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Duvalo "volcano" is a site of anomalous geogenic degassing close to Ohrid (North Macedonia) not related to volcanic activity, despite its name. CO2 flux measurements made with the accumulation chamber (321 sites over ∼50,000 m2) showed fluxes up to nearly 60,000 g m−2 d−1, sustaining a total output of ∼67 t d−1. Soil gas samples were taken at 50 cm depth from sites with high CO2 fluxes and analyzed for their chemical and isotope composition. The gas is mainly composed by CO2 (>90%) with significant concentrations of H2S (up to 0.55%) and CH4 (up to 0.32%). The isotope compositions of He (R/RA 0.10) and of CO2 (δ13C ∼ 0‰) exclude significant mantle contribution, while δ13C‐CH4 (∼−35‰) and δ2H‐CH4 (∼−170‰) suggest a thermogenic origin for CH4. The area is characterized by intense seismic activity and Duvalo corresponds to an active tectonic structure bordering the Ohrid graben. The production of H2S within the stratigraphic sequence may be explained by thermochemical reduction of sulfate. The uprising H2S is partially oxidized to sulfuric acid that, reacting with carbonate rocks, releases CO2. The tectonic structure of the area favors fluid circulation, sustaining H2S production and oxidation, CO2 production and allowing the escape of the gases to the atmosphere. In the end, Duvalo represents a tectonic‐related CO2 degassing area whose gases originate mostly, if not exclusively, in the shallowest part of the crust (<10 km). This finding highlights that even systems with trivial mantle contribution may sustain intense CO2 degassing (>1,000 t km−2 d−1). Plain Language Summary: The carbon cycle is an important piece of the puzzle of the present climate change. While anthropogenic sources of atmospheric carbon are reasonably constrained, geological sources are much less. Among the latter, carbon release to the atmosphere in seismically active areas, though known from decades, is less studied. Here we estimate the total emission of carbon dioxide from Duvalo, an area near Ohrid (North Macedonia). The obtained value (67 metric tons per day) is comparable with some active volcanic areas in the region. Although local inhabitants call this area Duvalo volcano, no recent volcanic activity is recognized here. The composition of the gas released by this system seems also to rule out geothermal activity or deep contributions from the Earth's mantle. This study shows that natural degassing systems with relatively shallow crustal sources (few kilometers), may sustain intense gas emissions from the soil. Key Points: The total CO2 output from Duvalo "volcano" has been estimated for the first timeGas compositions rule out significant mantle contributions or recent volcanic activity while geothermal activity is improbableAn active fault system favors indirect gas production and upflow to the Earth's surface [ABSTRACT FROM AUTHOR]

We report on the results of an extensive geochemical survey of fluids released in the Vardar zone (central-western Serbia), a mega-suture zone at the boundary between Eurasia and Africa plates. Thirty-one bubbling gas samples are investigated for their chemical and isotopic compositions (He, C, Ar) and cluster into three distinct groups (CO2-dominated, N2-dominated, and CH4-dominated) based on the dominant gas species. The measured He isotope ratios range from 0.08 to 1.19 Ra (where Ra is the atmospheric ratio), and reveal for the first time the presence of a minor (<20%) but detectable regional mantle-derived component in Serbia. δ13C values range from -20.2‰ to -0.1‰ (versus PDB), with the more negative compositions observed in N2-dominated samples. The carbon-helium relationship indicates that these negative δ13C compositions could be due to isotopic fractionation processes during CO2 dissolution into groundwater. In contrast, CO2-rich samples reflect mixing between crustal and mantle-derived CO2. Our estimated mantle-derived He flux (9.0 × 109 atoms m-2 s-1) is up to 2 orders of magnitude higher than the typical fluxes in stable continental areas, suggesting a structural/tectonic setting favoring the migration of deep-mantle fluids through the crust. [ABSTRACT FROM AUTHOR]

