Duvalo “Volcano” (North Macedonia): A Purely Tectonic‐Related CO 2 Degassing System

Duvalo “volcano” is a site of anomalous geogenic degassing close to Ohrid (North Macedonia) not related to volcanic activity, despite its name. CO 2 flux measurements made with the accumulation chamber (321 sites over ∼50,000 m 2 ) 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 CO 2 fluxes and analyzed for their chemical and isotope composition. The gas is mainly composed by CO 2 (>90%) with significant concentrations of H 2 S (up to 0.55%) and CH 4 (up to 0.32%). The isotope compositions of He ( R / R A 0.10) and of CO 2 (δ 13 C ∼ 0‰) exclude significant mantle contribution, while δ 13 C‐CH 4 (∼−35‰) and δ 2 H‐CH 4 (∼−170‰) suggest a thermogenic origin for CH 4 . The area is characterized by intense seismic activity and Duvalo corresponds to an active tectonic structure bordering the Ohrid graben. The production of H 2 S within the stratigraphic sequence may be explained by thermochemical reduction of sulfate. The uprising H 2 S is partially oxidized to sulfuric acid that, reacting with carbonate rocks, releases CO 2 . The tectonic structure of the area favors fluid circulation, sustaining H 2 S production and oxidation, CO 2 production and allowing the escape of the gases to the atmosphere. In the end, Duvalo represents a tectonic‐related CO 2 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 CO 2 degassing (>1,000 t km −2  d −1 ).