Processes in the benthic boundary layer at the Iberian continental margin and their implication for carbon mineralization

Abstract

This paper evaluates the role of biological processes in the cycling of particulate organic matter within the benthic boundary layer (BBL). It reveals that horizontal, advective fluxes of particulate matter exceed the vertically sinking fluxes through the water column. The data suggest that there is an important additional mechanism of carbon mineralization at hydrodynamically energetic continental margins, and that not the sediment surface but the near-bed fluid layer of the BBL is the major region for organic carbon mineralisation, and the amount of carbon finally buried depends on the time that aggregates are exposed to the BBL. Water and sediment samples were taken during three cruises to the North East Atlantic continental margin. The study site covered the area between 41°N, 9°W and 44°N, 13°W with transects perpendicular to and along the continental margin, resolving velocity and particle features of the benthic boundary layer [BBL] at 5–40 cm height above the sea floor. Particulate organic carbon in the BBL ranged from 29 to 102 mg m −3, total particulate matter from 1.2 to 6.2 g m −3 and Chloroplastic pigments from 0.01 to 0.26 mg m −3. Sediments on the continental margin consisted of an aggregated surface layer, which covered the underlying sediments. At all stations, surface erosion yielded BBL aggregates > 100 μm at critical friction velocities [u ∗c] of 0.4 to 1.2 cms −1. The erosion thresholds of underlying cohesive sediments typically increased from 1.1 to 1.8 cms −1 from shallow to deeper sites. Long-term flow velocity data reveal a tidally modulated flow field with repetitive cycling of particles between sea bed and suspension, representing altogether a situation of prolonged particle resuspension time, moderate residual currents and large variability of flow speed. The currents parallel and cross slope (north-south, on -and offslope) move the BBL aggregates with little net distance in one direction per tidal cycle. Within the ensuing particle transport loop alternating between bed and suspension mode, the material is redistributed. However, because of the low net downslope transport of these remoulded particles, little if any labile carbon reaches the steep lower continental slope via lateral advection within the study area.