Intense mid-slope resuspension of particulate matter in the Faeroe–Shetland Channel: short-term deployment of near-bottom sediment traps

An array of four moorings was deployed on a transect perpendicular to the south-eastern slope of the Faeroe–Shetland Channel to measure near-bottom fluxes during a 12-day period in spring 1999. Each mooring combined current meters and two sediment traps equipped with optical backscatter sensors (OBS) situated at 2 and 30 m above the bottom, at water depths of 470, 700, 800 and 1000 m. During the deployment, near-bottom current velocities at 470, 700 and 800 m water depth increased abruptly to values as high as 55 cm s−1 (within a few hours). The sudden change in the cross-slope component of the current at 470 m was immediately followed by a severe drop in temperature at this depth, and large fluxes were intercepted in the mid-slope traps (700 and 800 m) on the same day, probably associated with the strengthening of the along-slope component. Organic carbon and nitrogen content of the trap samples were, except for the shallowest mooring, much lower than in particulate matter suspended in the water column. During the high flux event, particles possessed lower organic carbon and nitrogen values than during lower flux periods. This indicates that sedimentary material entered the traps. A simple multi-component mixing model was applied to the trap samples to estimate the relative contribution to our total mass fluxes of material from different sources: primary settling from the water surface, rebound material, and the relatively aged sediment. It showed that fluff was the main contributor to the resuspended flux but that sediment proper was resuspended at mid-slope during the strong increase in the current velocities and represented up to 70% of the total mass flux at that time. This study shows that near-bottom resuspension on the slope may be intense and, although the scale is here short-term the phenomena described might be larger in time and space.