding occurred several days after the main pulse of downcanyon sediment transport suggest again, that erosion from the adjacent shelf was the main source of particles introduced in the canyon during mid-March (Martín et al, 2013). The turbid flow did not penetrate into the canyon deeper than about 350 m. The coarsening of the grain size of the particles collected at CC300 suggest that near-bottom currents during this event where strong enough to resuspend and transport coarse particles in suspension from the shelf to the canyon head (Fig.…”

Impact of storms on residence times and export of coastal waters during a mild autumn/winter period in the Gulf of Lion

“…The canyons located on the western part of the GoL (Cap de Creus, hereafter CCC, and Lacaze-Duthiers) play a predominant role in the export of dense coastal waters by guiding them across the continental slope (Canals et al, 2006;Palanques et al, 2008;Puig et al, 2008;Ulses et al, 2008b;Durrieu de Madron et al, 2013;Martín et al, 2013), allowing them to reach great depths in the coldest years. Canals et al (2006) and Ulses et al (2008c) showed that, during the extremely cold winter of 2005, more than two thirds of the GoL shelf waters (∼ 750 km 3 ) were exported to the deep ocean by cascading.…”

“…During the mild winter of 2010-2011, only brief, weak cascading was recorded (Rumín-Caparrós et al, 2013). In March, the CASCADE (CAscading, Storm, Convection, Advection and Downwelling Events) experiment documented the hydrological and hydrodynamic characteristics of the south-western GoL shelf and export zone during a period of low loss of buoyancy (Bourrin et al, 2015;Martín et al, 2013). Two intense easterly wind events resulted in coastal currents greater than 0.60 m s −1 over the 26 m deep water column of the south-western GoL shelf (Bourrin et al, 2015).…”

“…Two intense easterly wind events resulted in coastal currents greater than 0.60 m s −1 over the 26 m deep water column of the south-western GoL shelf (Bourrin et al, 2015). At the south-western tip of the GoL, this intense coastal current produced a downwelling of coastal waters into the upper part of the canyon, with intense currents (0.90 m s −1 ) and cold, low salinity waters near the bottom at about 300 m on the southern flank of the CCC (Martín et al, 2013). However, the absence of measurements in the surface layers did not allow to fully characterize the export of this relatively light water mass, especially across the narrow shelf along the Cap de Creus, which has never been instrumented.…”

External forcings, oceanographic processes and particle flux dynamics in Cap de Creus submarine canyon, NW Mediterranean Sea

“…Increased current speeds (up to 0.68 m s −1 ) and SSC (up to 7.37 mg L −1 ) were recorded. The fact that flooding occurred several days after the main pulse of downcanyon sediment transport suggest again, that erosion from the adjacent shelf was the main source of particles introduced in the canyon during mid-March 2011 (Martín et al, 2013). The turbid flow did not penetrate into the canyon deeper than about 350 m. The coarsening of the grain size of the particles collected at CC300 suggest that near-bottom currents during this event where strong enough to resuspend and transport coarse particles in suspension from the shelf to the canyon head (Fig.…”