Duvalo Volcano (North Macedonia): A Purely Tectonic-Related CO2 Degassing System

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 similar to 50,000 m(2)) showed fluxes up to nearly 60,000 g m(-2) d(-1), sustaining a total output of similar to 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/R-A 0.10) and of CO2 (delta C-13 similar to 0 parts per thousand) exclude significant mantle contribution, while delta C-13-CH4 (similar to-35 parts per thousand) and delta H-2-CH4 (similar to-170 parts per thousand) 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)).

Automated summary

Duvalo volcano is a site of anomalous geogenic degassing close to Ohrid (North Macedonia) not related to volcanic activity. The gases at Duvalo mainly consist of CO2, with significant concentrations of H2S and CH4. These gases originate mostly, if not exclusively, in the shallowest part of the crust, and their production is influenced by seismic activity and tectonic structures in the area.