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 deep basin, particularly in winter when storms and dense shelf water cascading coalesce to enhance the seaward export of shelf waters. <br><br> 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. <br><br> In the context of a mild and wet 2010–2011 winter, no remarkable dense shelf water formation was observed. On the other hand, the experimental setup allowed to study 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 of significant wave heights = 4–4.5 m. During this period, a plume of freshened, relatively cold and turbid water flowed at high speeds along the southern flank of CCC in an approximate depth range of 150–350 m. The density of this water mass only reached ~ 28.78 kg m<sup>−3</sup>, indicating that it did not cascade into the canyon and that merely downwelled into it forced by the accumulation of seawater along the coast during the storms and by the subsequent strong cyclonic circulation induced over the shelf. Suspended sediment load in this turbid intrusion was comparable at three heights above bottom where turbidimeters were installed (10, 75 and 115 m above bottom) on the southern canyon flank and oscillated between 10 and 50 mg L<sup>−1</sup>. Current speeds were also comparable in the depth range profiled by ADCPs (40 to 150 mab) and reached values up to 90 cm s<sup>−1</sup> during the peak of the strongest storm (13 March, <i>H</i><sub>s</sub> = 4.5 m). <br><br> Sediment transport at 75 mab on the southern canyon flank was estimated at 1–1.5 t m<sup>−2</sup> for the entire deployment while very close to the bottom (5 m above) in the canyon head it was less than 0.6 t m<sup>−2</sup> during the same period. We provide a rough estimation of 10<sup>5</sup> t of sediment transported through the canyon along its southern wall during a 3 day-long period of storm-induced downwelling. <br><br> 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. <br><br> This study highlights that remarkable sediment transport occurs in the CCC, an...

Natural vs. trawling-induced water turbidity and suspended sediment transport variability within the Palamós Canyon (NW Mediterranean)

“…11a). This sudden change in current flow, due to the compensation of the isopycnals, has been previously described as the reversal (relaxation) phase of cascading/downwelling events (Ulses et al 2008b), and described in detail in the Cap de Creus Canyon during a similar storm event in winter 2011 (Martín et al 2013). The second DSW pulse detected on February 21 was not associated to any storm event nor a flashflood river discharge (Fig.…”

Sources and exchanges of mercury in the waters of the Northwestern Mediterranean margin

“…Finer riverine material is exported farther on the Gulf of Lions shelf, undergoing a westward net transport through sedimentation/resuspension processes generated by infrequent easterly wind storm events (Durrieu de Madron et al, 2008;Ulses et al, 2008a;Guizien, 2009;Marion et al, 2010;Bourrin et al, 2015). These easterly storms induce downwelling at the southwestern end of the Gulf of Lions, especially in the Cap de Creus canyon (Palanques et al, 2006;2009;Ulses et al, 2008a;Martin et al, 2013), and a massive export of shelf water and resuspended particulate matter to the upper slope.…”

Integrated study of Mediterranean deep canyons: Novel results and future challenges

“…Current speed attenuation with increasing distance from shore and depth causes the selective deposition of particles along the canyon floor and axis according to their size, with coarser gravel and sand-sized particles being deposited in the canyon head and upper reaches and finer particles travelling to the mid and lower canyon and beyond (Pedrosa-Pamies et al, this volume;Sanchez-Vidal et al, 2008). Transport of fine particles during and after these events is not near-bottom restricted but distributed in a height range of 200-500 m, or even more, above the seafloor Martin et al, 2013), which results in the formation of a long-lasting benthic nepheloid layer covering the entire Western Mediterranean deep basin .…”

“…However those total mass fluxes likely are underestimations, as overfilling of the sediment trap cups and sediment trap saturation has been often observed. The downcanyon transport of shelf and upper canyon sedimentary particles frequently follows an in-echelon pattern Lopez-Fernandez et al, this volume-b;Martin et al, 2013). The first energetic event in autumn or winter leads to the remobilization of the loose, erodible top sediment layer on the shelf with subsequent transfer into the canyon head and upper-mid reaches, where it accumulates temporarily.…”