Francesco Frondini

CO2 degassing and energy release at Solfatara volcano, Campi Flegrei, Italy

In the present period of quiescence, the Solfatara volcano, 1 km far from Pozzuoli, releases 1500 t d−1 of hydrothermal CO2 through soil diffuse degassing from a relatively small area (0.5 km2). This amount of gas is comparable to that released by crater plume emissions of many active volcanoes. On the basis of the CO2/H2O ratio measured in high-temperature fumaroles inside the degassing area, we computed a total thermal energy flux of 1.19×1013 J d−1 (138 MW). Most of this energy is lost by shallow steam condensation and transferred to the atmosphere through the hot soil of the degassing area. The thermal energy released by diffuse degassing at Solfatara is by far the main way of energy release from the whole Campi Flegrei caldera. It is 1 order of magnitude higher than the conductive heat flux through the entire caldera, and, during the last 20 years, it was several times higher than the energy associated with seismic crises and ground deformation events. It is possible that changes in the energy flux from a magma body seated underneath Solfatara and/or argillification processes at relatively shallow depths determine pressurization events in the hydrothermal system and consequently ground deformation and shallow seismic swarms, as recorded during the recent episodes of volcanic unrest centered at Pozzuoli.

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 km2) 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 6.5×1010 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 (δ13C ≅ −3‰). The calculated deep CO2 influx rate ranges from 105 to 107 mol yr−1 km−2, increasing regionally from east to west in the study area.

Quantification of deep CO2 fluxes from Central Italy. Examples of carbon balance for regional aquifers and of soil diffuse degassing

Abstract In Central Italy non-volcanic CO 2 is discharged by focused degassing (strong diffuse emission and vents) and by high-CO 2 groundwater. 3 He / 4 He data and the carbon isotopic composition of CO 2 are compatible with derivation from mantle degassing and/or metamorphic decarbonation. The gases produced at depth accumulate in permeable reservoirs composed of Mesozoic carbonates. When total pressure (roughly corresponding to p CO 2 ) of the reservoir fluid exceeds hydrostatic pressure, a free gas phase forms gas reservoirs within the permeable host rocks from which gases may escape toward the surface. This process generates both the focused vents and the CO 2 -rich springs which characterise the study area. The storage and expulsion of CO 2 is controlled by fractures and faults and/or structural highs of permeable carbonate formations. Influx of deep CO 2 into the overlying groundwater yields a widespread elevated p CO 2 anomaly in the Tyrrhenian Central Italy aquifers. These aquifers release CO 2 to the atmosphere when groundwater is discharged at the surface from springs. The groundwater degassing flux is estimated from the carbon balance of regional aquifers computed by coupling aquifer geochemistry with isotopic and hydrogeological data. The resulting production rate of deep CO 2 ranges from 4×10 5 to 9×10 6 mol y −1 km −2 . In concert with the regional geologic setting, the deep CO 2 production rate increases westward. In the aquifers with anomalously high p CO 2 , the average CO 2 influx rate of the anomalous areas is several times higher than the value 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 (1995) 285–293.] as baseline for CO 2 emission from areas of high heat flow. The flux of CO 2 lost to the atmosphere from water emitted from springs is of the same order of magnitude as the influx of deep CO 2 into the aquifer.

Carbon dioxide degassing from the Albani Hills volcanic region, Central Italy

The Albani Hills are affected by strong CO2 degassing processes. The amount of CO2, which rises from the depths and subsequently dissolves into shallow groundwater, has been estimated to be more than 4.2×109 mol year−1. Most of the gas is released in localised anomalous spots (Lavinio, Solforata, and Ciampino–Albano–Nemi areas), generated by fluid leakage processes from buried pressurized reservoirs hosted by structural highs of the calcareous–siliceous marly basement. The groundwater circulating in the anomalous areas becomes CO2 oversaturated, generating observable gas manifestations whose composition is mainly controlled by gas–liquid separation processes. A gas flow rate of 6.1×108 mol year−1 has been measured from the two larger manifestations using the accumulation chamber technique. The presence at shallow level of CO2 oversaturated groundwater can explain several episodes of sudden release of gas which was documented by historical chronicles from Roman times, until now.

