The remineralization of organic carbon on the North Carolina continental slope

The sources and fates of metabolizable organic carbon were examined at three sites on the North Carolina slope positioned offshore of Cape Fear, Cape Lookout and Cape Hatteras. The¹³C/¹²C compositions (δ¹³C) of the solid phase organic matter, and the dissolved inorganic carbon (ΣCO₂) produced during its oxidation, suggested that the labile fraction was predominantly marine in origin. The ΣCO₂ concentration gradient across the sediment-water interface, and by inference the ΣCO₂ flux and production rate, increased northward from Cape Fear to Cape Hatteras. Methane distributions and ΣCO₂ δ¹³C values suggest that the rate of anaerobic diagenesis increased northward as well. The differences in sedimentary biogeochemistry are most likely driven by an along-slope gradient of reactive organic carbon flux to the seabed. This trend in reactive organic carbon flux correlates well with macrofaunal densities previously observed at the three sites. Proximity to the shelf and the transport of particulate material by surface boundary currents may control the deposition of metabolizable material on the Carolina slope.Evidence for methanogenesis was found only on the Cape Hatteras slope. The methane, which was produced at a depth of approximately 1 m in the seabed, was consumed nearly quantitatively in the biologically mixed layer as it diffused upward. Irrigation of the sediments by infauna may have provided the necessary oxidant for the consumption of the methane.