Role of the climatological and current variability on shelf-slope exchanges of particulate matter: Evidence from the Rhone continental margin (NW Mediterranean)

Climatological, current and particulate flux data were gathered in the Gran d-Rhone canyon on the Gulf of Lions continental margin for one year (Jan. 1 988-Jan 1989). Time series were analyzed to determine the influence of phys ical exchange processes on particulate matter at the shelf-edge, with a spe cial 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 ma ximum values in early spring. Peaks of particulate fluxes in the upper trap s were little affected by large river and atmospheric inputs, but rather by enhanced shelf-slope exchanges at the shelf edge due to intense cross-slop e fluctuations of the Northern Current. These fluctuations caused cross-iso bath flows near the bottom, which appeared to be a potential mechanism in t ransporting particles off the shelf. At 900 m depth, high-flux events measu red by sediment traps were primarily linked to periods of higher cross-slop e current oscillations. This correlation suggests that vertical motions cau sed by these oscillations contribute to the suspended particulate matter tr ansport through the process of bringing higher suspended material concentra tions from above to greater depths. Vertical velocity estimates were derive d through temperature fluctuations combined with vertical temperature gradi ent and from the kinematic boundary condition. A simple diffusion model ind icates 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. (C) 1999 Elsevie r Science Ltd. All rights reserved.