Rates of organic carbon oxidation in deep sea sediments in the eastern North Atlantic from pore water profiles of O2 and the δ13C of dissolved inorganic carbon

The remineralization rate of sedimentary organic carbon (Rorg) and the depth-integrated, diffusion-supplied O2 consumption rate (IOC) during microbial metabolism in sediments was investigated in three deep-sea sites at 1100, 2000 and 3500 m water depth in the eastern north Atlantic during the spring and summer 1998. In-situ pore water O2 profiles yielded an IOC of 0.45±0.07 mmol O2 m−2 day−1 at the deepest site (n=3) and ca. 1–1.5 mmol O2 m−2 day−1 at the shallowest site (n=2). The Rorg was independently estimated at all three sites from ex-situ pore water profiles of the isotopic composition of total dissolved inorganic carbon (δ13CT), assuming that the concentration and isotopic composition of pore water CT with depth in the sediment was controlled only by microbial oxidation of isotopically depleted sedimentary organic carbon. The Rorg was thus estimated to be ca. 0.5–0.6 mmol C m−2 day−1 at the shallowest site and ca. 0.3–0.4 mmol C m−2 day−1 at the two deeper sites. Stoichiometric and isotopic constraints indicated that oxic remineralization of sedimentary organic matter was the dominant metabolic pathway in the sediments at 3500 m water depth. Similarly, stoichiometric and isotopic constraints suggested that the Rorg estimates from the ex-situ pore water δ13CT profiles from 1100 and 2000 m water depth were likely to be minimum values and provided evidence for the occurrence of post-oxic remineralization processes. Post-oxic metabolism in the sediments of these sites could be linked to, or even augmented by, the non-diffusive mode of supply of organic matter mediated by infaunal organisms below the oxic sediment layer.