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Massive emissions of deep CO2 coupled to the 2009–2018 Central Italy earthquakes., Deep CO2 emissions characterize many nonvolcanic, seismically active regions worldwide, and the involvement of deep CO2 in the earthquake cycle is now generally recognized. However, no long-time records of such emissions have been published, and the temporal relations between earthquake occurrence and tectonic CO2 release remain enigmatic. Here, we report a 10-year record (2009–2018) of tectonic CO2 flux in the Apennines (Italy) during intense seismicity. The gas emission correlates with the evolution of the seismic sequences: Peaks in the deep CO2 flux are observed in periods of high seismicity and decays as the energy and number of earthquakes decrease. We propose that the evolution of seismicity is modulated by the ascent of CO2 accumulated in crustal reservoirs and originating from the melting of subducted carbonates. This large-scale, continuous process of CO2 production favors the formation of overpressurized CO2-rich reservoirs potentially able to trigger earthquakes at crustal depth.

Both the shallow (organic-derived) and deep (mantellic–magmatic–metamorphic) fluxes of CO2 [ΦCO2, mass time−1] and specific fluxes of CO2 [ϕCO2 mass time−1 surface−1] dissolving in the shallow groundwaters of the volcanic areas of Amiata, Vulsini-Vico-Sabatini, Albani, Roccamonfina, Vesuvio, Vulture, and Etna were evaluated by partitioning the composed population of total dissolved inorganic carbon in two individual populations and subsequent subtraction of local background population. The flux of deep CO2 released from the geothermal fields of Piancastagnaio (Amiata), Torre Alfina, Latera, Marta, Bracciano south, Cesano, and Mofete and from the Overall Northern Latium Hydrothermal Reservoir were also evaluated by means of the total surface heat flux and the enthalpy and CO2 molality of the single liquid phase circulating in each geothermal reservoir. These data suggest that the ϕCO2 released to the atmosphere varies from 9.5×106 to 3.0×106 mol year−1 km−2, over the geothermal fields of Bracciano south and Cesano, respectively, and that a total ΦCO2 of 3.8×108 mol year−1 is cumulatively released from the geothermal fields of Torre Alfina, Latera and Cesano extending over an area of only 66 km2. In addition, a flux of ∼2.2×1011 to 3.8×1011 mol year−1 of gaseous CO2 entering the atmosphere is obtained for the entire anomalous area of central Italy, extending from the Tyrrhenian coastline to the Apennine chain (45,000 km2). Thus terrestrial CO2 emission in central-southern Italy appears to be a significant carbon source.

We describe the results of a detailed hydrogeochemical campaign on the groundwater circulating in two regional aquifers located in the area of the Abruzzo 2009 earthquakes. The influx of deeply derived CO2 rich gases into the two aquifers is highlighted by the 13C isotopic composition of dissolved carbon species. The source of the gas is roughly localised beneath the epicentral area of the earthquakes where the presence of sources of fluids under high pressure is suggested by seismological investigations. The carbon isotopic-mass balance of the aquifers indicates that the amount of the deep CO2 dissolved and transported by the groundwaters is ~ 530 t/day. The chemical and isotopic composition of the gas entering the aquifers, named Abruzzo gas, has been derived by comparing the data measured in the springs with the results of a gas–water–rock reaction model, that simulates the evolution of the chemical and isotopic composition of groundwater affected by the input of a deeply-derived CO2 rich gas phase. The composition of Abruzzo gas is compared to that of 40 large gas emissions located in central Italy. The gas becomes progressively richer in radiogenic elements (4He and 40Ar) and in N2, from the volcanic complexes in the west to the Apennines in the east. The Abruzzo gas, in agreement with its location, well matches the composition of the gases emitted in the pre-Apennine region. These geochemical features, consistent with the structural setting of the region, indicate increasing residence times of the gas in the crust moving from west to east. In particular we suggest that the strong increase in radiogenic crustal gases reflects the occurrence of deep traps where the gas is stored at high pressures for a long time and that such high pressure gas pockets play a major role in the generation of Apennine earthquakes.