Measuring and interpreting CO2 fluxes at regional scale: the case of the Apennines, Italy

Tectonically active regions are often characterized by large amounts of carbon dioxide degassing, and estimation of the total CO2 discharged to the atmosphere from tectonic structures, hydrothermal systems and inactive volcanic areas is crucial for the definition of present-day global Earth degassing. The carbon balance of regional aquifers is a powerful tool to quantify the diffuse degassing of deep inorganic carbon sources because the method integrates the CO2 flux over large areas. Its application to peninsular Italy shows that the region is characterized by specific CO2 fluxes higher than the baseline determined for the geothermal regions of the world, and that the amount of endogenous CO2 discharged through diffuse regional degassing (c. 2.1 × 1011 mol a−1) is the major component of the geological CO2 budget of Italy, definitely prevailing over the CO2 discharged by Italian active volcanoes and volcanoes with hydrothermal activity. Furthermore, the positive correlation between geothermal heat and deep CO2 dissolved in the groundwater of central Italy suggests that (1) the geothermal heat is transported into the aquifers by the same hot CO2-rich fluids causing the Italian CO2 anomaly and (2) the advective heat flow is the dominant form of heat transfer of the region. Supplementary material: The location, flow rate, extent of the hydrogeological basin, chemical and isotopic analyses of the 160 springs considered in this study, and the results of the carbon mass balance are reported in a table available at https://doi.org/10.6084/m9.figshare.c.423702

Preliminary study on geogenic degassing through the big karstic aquifers of Greece

Non-volcanic degassing contributes to the C-cycle by providing on a global scale a significant amount of CO2 emitted through diffuse earth degassing processes (Kerrick et al 1995). Due to the elevated solubility of the CO2 in water, in the areas where high CO2 fluxes directly affect regional aquifers, most of it can be dissolved, transported and released by groundwaters. Therefore, quantification of this contribution to the atmosphere has a substantial implication for modeling the global carbon cycle. According to Chiodini et al. (2000), total dissolved inorganic carbon (TDIC) concentrations and δ13CTDIC values of groundwaters are useful tools to both quantify the geogenic degassing and distinguish the different carbon sources. This approach was proved to be valid for central Italy and can possibly work for continental Greece; due to similar geodynamic history. Greece is considered one of the most geodynamically active regions and is characterized by intense geogenic degassing. The main source of degassing in the Hellenic area is concentrated on hydrothermal and volcanic environments (Daskalopoulou et al., 2019), however, the impact of geogenic CO2 released by tectonically active areas shouldn’t be disregarded. Aim of this work is to quantify the CO2 degassing through aquifers hosted in the carbonate successions in the Hellenic region. 95 karst, thermal and cold waters were collected in the northern and central part of Greece with some of which being characterized by bubbling of CO2-rich gases. Results show that karst waters have a typical Ca-HCO3 composition. Thermal and cold waters show two different compositions: some samples are characterized by Ca-HCO3 composition suggesting the presence of a carbonate basement, whilst others have a prevailing Na-HCO3 composition. On the basis of TDIC concentrations and δ13CTDIC values, the springs are divided into two groups. The first group includes karst waters and some of thermal waters and is characterized by low TDIC concentrations and negative δ13CTDIC values. This group shows no evidence of deep CO2 contributions, whereas the carbon of these waters derives from dissolution of carbonate minerals by organic derived CO2. Remaining samples belong to the second group and present intermediate to high TDIC concentrations and δ13CTDIC values, indicating a possible input of inorganic CO2. Some of these springs are characterized by gas bubbling at discharge, suggesting an extensive degassing

