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

Central Italy is characterized by an anomalous flux of deeply derived CO2. In the western Tyrrhenian sector of central Italy, CO2 degassing occurs mainly from focused emissions (vents and strong diffuse degassing) and thermal springs, whereas in the eastern Apennine area, deep CO2 is dissolved in "cold" groundwaters of regional aquifers hosted by Mesozoic carbonate-evaporite formations. Influx of deep CO2 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 6.5x10(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 (delta(13)C congruent to-3 parts per thousand). The calculated deep CO2 influx rate ranges from 10(5) to 10(7) mol yr(-1) km(-2), increasing regionally from east to west in the study area.