Carbon dioxide degassing from Tuscany and Northern Latium (Italy)

Abstract

The CO 2 degassing process from a large area on the Tyrrhenian side of central Italy, probably related to the input into the upper crust of mantle fluids, was investigated in detail through the geochemical study of gas emissions and groundwater. Mass-balance calculations and carbon isotopes show that over 50% of the inorganic carbon in regional groundwater is derived from a deep source highlighting gas−liquid separation processes at depth. The deep carbonate−evaporite regional aquifer acts as the main CO 2 reservoir and when total pressure of the reservoir fluid exceeds hydrostatic pressure, a free gas phase separates from the parent liquid and escapes toward the surface generating gas emissions which characterise the study area. The distribution of the CO 2 flux anomalies and the location of high PCO 2 springs and gas emissions suggest that the storage and the expulsion of the CO 2 toward the atmosphere are controlled by the geological and structural setting of the shallow crust. The average CO 2 flux and the total amount of CO 2 discharged by the study area were computed using surface heat flow, enthalpy and CO 2 molality of the liquid phase circulating in the deep carbonate−evaporite aquifer. The results show that the CO 2 flux varies from 1 × 10 4 mol y − 1 km − 2 to 5 × 10 7 mol y − 1 km − 2 , with an average value of 4.8 × 10 6 mol y − 1 km − 2 , about five times higher than the value of 1 × 10 6 mol y − 1 derived by Kerrick et al. [Kerrick, D.M., McKibben, M.A., Seward, T.M., Caldeira, K., 1995. Convective hydrothermal CO 2 emission from high heat flow regions. Chem. Geol. 121, 285–293] as baseline for terrestrial CO 2 emissions. The total CO 2 discharged from the study area is 0.9 × 10 11 mol y − 1 , confirming that Earth degassing from Tyrrhenian central Italy is a globally relevant carbon source.