Burst resuspension of seabed material at the foot of the continental slope in the Rockall Channel

To better understand the relationship between near-bottom hydrodynamics and sediment resuspension in the transition zone between the continental slope and the abyssal plain, an array of three moorings equipped with near-bottom sediment traps and current meters was deployed in a triangular configuration of at the foot of the south-east slope of the Rockall Channel, North Atlantic. In addition a fast sampling ADCP was moored in the centre of the triangle. Microscopic observations combined with grain-size analysis revealed that the material intercepted by the traps was mainly composed of loose phytodetritus and aggregates. Organic carbon and δ13Corg analyses further indicate that this material is relatively fresh and hence probably only transported over short distances. The total mass flux measured by the traps suggests input from either laterally advected or resuspended material during the deployment period. The measured flux was variable in time and space with a strong increase at the deepest site at the very foot of the slope. The measured flux did not correlate significantly with daily averaged current speed suggesting that processes other than those related to elevated bed shear stress are involved in the resuspension mechanism. Rapid changes in the hydrodynamics probably related to internal motions are evidenced by abrupt bursts in the current velocity and in the near-bottom turbidity. Their occurrence at slack tidal waters appears to be at the origin of the resuspension observed at the foot of the slope of the Rockall Channel. High resolution current velocity measurements suggest that resuspension is dominated by energetic, highly localised and short-term resuspension events, despite the abyssal depths. Part of these events resemble in many ways the solibores which have been previously observed to enhance sediment transport at much shallower depths over the continental slope while others rather appear to be associated to gravity waves.