OXYGEN AND ORGANIC CARBON FLUXES IN SEDIMENTS OF THE BAY OF BISCAY

Abstract : The relationship between particulate organic carbon (POC) measured in modern sediment and fluxes of exported POC to the sediment surface needs to be understood in order to use POC as a paleo-environmental proxy. Our objective was to compare POC concentrations, POC mineralization rates calculated from O2 consumption and POC burial rates. Benthic O2 distributions were determined in 58 sediment cores collected at different periods at 14 stations in the Bay of Biscay with depths ranging from 140 m to 2,800 m. Depth-dependent volume-specific oxygen consumption rates were used to assess rates of aerobic oxidation of organic matter (OM). O2 fluxes showed that the fast-decaying OM flux was much higher than buried POC. Sedimentary POC burial was calculated from mass accumulation rate and the POC content measured at the top of sediments. The proportion of buried POC relative to total exported POC ranged between 50% and 10%, depending on station location. The most labile fraction of exported POC engenders a steep decrease in the upper 5 mm of vertical O2 profiles. This rupture may be related to the bioturbation-induced mixing depth of fast-decaying carbon. A second class of POC would consist of slow-decaying OM that is mineralized within the upper decimetres of sediment through oxic and anoxic processes. At sites located below 500 m water depth, the fast-decaying carbon did not reach the anoxic sediment. When O2 is not entirely consumed in the presence of the fast pool of OM, the slow-decaying pool may control the O2 penetration depth.