Benthic dynamics and carbon fluxes on the NW European continental margin

Across the Goban Spur on the NW European continental margin, laterally dire cted, intermittent, off-slope transport of particulate matter takes place b y intermediate and bottom nepheloid layers (BNLs). These are generated by s emidiurnal 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-slo pe (during 15-20% of the tidal cycle). BNLs are semi-permanently present, i ncreasing in thickness above the seabed in downslope direction but decreasi ng in particle density. Near-bed currents measured on the upper slope are stronger in autumn than d uring summer, and both long- and short-term records suggest interannual var iability. Aggregate formation in the benthic boundary layer (BBL) is consid ered the dominant process controlling particle accumulation. The organic fr action 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 cont inental rise, with minima on the upper slope. Flux values at comparable dep ths 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 fl uxes range between 0.05 and > 0.16 g C m (-2) a (-1) on Goban Spur, and up to 0.41 g C m (-2) a (-1) on Meriadzek Terrace. Over 90% of the organic car bon mineralization at the sediment-water interface and directly below the s eabed is driven by oxygen, as shown by pore water modelling and in situ oxy gen measurements. Denitrification is of only minor (< 5%) importance for th e organic carbon mineralization; anoxic mineralization plays a (minor) role on shallow stations. Inventories and fluxes of Pb-210(xs) in surface sediments on Goban Spur ind icate that the slope below 1500m receives only about half of the amount of relatively young sedimentary material compared to the upper slope and shelf . Yet total sediment fluxes increase from the upper slope downward, indicat ing a significant contribution of reworked sediment in lower-slope sediment s. Pb-210-derived mixing coefficients correlate with macro- and meiofaunal density and biomass, decreasing with increasing depth downslope. Fluxes of lithogenic material, CaCO3 and Pb-210(xs) on the lower slope agre e reasonably well with fluxes recorded in deep-water sediment traps, sugges ting that the bulk of the sediment may be supplied via vertical settling th rough midwater depth. Benthic fluxes of organic carbon, however, are three times higher than deep-water trap fluxes, emphasizing the importance of lat eral transport of organic matter over the slope. At present, the NW Europea n continental margin at Goban Spur is not a major carbon depocenter. (C) 20 01 Elsevier Science Ltd. All rights reserved.