A MASS BALANCE OF 13C AND 12C IN AN ORGANIC-RICH METHANE-PRODUCING MARINE SEDIMENT

Abstract

The sedimentary budgets of 12C and 13C were determined for the well-characterized coastal site of Cape Lookout Bight, North Carolina, USA. Degradation of organic carbon with a δ13C value of −19.2 ± 0.2‰ proceeds via sulfate reduction and methanogenesis. Neither process detectably alters the 13C/12C composition of the solid phase organic fraction that is ultimately buried. The remineralized carbon (ΣCO2 plus CH4) has an annual mean δ13C of −18.7 ± 2.2‰, indicating that the breakdown of solid phase organic material to dissolved intermediates does not discriminate between 13C and 12C.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 δ13C value of −58.9 ± 1.4‰. The ΣCO2 is enriched in 13C (−11.0 ± 1.2‰) because of the preferential synthesis of 12CH4. The relative rate of production of the ΣCO2 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 13C/12C content of the two diagenetic endproducts.