The present investigation focused on the origin of CO2 and CH4 in the soil gas and two gas vents present in the Fucino plain, a tectonically active intramontane depression of central Italy. The soil gases have concentrations of CO2 ranging from 1 to 10% and CH4 in the order of a few parts per million. Using carbon isotopes, it was possible to identify two different sources of CO2 in the soil gas: biogenic CO2(δ13CCO220) produced in the fluvio-lacustrine sediments of the plain and CO2 produced from CH4 oxidation in soil (δ13CCO234/38). The two gas vents yield CO2 contents of 2.63 and 15.33% and CH4 of 94.7 and 8.5%. In the CH4-dominated gas vent, the carbon isotopes of methane (δ13CCH2= 64) point to a gas of microbial origin. The other gas vent, with a δ13CCH4 of −52.8‰, may be affected by a microbial oxidation process.

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