Role of the climatological and current variability on shelf-slope exchanges of particulate matter: Evidence from the Rhone continental margin (NW Mediterranean)
Résumé
Climatological, current and particulate flux data were gathered in the Grand-Rhone canyon on the Gulf of Lions continental margin for one year (Jan. 1988-Jan 1989). Time series were analyzed to determine the influence of physical exchange processes on particulate matter at the shelf-edge, with a special emphasis on the Northern Current variability. The synoptic variability of the Northern Current was linked to meanders of 2-5 day period. Its meso-scale activity presented a seasonal signal with maximum values in early spring. Peaks of particulate fluxes in the upper traps were little affected by large river and atmospheric inputs, but rather by enhanced shelf-slope exchanges at the shelf edge due to intense cross-slope fluctuations of the Northern Current. These fluctuations caused cross-isobath flows near the bottom, which appeared to be a potential mechanism in transporting particles off the shelf. At 900 m depth, high-flux events measured by sediment traps were primarily linked to periods of higher cross-slope current oscillations. This correlation suggests that vertical motions caused by these oscillations contribute to the suspended particulate matter transport through the process of bringing higher suspended material concentrations from above to greater depths. Vertical velocity estimates were derived through temperature fluctuations combined with vertical temperature gradient and from the kinematic boundary condition. A simple diffusion model indicates that the vertical turbulent mixing of suspended particulate matter, induced by the cross-slope current oscillations, yields downward fluxes of particles comparable to those collected by sediment traps