Dissolved organic carbon (DOC) fluxes of deep-sea sediments from the Arabian Sea and NE Atlantic

Diffusive flux of dissolved organic carbon (DOC) from deep-sea sediments is a poorly constrained component of the global marine carbon cycle, but may play an important role for DOC concentrations in the oceans. During six cruises between the years 1995 and 2000 we measured DOC concentrations in pore-waters of surface sediments and the ambient bottom waters at five stations in the deep Arabian Sea (water depths: 3190–4420 m) and at two sites in the NE Atlantic (4500–4800 m). Pore-water DOC concentrations were usually one order of magnitude greater than in bottom waters, with maximum DOC contents between 509 and 1526 μmol/L in the uppermost sediment interval (0–0.5 cm). DOC values generally decrease towards ∼3 cm core depth with slightly increasing concentrations in deeper layers under sub-oxic to anoxic conditions. Calculated benthic DOC effluxes into the bottom water caused by concentration gradients ranged between 59.8 and 223.9 μmol m −2 d −1 in the Arabian Sea and 47.2 and 121.2 μmol m −2 d −1 in the NE Atlantic. DOC efflux rates were highly correlated with the annual vertical particle flux rates at the sites and thus appear to be a function of the regional productivity. Taking an average global oceanic DOC efflux rate between 50.5 and 78.1 μmol m −2 d −1, we calculated that the DOC release is in the order of 70–103×10 12 g C yr −1 from sediments below 2000 m water depth. Thus, deep-sea sediments have the potential to represent a significant source of DOC in the oceans.