Natural sources of carbon dioxide and methane from the fluvio-lacustrine sediments of the Fucino Plain (Central Italy): evidence from stable carbon isotopes

Abstract

The present investigation focused on the origin of CO2 and CH4in the soil gas and two gas vents present in the Fucino plain, a tectonicallyactive intramontane depression of central Italy. The soilgases have concentrations of CO2 ranging from 1 to 10% and CH4 inthe order of a few parts per million. Using carbon isotopes, it waspossible to identify two different sources of CO2 in the soil gas: biogenicCO2 (d13CCO2~ –20‰) produced in the fluvio-lacustrine sedimentsof the plain and CO2 produced from CH4 oxidation in soil(d13CCO2 –34/–38‰). The two gas vents yield CO2 contents of 2.63and 15.33% and CH4 of 94.7 and 8.5%. In the CH4-dominated gasvent, the carbon isotopes of methane (d13CCH4 = –64‰) point to agas of microbial origin. The other gas vent, with a d13CCH4 of–52.8‰, may be affected by a microbial oxidation process.