SATELLITE AND IN SITU GROUND TEMPERATURE

Ground thermal anomalies in volcanic-hydrothermal systems, where the outflow of hot fluids gives rise to fumarolic fields, soil degassing, and hot soils, have, up to now, rarely been investigated by using satellite. Here we report a comparison between surface temperature derived by satellite data and a large data set of measured soil temperatures and CO2 fluxes for a volcanic-hydrothermal system, the Solfatara of Pozzuoli (Campi Flegrei, Italy). Surface temperatures derived from ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data are compared with soil temperatures and CO2 fluxes from four surveys performed in 2003, 2010, and in 2014. The good match between the spatial distributions of computed and measured temperatures suggests the adequacy of satellite data to describe the Solfatara thermal anomaly, while the correspondence between temperatures and CO2 fluxes, evidences the link between degassing and heating processes. The ASTER derived surface temperatures (14–37°C) are coherent with those measured in the soil (10–97°C at 10 cm depth), considering the effect of the thermal gradients which characterize the degassing area of Solfatara. This study shows that satellite data can be a very powerful tool with which to study surface thermal anomalies, and can provide a supplementary tool to monitor thermal evolution of restless volcanoes

Chemical weathering and consumption of atmospheric carbon dioxide in the Alpine region

To determine the CO2 consumption due to chemical weathering in the Alps, water samples from the 32 main Alpine rivers were collected and analysed in two periods, spring 2011 and winter 2011/2012. Most of the river waters are characterized by a bicarbonate earth-alkaline composition with some samples showing a clear enrich-ment in sulphates and other samples showing a slight enrichment in alkaline metals. The amount of total dissolved solids (TDS) ranges between 96 and 551 mg/L. Considering the major ion composition and the Sr isotopic composition of water samples, coherently with the geological setting of the study area, three major reservoirs of dissolved load have been recognized: carbonates, evaporites and silicates. Based on a chemical mass balance, the flux of dissolved solids, and the flux of carbon dioxide consumed by chemical weathering have been computed for each basin and for the entire study area. Results show that the flux of dissolved solids, ranges from 8 × 103 to 411 × 103 kg km−2 y−1, with an average value of 127 × 103 kg km−2 y−1, while the flux of carbon dioxide consumed by chemical weathering in the short-term (b1 Ma) is 5.03 × 105 mol km−2 y−1 1 on average. Since part of the CO2 is returned to the atmosphere through carbonate precipitation and reverse weathering once river water reaches the ocean, the CO2 removed from the atmosphere/soil system in the long-term (N1 Ma) is much smaller than the CO2 consumed in the short-term and according to our calculations amounts to 2.01 × 104 mol km−2 y−1 on average. This value is almost certainly a minimum estimate of the total amount of CO2 fixed by weathering on the long-term because in our calculations we assumed that all the alkaline metals deriving from rock weathering in the continents are rapidly involved in the process of reverse weathering in the oceans, while there are still large uncertainties on the magnitude and significance of this process. The values of CO2 flux consumed by weathering are strongly correlated with runoff while other potential controlling factors show only weak correlations or no correlation. Our estimation of the CO2 consumed by weathering in the Alpine basins is in the same order of magnitude, but higher than the world average and is consistent with previ-ous estimations made in river basins with similar climatic conditions and similar latitudes

Impact of geogenic degassing on C-isotopic composition of dissolved carbon in karst systems of Greece

The Earth C-cycle is complex, where endogenic and exogenic sources are interconnected, operating in a multiple spatial and temporal scale (Lee et al., 2019). Non-volcanic CO2 degassing from active tectonic structures is one of the less defined components of this cycle (Frondini et al., 2019). Carbon mass-balance (Chiodini et al., 2000) is a useful tool to quantify the geogenic carbon output from regional karst hydrosystems. This approach has been demonstrated for central Italy and may be valid also for Greece, due to the similar geodynamic settings. Deep degassing in Greece has been ascertained mainly at hydrothermal and volcanic areas, but the impact of geogenic CO2 released by active tectonic areas has not yet been quantified. The main aim of this research is to investigate the possible deep degassing through the big karst aquifers of Greece. Since 2016, 156 karst springs were sampled along most of the Greek territory. To discriminate the sources of carbon, the analysis of the isotopic composition of carbon was carried out. δ13CTDIC values vary from -16.61 to -0.91‰ and can be subdivided into two groups characterized by (a) low δ13CTDIC, and (b) intermediate to high δ13CTDIC with a threshold value of -6.55‰. The composition of the first group can be related to the mixing of organic-derived CO2 and the dissolution of marine carbonates. Springs of the second group, mostly located close to Quaternary volcanic areas, are linked to possible carbon input from deep sources

