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

Abstract

Within the framework of the Ocean Margin Exchange Programme (OMEX), benthic carbon mineralisation was determined along the Goban Spur shelf-slope transition (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 combination with fluxes of solutes across the sediment–water interface. Pore water profiles of oxygen were obtained in situ with a benthic lander and on-deck 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 increased from 0.2 to 0.7% (wt/wt). At stations deeper than 1500 m, no further trends with depth were found. Carbon burial efficiencies in this low-sedimentation continental margin were not related to water depth and ranged between 0.8 and 2.3%. We conclude from these data that there is no distinct carbon depocenter at the Goban Spur continental slope, this in contrast to the slope at the western North Atlantic margin ( Anderson, Rowe, Kemp, Trumbore, & Biscaye (1994). Carbon budget of the Middle Atlantic Bight. Deep-Sea Research I, 41, 669–703.). Integrated carbon mineralisation rates indicated that oxic respiration accounted for more than 70% of the total carbon oxidation at all stations. Substantial anoxic mineralisation was identified only on the upper slope, while the contribution of denitrification never exceeded 10% along the entire transect. Benthic oxygen fluxes showed no direct response to pulses of organic material settling on the sea floor, as appearing in sediment traps, suggesting that the organic material deposited is dominated by refractory compounds. This finding was supported by steady-state modelling of pore water oxygen profiles which showed that the organic matter 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 pore water profiles indicated that within the experimental errors there was no significant contribution by bioirrigating organisms to the sediment–water exchange fluxes.