Comparison of horizontal and downward particle fluxes across canyons of the Gulf of Lions (NW Mediterranean): Meteorological and hydrodynamical forcing

Abstract

In the framework of the European project E urostrataform, an array of six near-bottom mooring lines was deployed in the heads of the major submarine canyons incising the continental slope of the Gulf of Lions (NW Mediterranean). All moorings were equipped with sediment traps, current meters and turbidity Optical Backscatters Sensors (OBS) situated at few meters above the bottom. Particulate mass fluxes were recorded weekly by the sediment traps between November 2003 and May 2004 and compared with horizontal export fluxes obtained from the turbidity and current meters. Results show that near-bottom vertical particulate mass fluxes generally increase along slope between the northeastern-most and the southwestern region of the Gulf of Lions. Horizontal export fluxes also show some increase southwestward with flux in the southwestern-most canyon up to two-orders of magnitude higher than in northeast canyons. Furthermore, most of the export flux is driven by energetic and short events such as marine storms and dense shelf water cascading. Although the consequence of storms and cascading was obvious from traps and OBS measurements, substantial differences between vertical and horizontal fluxes appeared. While sediment traps recorded the largest vertical flux during the autumn storm (4 December) with contributions from river discharge and resuspended material, horizontal flux as obtained from OBS data was highest in winter (21–24 February). We believe that such discrepancies are partly due to measurement artifacts originating from the particle size effects and/or the different height above the bottom occupied by the traps and OBS which complicate comparisons between vertical and horizontal fluxes. However, different water-column conditions (stratified versus unstratified) together with the hydrodynamic regime during the two periods appear as a likely explanation for the flux differences between fall and winter storms.