A mass balance of C-13 and C-12 in an organic-rich methane-producing marine sediment

The sedimentary budgets of C-12 and C-13 were determined for the well-characterized coastal site of Cape Lookout Eight, North Carolina, USA. Degradation of organic carbon with a delta(13)C value of -19.2 +/- 0.2 parts per thousand proceeds via sulfate reduction and methanogenesis. Neither process detectably alters the C-13/C-12 composition of the solid phase organic fraction that is ultimately buried. The remineralized carbon (Sigma CCO2 plus CH4) has an annual mean delta(13)C of -18.7 +/- 2.2 parts per thousand, indicating that the breakdown of solid phase organic material to dissolved intermediates does not discriminate between C-13 and C-12. The dominant carbon isotope effect expressed during the diagenesis of the organic matter is associated with methanogenesis. Methane produced at this site has a measured annual mean delta(13)C value of -58.9 +/- 1.4 parts per thousand. The Sigma CO2 is enriched in C-13 (-11.0 +/- 1.2 parts per thousand) because of the preferential synthesis of (CH4)-C-12. The relative rate of production of the Sigma CO2 and CH4, which is controlled by the depositional flux of metabolizable organic carbon, along with the fractionation factor associated with methanogenesis and the isotopic composition of the source carbon, determine the C-13/C-12 content of the two diagenetic endproducts.