Molecular (C1/C2+) and isotopic (¦Ä13CCH4, ¦ÄDCH4, ¦Ä13CC2H6) indicators delineate a microbial source for the CH4 and C2H6. In Core UBGH1-10B-5R, the carbon isotopic fractionation factor between CO2 and CH4 (¦Åc ¡Ö ¦Ä13CCO2 ? ¦Ä13CCH4) was observed to increase from 50 to 68 as pressure was reduced during our experiments. These changes illustrate the isotope fractionation that results from the preferential release of 13C depleted CO2 during the early stages of depressurization.
The constant carbon isotope separation (¡«10-20¡ë) between CO2 in void gases and in the dissolved inorganic carbon (DIC) pool shown in cores from the Ulleung Basin, has also been observed in gas hydrate-bearing sediments elsewhere. It is important to consider this effect when using carbon isotopic data in carbon cycling models, specifically when unraveling metabolic pathways of methane generation. In situations where partial CO2 degassing of a core is suspected, we recommend the use of the carbon isotope ratio of DIC instead of the gas measurements to best delineate the methane cycling pathways in marine systems.