Carbon degassing through karst hydrosystems of Greece

Estimation of CO2 degassing from active tectonic structures and regional hydrothermal systems is essential for the quantification of presentday Earth degassing [Frondini et al., 2019 and references therein]. Due to the high solubility of CO2 in water, great amounts of deep inorganic carbon can be dissolved, transported, and released from regional aquifers. By applying a massbalance approach [Chiodini et al., 2000], different sources of the dissolved CO2 can be discriminated. The main source of degassing in Greece is concentrated in hydrothermal and volcanic areas. However, deep CO2 from active tectonic areas has not yet been quantified. A key point of this research is to investigate the possible deep CO2 degassing through the big karst aquifers of Greece. From May 2016, 156 karst springs were sampled along the greatest part of the Hellenic region. To discriminate the different carbon sources, we analyzed the chemical and isotopic composition of total dissolved inorganic carbon (TDIC). Results yield TDIC values from 1.89 to 21.7 mmol/l and δ13CTDIC from 16.61 to 0.91 ‰. On this basis, karst springs are clustered into two groups: (a) low TDIC and δ13CTDIC values and (b) intermediate TDIC and δ13CTDIC values. The carbon of the first group derives from organic source and dissolution of carbonates; whilst the second group shows a possible carbon input from deep source. This geogenic carbon is mostly related to high heat flux areas, often near active or recent (Quaternary) volcanic systems

Duvalo (North Macedonia): A "volcano" without volcanic activity

T he Duvalo locality is located in the SW of the Republic of North Macedonia, in the Ohrid region, near the village of Kosel. It is an area of strong soil degassing, called “volcano” by the local people despite volcanic activity has never been documented in the recent geologic history of the area [1]. A large area (thousands of sqm) shows signs of strong alteration and is devoid of vegetation. Until the 19thcentury sulphur was mined from this area [1]. In August 2019, a campaign of soil CO2 flux measurements and soil gas sampling was made. Duvalo is sometimes referred to as an active geothermal feature but no signs of enhanced geothermal gradient were found and the soil temperatures at 50 cm depth in this campaign were always within the range of local mean air temperatures. Soil CO2 flux values ranged from 1.3 to 59,000 g/m2/d and can be modelled with the overlapping of 3 or 4 flux populations. A possible biological background is estimated in 6.8±1.8 g/m2/d while the other populations are characterized by an anomalous average flux ranging from 180 to 33,000 g/m2/d. The CO2 total emission, estimated both with a statistical and geostatistical approach, provided similar values in the order of 50 t/d. This has to be considered as a minimum value because only areas with evident signs of alteration have been investigated. Nevertheless, the estimated output is quite high for an area unrelated with recent volcanism or geothermal activity. The chemical composition of soil gases shows: CO2 (96.6%), N2 (1.8%), H2S (0.6%) and CH4 (0.3%) as the main gases. The present composition is almost indistinguishable from previous analyses made in 1957 and 1977 [1] pointing to a stability of the system in last decades. The isotope compositions indicate for CO2 (δ13C -0.2 ‰) a pure carbonate rock origin, for CH4 (δ13C -34.4 ‰ and δ2H -166 ‰) a thermogenic origin and for He (R/RA 0.10) a pure crustal origin. The H2S released at Duvalo may be produced by either microbial or thermochemical sulphate reduction favoured by hydrocarbons whose presence can be inferred by the uprise of thermogenic methane. Partial oxidation of H2S during its upflow, producing sulphuric acid, may be responsible of the production of abundant CO2 through dissolution of carbonate rocks. Similar processes have been evidenced also in other parts of North Macedonia [2]. These gases rise up through the N–S trending normal faults bordering the seismically active Ohrid basin graben [3] being released to the atmosphere through the soils of Duvalo “volcano”

Preliminary geochemical characterization of gas manifestations in North Macedonia