Flux measurements of nonvolcanic CO2 emission from some vents in central Italy

In central Italy, nonvolcanic CO2 is discharged from many areas of focused degassing (vents and associated strong diffuse emission) and from high-pCO2 groundwater. We estimate that there are at least 100 CO2 vents in central Italy, many of which have a history of lethality to animals and humans. The gas vents are extremely efficient pathways for transporting large amounts of deeply derived CO2 from gas reservoirs to the surface. As CO2 fluxes from these vents were heretofore unmeasured, several complementary methods were developed to measure the CO2 flux from seven of these vents, which cumulatively discharge 3.1×109 mol yr−1 CO2 (472 t d−1). Considering all of the vents in central Italy, the total CO2 flux from vents alone may be similar to the CO2 emission from the Taupo Volcanic Zone in New Zealand (∼1010 mol yr−1; ∼1200 t d−1) and other areas of high heat flow. Summing the CO2 discharged from vents, areas of diffuse CO2 degassing, and degassing of high-pCO2 groundwater, we provisionally estimate that the total nonvolcanic CO2 flux to the atmosphere in central Italy may be >1011 mol yr−1 (∼12,000 t d−1).

Fumarolic and diffuse soil degassing west of Mount Epomeo, Ischia, Italy

Abstract Fumarolic fluid compositions and diffuse soil emissions of hydrothermal fluids of the Donna Rachele area (0.86 km 2 , western flank of Mt. Epomeo, Ischia Island) have been studied in order to develop a conceptual geochemical model of the hydrothermal system. The degassing area was mapped and the total release of hydrothermal gas and heat associated with the diffuse emission of hydrothermal fluids was estimated. A mesostructural study was carried out in order to investigate the relations between the brittle structures and the main pathways of the uprising vapor. The fumarolic compositions are typical of hydrothermal fluids and water (>99%) represents the major component. All gas species in the H 2 O–H 2 –CO–CH 4 –CO 2 –H 2 S system are close to equilibrium concentrations at temperatures of ∼300°C and at redox conditions slightly more oxidizing than expected. The compositions of the Donna Rachele fumarolic gas approach the pure liquid equilibrium composition. This indicates a high fraction of separated vapor and suggests the presence of a highly energetic hydrothermal system at depth. The pure liquid equilibrium compositions of the Donna Rachele fumaroles, along with the historical records of shallow seismicity, the ‘explosion’ of a well in 1995, the occurrence of intense acoustic phenomena and of shallow wells discharging vapor indicate that the internal pressure of the hydrothermal system is occasionally larger than the hydrostatic pressure. To quantify the energy dissipated in the Donna Rachele area by the emission of fumarolic fluids, the hydrothermal diffuse degassing was studied by means of 336 soil CO 2 flux measurements. The highest CO 2 fluxes were measured in hydrothermally altered areas along the faults that border Mt. Epomeo. Structural data indicate that the vapor rises up along NW–SE striking normal faults related to gravity-induced stresses and affecting highly fractured lavas. The older faults, which are related to the Mt. Epomeo resurgence, act as a permeability barrier and bound the Donna Rachele diffuse degassing structure. The total hydrothermal CO 2 output was estimated to be ∼9 t d −1 . Assuming that the H 2 O/CO 2 ratio of the fluids that feed the diffuse degassing is the same as that of fumarolic effluents, the calculated heat flux is ∼40 MW. This value, which represents an important energy release, is only a part of the total thermal energy release of Ischia, where other fumarolic areas occur. The results obtained at Ischia indicate the importance of thermal energy released by diffuse degassing structures in the energy balance of quiescent volcanoes. Values of the thermal energy release from the Ischia hydrothermal system are comparable with those estimated on other quiescent volcanoes.

