On the oxidation and burial of organic carbon in sediments of the Iberian margin and Nazaré Canyon (NE Atlantic)

Abstract

As a contribution to the EC-OMEX-II program, sediment carbon and nitrogen budgets are presented for the Iberian Margin (northeastern Atlantic). The budgets for degradable organic carbon and associated nitrogen were calculated from sediment and pore water properties, using a steady-state version of a numerical coupled diagenetic model, OMEXDIA. Data were collected throughout the major upwelling period along five transects, four of which were located on the open margin and one positioned in a major submarine canyon, the Nazaré Canyon. A comparison of in situ oxygen profiles measured with monocathodic microelectrodes and with Clark type microelectrodes showed that monocathodic electrodes overestimate the oxygen concentration gradient near the sediment–water interface. This artifact probably results from the loss in sensitivity of the monocathodic microelectrode during profiling. Shipboard time course measurements with Clark type electrodes demonstrated transient conditions upon sediment retrieval on deck and indicated enhanced rates of oxygen consumption in the surface sediment, presumably as a result of lysis or exudation of oxidisable substrates by infauna. As a result, oxygen fluxes calculated from shipboard oxygen profiles overestimated in situ fluxes by up to a factor of 5 for water depths >1000 m. The sediments from the canyon and from a depositional area on the shelf were enriched in organic carbon (3–4.5 wt%) relative to the open margin stations (0.5–2 wt%) and showed C/N ratios exceeding Redfield stoichiometry for marine organic matter, indicating there was deposition of organic carbon of terrestrial origin in these areas. The oxidation of organic carbon on the open margin declined from ~11 gCm −2y −1 on the shelf to 2 gCm −2y −1 at 5000 m water depth, and was dominated by aerobic oxidation. The reactivity of the degradable organic carbon at the time of deposition was <2.5 y −1 on the shelf, and declined to <0.5 y −1 offshore. The burial of refractory organic carbon at the stations along the open margin transects also declined with increasing water depth from ~5 gCm −2y −1 on the shelf to <1 gCm −2y −1 at 2000 m depth, whereas the burial of particulate inorganic carbon declined from ~20 gCm −2y −1 to <5 gCm −2y −1. A comparison of the estimated total organic carbon deposition and predicted delivery for the shelf suggest that 58 to 165 gCm −2y −1 is oxidized in the water column, laterally advected, or focused into one of the canyons. Anaerobic oxidation, denitrification and, therefore, total oxidation of organic carbon was enhanced within the canyon relative to the open margin. Total organic carbon oxidation decreased with water depth from 22 gCm −2y −1 at the head of the canyon to 3 gCm −2y −1 over its fan. The reactivity of the organic carbon deposited in the canyon was lower than those of the shelf stations, suggesting that the canyon is being enriched in older, laterally advected organic matter. The burial of refractory organic carbon in sediments from the Nazaré Canyon was considerably higher than in the sediments from the open margin; it also decreased with depth from 20 gCm −2y −1 at 343 m to ~2.5 gCm −2y −1 at 4298 m water depth. The burial of particulate inorganic carbon was slightly lower than that of refractory organic carbon. The burial of refractory organic carbon and the deposition of degradable organic carbon were both positively correlated with the sedimentation rates for the Iberian Margin, and indicated burial efficiencies were 0.6 to 48%. A single trend for burial efficiency versus sedimentation rate for both the canyon and the open margin indicates that the sedimentation rate was the master variable for the geographical distribution of organic carbon oxidation and carbon preservation on the NW Iberian Margin.