Benthic dynamics and carbon fluxes on the NW European continental margin

Across the Goban Spur on the NW European continental margin, laterally directed, intermittent, off-slope transport of particulate matter takes place by intermediate and bottom nepheloid layers (BNLs). These are generated by semidiurnal tidal currents, which on the upper slope reach maximum near-bed speeds of up to 20 cm s−1, and which are directed predominantly off-slope (during 15–20% of the tidal cycle). BNLs are semi-permanently present, increasing in thickness above the seabed in downslope direction but decreasing in particle density. Near-bed currents measured on the upper slope are stronger in autumn than during summer, and both long- and short-term records suggest interannual variability. Aggregate formation in the benthic boundary layer (BBL) is considered the dominant process controlling particle accumulation. The organic fraction has low settling velocities and high residence times within the BBL. The flux of lithogenic material into the sediment on Goban Spur decreases from >44 g m−2 a−1 on the shelf edge to 6.9 and 4.9 g m−2 a−1 on the upper slope, then increases to a maximum of 19.1 g m−2 a−1 on the continental rise. CaCO3 flux increases with depth from about 13 g m−2 a−1 on the shelf edge to a maximum of 30.7 g m−2 a−1 on the continental rise, with minima on the upper slope. Flux values at comparable depths on Meriadzek Terrace are considerably higher. Mineralization of organic carbon on Goban Spur, representing more than 97.7% of the deposition flux, decreases with depth from 19.13 g C m−2 a−1 on the shelf edge, to 4.39 g C m−2 a−1 on the continental rise, and 1.10 g C m−2 a−1 on Porcupine Abyssal Plain. Organic carbon burial fluxes range between 0.05 and >0.16 gC m−2 a−1 on Goban Spur, and up to 0.41 gC m−2 a−1 on Meriadzek Terrace. Over 90% of the organic carbon mineralization at the sediment–water interface and directly below the seabed is driven by oxygen, as shown by pore water modelling and in situ oxygen measurements. Denitrification is of only minor (