Carbon dioxide degassing at Latera caldera (Italy): Evidence of geothermal reservoir and evaluation of its potential energy

has been evaluated at 350 t d � 1 from an area of 3.1 km 2 . It has been estimated that such a CO2 release would imply a geothermal liquid flux of 263 kg s � 1 , with a heat release of 239 MW. The chemical and isotopic composition of the gas indicates a provenance from the geothermal reservoir and that CO2 is partly originated by thermal metamorphic decarbonation in the hottest deepest parts of the system and partly has a likely mantle origin. The ratios of CO2 ,H 2 ,C H4, and CO to Ar were used to estimate the T-P conditions of the reservoir. Results cluster at T � 200–300C and PCO2 � 100–200 bars, close to the actual well measurements. Finally, the approach proved to be an excellent tool to investigate the presence of an active geothermal reservoir at depth and that the H2-CO2CH4-CO-Ar gas composition is a useful T-P geochemical indicator for such CO2 rich geothermal systems.

Theoretical geothermometers andPCO2 indicators for aqueous solutions coming from hydrothermal systems of medium-low temperature hosted in carbonate-evaporite rocks. Application to the thermal springs of the Etruscan Swell, Italy

In order to deriveT-PCO2 geoindicators suitable for thermal waters coming from the hydrothermal systems of medium-low temperature which are hosted in carbonate-evaporite rocks, the theoretical concentration of Ca, Mg, HCO3, SO4, F and their complexes in aqueous solutions in equilibrium with a mineral assemblage made up of calcite, dolomite, anhydrite and fluorite has been calculated at temperatures between 0 and 150°C, atPco2; in the 0.1–100 bar interval and for total molality of mobile species Na and CI ranging between 0.0001 and 0.3. The results of such calculations indicated that: (1) the Ca, Mg, S04 and F contents and theS04(F)2 andCaMg ratios are potential geothermometers; (2) the(HC03)2SO4 ratio is a Pco, indicator; (3) theHC03F ratio and the HC03 content arc indicators of bothPco2 andT. A first geothermometric and geobarometric evaluation of the thermal springs of the Etruscan Swell generally shows equilibrium temperatures in the interval 50–100°C and Pco, from about 1 bar to bars. These estimations are consistent with the general knowledge of the hydrothermal systems of medium-low temperature present in the region.

Monitoring diffuse volcanic degassing during volcanic unrests: the case of Campi Flegrei (Italy)

In volcanoes with active hydrothermal systems, diffuse CO2 degassing may constitute the primary mode of volcanic degassing. The monitoring of CO2 emissions can provide important clues in understanding the evolution of volcanic activity especially at calderas where the interpretation of unrest signals is often complex. Here, we report eighteen years of CO2 fluxes from the soil at Solfatara of Pozzuoli, located in the restless Campi Flegrei caldera. The entire dataset, one of the largest of diffuse CO2 degassing ever produced, is made available for the scientific community. We show that, from 2003 to 2016, the area releasing deep-sourced CO2 tripled its extent. This expansion was accompanied by an increase of the background CO2 flux, over most of the surveyed area (1.4 km2), with increased contributions from non-biogenic source. Concurrently, the amount of diffusively released CO2 increased up to values typical of persistently degassing active volcanoes (up to 3000 t d−1). These variations are consistent with the increase in the flux of magmatic fluids injected into the hydrothermal system, which cause pressure increase and, in turn, condensation within the vapor plume feeding the Solfatara emission.

Innovative monitoring tools for the complex spatial dynamics of river chemistry: case study for the Alpine region

Chemical reactions in aqueous geochemical systems are driven by nonequilibrium conditions, and their dynamics can be deduced through the distributional analysis (identification of probability laws) of complex compositional indices. In this perspective, compositional data analysis offers the possibility to investigate the behavior of the composition as a whole instead of isolated chemical species, with the awareness that multispecies systems are characterized by the simultaneous interactions among all their parts. We addressed this problem using D − 1 isometric log-ratio coordinates describing the D compositional dataset of the river chemistry of the Alpine region (D number of variables), thus working in the