The carbon isotope biogeochemistry of a methanogenic marine sediment

This study was undertaken to quantify the rates of processes controlling the {dollar}Sigmarm COsb2{dollar} profile in the porewaters of Cape Lookout Bight, North Carolina. A carbon isotope mass balance was determined by measuring the diffusive, ebullitive and burial fluxes and isotopic signatures of {dollar}Sigmarm COsb2{dollar} and CH{dollar}sb4{dollar}. The flux-weighted isotopic signature of the remineralized carbon {dollar}(-18.9pm 2.7{dollar} per mil) agreed with the isotopic composition of the remineralized organic carbon determined from the particulate organic carbon (POC) {dollar}deltasp{lcub}13{rcub}{dollar}C profiles {dollar}(-19.2pm 0.2),{dollar} verifying the flux and isotopic signature estimates.; The measured {dollar}deltasp{lcub}13{rcub}{dollar}C values of the {dollar}Sigmarm COsb4{dollar} and CH{dollar}sb4{dollar} diffusive fluxes were different from those calculated from porewater gradients, and appear to be controlled by methane oxidation at the sediment-water interface, although other processes cannot be excluded.; The absence of downcore change in the {dollar}deltasp{lcub}13{rcub}{dollar}C value of the POC fraction and the identical isotopic composition of the POC and the products of remineralization indicate that no isotopic fractionation is expressed during the initial breakdown of the POC, despite its heterogeneous composition.; To investigate the relationship between {dollar}Sigmarm COsb2 deltasp{lcub}13{rcub}{dollar}C values and rates of the dominant remineralization processes, the {dollar}Sigmarm COsb2 deltasp{lcub}13{rcub}{dollar}C was measured seasonally. Porewater {dollar}Sigmarm COsb2 deltasp{lcub}13{rcub}{dollar}C profiles appear to be controlled by the rates of production from the two dominant sources, sulfate reduction and methanogenesis, by surficial processes including methane oxidation, and the oxidation state of the organic matter being remineralized.; A tube incubation experiment was performed to determine the isotopic signature of the {dollar}Sigmarm COsb2{dollar} produced by sulfate reduction and methanogenesis. The {dollar}deltasp{lcub}13{rcub}{dollar}C of the {dollar}Sigmarm COsb2{dollar} produced in the sulfate reduction zone determined from the tube incubation was {dollar}-14.3pm 1.9,{dollar} a value enriched in {dollar}sp{lcub}13{rcub}{dollar}C relative to the labile organic fraction, and may be caused by methanogenesis occurring in the sulfate reduction zone. The {dollar}deltasp{lcub}13{rcub}{dollar}C of the {dollar}Sigmarm COsb2{dollar} produced in the methanogenic zone was estimated to be +44 per mil, whereas the co-produced methane was {dollar}-{dollar}65 per mil. The measured concentration and {dollar}Sigmarm COsb2 deltasp{lcub}13{rcub}{dollar}C was reproduced using a diagenetic model and measured rates of sulfate reduction, {dollar}Sigmarm COsb2{dollar} production, and the isotopic signature of the {dollar}Sigmarm COsb2{dollar} production in the two biogeochemical zones.