Changing dormancy in the Monts Dore volcanic province, Massif Central? - CO2 output and isotopic composition (C, He) at Escarot

CO₂ flux measurements have become an essential tool in volcanic monitoring, yet standardized methodologies remain critical for cross-study comparability. Here, we present a newly acquired CO₂ flux dataset from the Escarot mofette field, processed using a partially automated geostatistical web application that minimizes user subjectivity. We calculate a CO₂ emission rate of 1065 ± 110 t yr −1 over an area of 35,000 m 2 . A direct comparison with a CO 2 flux study by Gal et al. (2018) initially suggests a substantial decrease in emissions by roughly a factor of eight (8100 ± 1800 t yr −1 ). However, this apparent decrease largely reflects differences in data treatment and output calculation. Threshold definition and outlier treatment strongly affect these estimates and must be applied consistently to ensure comparability between studies. Recalculating the 2018 dataset with the same geostatistical methodology as applied here yields a CO 2 output of 692 t yr −1 , revealing an increase of ~370 t (~54%) over eight years. In addition, air-corrected 3 He/ 4 He ratios of 5.93–6.38 Rₐ confirm a subcontinental lithospheric mantle (SCLM) signature, while δ 13 C CO2 values of −1.67 to −1.62‰ suggest secondary chemical and physical fractionation processes potentially driven by enhanced deep-sourced fluid flux. Together, these results indicate that structural control at Escarot governs both the origin of the gases and the spatial distribution and degassing rates of surface CO 2 emissions. Based on the results we assume that the system may have been in geochemical disequilibrium over the past decade, potentially coupled with the contemporaneous regional seismic activity. Continuous, long-term fluid monitoring is needed to reliably distinguish real system change from natural background variability. Plain language summary A volcanic system that appears dormant, like the Monts Dore volcanic province, which shows no clear signs of increased activity, is not necessarily truly inactive. By applying a new, semi-automatic geostatistical tool to both an older (2018) and a more recent (2025) CO₂ flux dataset measured at the Escarot degassing site in the French Massif Central, we show that the system's CO₂ output increased by approximately 54% over the past eight years. Furthermore, the geochemical fingerprint of the gases, defined by both their carbon and helium isotopic signatures, points to a subcontinental lithospheric mantle source. Thus, gases must have migrated over long distances (>25 km) through the Earth's crust, along permeable structures, before reaching the surface. This is further supported by the widespread degassing observed across Escarot, most clearly expressed by bubbling mofettes and, at times, accompanied by the sound of escaping gas.