L ike most of the Balkan Peninsula, North Macedonia is a geodynamically active area. As such it has many hydrothermal features and gas manifestations. Until now, no systematic study about the geochemical characterization of the geogenic gases was made before in this country. In August 2019, 24 gas samples were collected in the study area. All, except one collected at Duvalo (soil gas), are gases bubbling or dissolved in thermomineral waters (temperatures from 12 to 66 \ub0C). They were analysed in the laboratory for their chemical (He, Ne, Ar, O2 , N2 , H2 , H2S, CH4 and CO2) and isotopic composition (\u3b413C-CO2, \u3b413C-CH4, \u3b42H-CH4 and R/RA). Most of the gases have CO2 as the main component (400-998,000 ppm) while the remaining are enriched in N2 (1300-950,000 ppm). Helium ranges from 0.3 to 2560 ppm while CH4 from 1.6 to 20,200 ppm. R/RA and 4He/20Ne ratios indicate a generally low atmospheric contamination, a prevailing crustal contribution and mantle contributions between 1 and 20% considering a MORB endmember. The highest mantle contributions are found in the SE part of the country very close to the sites that show the highest R/RA values in continental Greece [1]. This area is characterised by extensional tectonics and Plio- Pleistocene volcanism. A quite high mantle contribution (about 15%) is also found in two manifestations in the NW part of the country along a main normal fault system. With the exception of the sample of Smokvica, which has very low CO2 (1400 ppm) and \u3b413C-CO2 (-15.7 \u2030 V-PDB), all free gases show a relatively narrow range in \u3b413C-CO2 values (-4.6 to +1.0 \u2030 V-PDB) indicating the mixing between a mantle and a carbonate rock source. The isotope composition allows us to assign the CH4 origin to three sources. The largest group can be attributed to a hydrothermal origin (\u3b413C-CH4 around -20 \u2030 V-PDB and \u3b42H-CH4 around -100\u2030). Three samples collected in the SW part of the country have a thermogenic origin (\u3b413C-CH4 around -35 \u2030 V-PDB and \u3b42H-CH4 around -160\u2030 V-SMOW). Finally, one sample (Smokvica) with the highest values (\u3b413C-CH4 -7.2 \u2030 V-PDB and \u3b42H-CH4 -80\u2030 V-SMOW) may be attributed to abiotic processes in a continental serpentinization environment or to methane oxidation

Carbon dioxide diffuse emission and thermal energy release from hydrothermal systems at Copahue-Caviahue Volcanic Complex (Argentina).

The north-western sector of Caviahue caldera (Argentina), close to the active volcanic system of Copahue, is characterized by the presence of several hydrothermal sites that host numerous fumarolic emissions, anomalous soil diffuse degassing of CO2 and hot soils. In March 2014, measurements of soil CO2 fluxes in 5 of these sites (namely, Las M\ue1quinas, Las Maquinitas I, Las Maquinitas II, Anfiteatro, and Termas de Copahue) allowed an estimation that ~165t of deeply derived CO2 is daily released. The gas source is likely related to a relatively shallow geothermal reservoir containing a single vapor phase as also suggested by both the geochemical data from the 3 deep wells drilled in the 1980s and gas geoindicators applied to the fumarolic discharges. Gas equilibria within the H-C-O gas system indicate the presence of a large, probably unique, single phase vapor zone at 200-210\ub0C feeding the hydrothermal manifestations of Las M\ue1quinas, Las Maquinitas I and II and Termas de Copahue. A natural thermal release of 107MW was computed by using CO2 as a tracer of the original vapor phase. The magmatic signature of the incondensable fumarolic gases, the wide expanse of the hydrothermal areas and the remarkable high amount of gas and heat released by fluid expulsion seem to be compatible with an active magmatic intrusion beneath this portion of the Caviahue caldera

Discharge variations of springs induced by strong earthquakes: the case of the mw 6.5 Norcia event (Italy, October 30 th 2016)

Earthquakes, dynamic strain, groundwater circulation, structural permeability