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 mai nly from focused emissions (vents and strong diffuse degassing) and thermal springs, whereas in the eastern Apennine area, deep CO2 is dissolved in "c old" groundwaters of regional aquifers hosted by Mesozoic carbonate-evapori te 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 cou ples aquifer geochemistry with isotopic and hydrogeological data. Mass bala nce calculations estimate that 6.5x10(10) mol yr(-1) of inorganic carbon ar e dissolved in the studied aquifers. Approximately 23% of this amount deriv es from biological sources active during the infiltration of the recharge w aters, 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 ea st to west in the study area.