Spatial and temporal variability of downward particle fluxes on a continental slope: Lessons from an 8-yr experiment in the Gulf of Lions (NW Mediterranean)

Abstract

A long-term experiment of downward particle fluxes and currents has been initiated in 1993 on the continental slope of the Gulf of Lions (NW Mediterranean) and pursued within the frame of several French and European projects (PNEC, Euromarge-NB, MTP II-MATER, EUROSTRATAFORM). Sediment traps and current meters were deployed at several locations on this slope deeply incised by numerous canyons, with an extensive spatial coverage for the first 2 years (canyons at the entrance, middle and exit of the gulf with respect to the general along-slope circulation, head and mid-canyon depths, adjacent open slope). From late 1995 onwards, this design was reduced to the two mid-canyon moorings at the entrance and exit of the gulf. Monthly fluxes and hourly temperatures and currents were recorded at 500 m (30 m above bottom, mab) in the canyon heads, at 500 and 1000 m (respectively 530 and 30 mab) nominal depths at the mid-canyon sites, and at 750 m (30 mab) open slope. This study aims at describing the spatial, seasonal and interannual variability of flux intensity and composition of settling particles, and at analyzing the role of diverse forcings in the control of particle exchange across the margin. Results from the first 8 years (1993–2001) show that total mass fluxes – in the 10 1–10 4 mg m − 2 d − 1 range – increase along slope, particularly for the near-bottom traps, between the NE (Planier Canyon) and the SW (Lacaze-Duthiers Canyon) limits of the Gulf of Lions, indicating an increased shelf export of particulate matter in the western part of the system. Bulk chemical composition (organic matter, carbonate, opal and lithogenic fraction) remained rather stable during the course of the experiment, tending towards values typical of superficial shelf sediments at higher mass fluxes. First-order calculations using a simple two-component mixing model suggest a decreasing contribution of primary particles settling out of the overlying waters to the total flux from the entrance towards the exit of the system. Particulate material transferred to the deeper slope in the southwestern part of the Gulf of Lions appears therefore to predominantly originate in resuspended shelf and/or upper slope sediment. Downward particle fluxes and potential forcing parameters exhibit a high seasonal variability, with higher values from late autumn to early spring. Furthermore, unprecedented winter flux peaks observed in 1999 dominated the interannual differences, which otherwise were quite limited. Correlations between sources of particulate material on the shelf (i.e., river and atmospheric inputs, phytoplankton biomass and sediment resuspension), cross-slope exchange mechanisms (derived from in situ temperature and current records) and flux data indicate a predominant effect of dense cold water cascading on the exchange of particulate matter between the shelf and the slope.