Nisyros Island (Greece) is an active volcano hosting a high-enthalpy geothermal system. During June 2013, an extensive survey on Hg concentrations in different matrices (fumarolic fluids, atmosphere, soils, and plants) was carried out at the Lakki Plain, an intracaldera area affected by widespread soil and fumarolic degassing. Concentrations of gaseous elemental mercury (GEM), together with H2S and CO2, were simultaneously measured in both the fumarolic emissions and the atmosphere around them. At the same time, 130 samples of top soils and 31 samples of plants (Cistus creticus and salvifolius and Erica arborea and manipuliflora) were collected for Hg analysis. Mercury concentrations in fumarolic gases ranged from 10,500 to 46,300 ng/m3, while Hg concentrations in the air ranged from high background values in the Lakki Plain caldera (10-36 ng/m3) up to 7100 ng/m3 in the fumarolic areas. Outside the caldera, the concentrations were relatively low (2-5 ng/m3). The positive correlation with both CO2 and H2S in air highlighted the importance of hydrothermal gases as carrier for GEM. On the other hand, soil Hg concentrations (0.023-13.7 μg/g) showed no significant correlations with CO2 and H2S in the soil gases, whereas it showed a positive correlation with total S content and an inverse one with the soil pH, evidencing the complexity of the processes involving Hg carried by hydrothermal gases while passing through the soil. Total Hg concentrations in plant leaves (0.010-0.112 μg/g) had no direct correlation with soil Hg, with Cistus leaves containing higher values of Hg with respect to Erica. Even though GEM concentrations in the air within the caldera are sometimes orders of magnitude above the global background, they should not be considered dangerous to human health. Values exceeding the WHO guideline value of 1000 ng/m3 are very rare (<0.1%) and only found very close to the main fumarolic vents, where the access to tourists is prohibited. [ABSTRACT FROM AUTHOR]

This study is focused on the (micro)biogeochemical features of two close geothermal sites ( FAV1 and FAV2), both selected at the main exhalative area of Pantelleria Island, Italy. A previous biogeochemical survey revealed high CH4 consumption and the presence of a diverse community of methanotrophs at FAV2 site, whereas the close site FAV1 was apparently devoid of methanotrophs and recorded no CH4 consumption. Next-Generation Sequencing ( NGS) techniques were applied to describe the bacterial and archaeal communities which have been linked to the physicochemical conditions and the geothermal sources of energy available at the two sites. Both sites are dominated by Bacteria and host a negligible component of ammonia-oxidizing Archaea (phylum Thaumarchaeota). The FAV2 bacterial community is characterized by an extraordinary diversity of methanotrophs, with 40% of the sequences assigned to Methylocaldum, Methylobacter (Gammaproteobacteria) and Bejerickia (Alphaproteobacteria); conversely, a community of thermo-acidophilic chemolithotrophs ( Acidithiobacillus, Nitrosococcus) or putative chemolithotrophs ( Ktedonobacter) dominates the FAV1 community, in the absence of methanotrophs. Since physical andchemical factors of FAV1, such as temperature and pH, cannot be considered limiting for methanotrophy, it is hypothesized that the main limiting factor for methanotrophs could be high NH4+ concentration. At the same time, abundant availability of NH4+ and other high energy electron donors and acceptors determined by the hydrothermal flux in this site create more energetically favourable conditions for chemolithotrophs that outcompete methanotrophs in non-nitrogen-limited soils. [ABSTRACT FROM AUTHOR]

Methane soil flux measurements have been made in 38 sites at the geothermal system of Sousaki (Greece) with the closed chamber method. Fluxes range from −47.6 to 29 150 mg m day, and the diffuse CH output of the system has been estimated at 19 t a. Contemporaneous CO flux measurements showed a moderate positive correlation between CO and CH fluxes. Comparison of the CO/CH soil flux ratios with the CO/CH ratio of the gases of the main gas manifestations provided evidence for methanotrophic activity within the soil. Laboratory CH consumption experiments confirmed the presence of methanotrophic microorganisms in soil samples collected at Sousaki. Consumption was generally in the range from −4.9 to −38.9 pmolCH h g but could sometimes reach extremely high values (−33 000 pmolCH h g). These results are consistent with recent studies on other geothermal systems that revealed the existence of thermoacidophilic bacteria exerting methanotrophic activity in hot, acid soils, thereby reducing methane emissions to the atmosphere. [ABSTRACT FROM AUTHOR]