Recycling of organic matter along a shelf-slope transect across the NW European Continental Margin (Goban Spur)

Within the framework of the Ocean Margin Exchange Programme (OMEX), benthic carbon mineralisation was determined along the Goban Spur shelf-slope tran sition (200-4500 m water depth) at the eastern margin of the North Atlantic . Carbon oxidation rates were derived from the pore water distributions of oxygen, nitrate, ammonium, dissolved manganese and dissolved iron in combin ation with fluxes of solutes across the sediment-water interface. Pore wate r profiles of oxygen were obtained in situ with a benthic lander and on-dec k in sediment cores retrieved by multi-coring. With water depths increasing from 200 to 1500 m benthic carbon oxidation rates decreased from 4.3 to 1. 5 mmol C m(-2) d(-1), while the interfacial organic carbon concentrations i ncreased from 0.2 to 0.7% (wt/wt). At stations deeper than 1500 m, no furth er trends with depth were found. Carbon burial efficiencies in this low-sed imentation continental margin were not related to water depth and ranged be tween 0.8 and 2.3%. We conclude from these data that there is no distinct c arbon depocenter at the Goban Spur continental slope, this in contrast to t he slope at the western North Atlantic margin (Anderson, Rowe, Kemp, Trumbo re, & Biscaye (1994). Carbon budget of the Middle Atlantic Eight. Deep-Sea Research I, 41, 669-703.). Integrated carbon mineralisation rates indicated that oxic respiration accounted for more than 70% of the total carbon oxid ation at all stations. Substantial anoxic mineralisation was identified onl y on the upper slope, while the contribution of denitrification never excee ded 10% along the entire transect. Benthic oxygen fluxes showed no direct r esponse to pulses of organic material settling on the sea floor, as appeari ng in sediment traps, suggesting that the organic material deposited is dom inated by refractory compounds. This finding was supported by steady-state modelling of pore water oxygen profiles which showed that the organic matte r being mineralised at stations deeper than 200 m had very low degradation rate constants ( < 1 y(-1)). Comparison of the measured oxygen and nutrient fluxes with the diffusive fluxes calculated from porewater profiles indica ted that within the experimental errors there was no significant contributi on by bioirrigating organisms to the sediment-water exchange fluxes. (C) 19 98 Elsevier Science Ltd. All rights reserved.