Rate of diffuse carbon dioxide Earth degassing estimated from carbon balance of regional aquifers: The case of central Apennine, Italy

Abstract

Central Italy is characterized by an anomalous flux of deeply derived CO 2 . In the western Tyrrhenian sector of central Italy, CO 2 degassing occurs mainly from focused emissions (vents and strong diffuse degassing) and thermal springs, whereas in the eastern Apennine area, deep CO 2 is dissolved in “cold” groundwaters of regional aquifers hosted by Mesozoic carbonate‐evaporite formations. Influx of deep CO 2 into 12 carbonate aquifers (12,500 km 2 ) of the central Apennine is computed through a carbon mass balance that couples aquifer geochemistry with isotopic and hydrogeological data. Mass balance calculations estimate that 6.5×10 10 mol yr −1 of inorganic carbon are dissolved in the studied aquifers. Approximately 23% of this amount derives from biological sources active during the infiltration of the recharge waters, 36% comes from carbonate dissolution, while 41% is representative of deep carbon sources characterized by a common isotopic signature (δ 13 C ≅ −3‰). The calculated deep CO 2 influx rate ranges from 10 5 to 10 7 mol yr −1 km −2 , increasing regionally from east to west in the study area.