Soil CO2 fluxes measured in the Acoculco Geothermal System, Mexico: Baseline emissions from a long-term prospection programme

The Acoculco Caldera Complex is considered a promissory hidden high-temperature geothermal system in Mexico. To support the geothermal prospection of this anomalous area, a comprehensive programme of soil CO₂ flux measurements was performed. A long-term measurement programme was conducted to determine the baseline of natural soil CO₂ effluxes. Significant efforts were devoted both to measuring the CO₂ fluxes between 2015 and 2022 and interpreting their origin. Eighteen soil gas surveys of CO₂ were carried out by using the accumulation chamber method. >1200 diffuse CO₂ fluxes were measured in six different areas of the Acoculco Caldera. Two areas (Los Azufres and Alcaparrosa) exhibited cold degassing sites, acid-sulphate springs, and gas bubbling in surface water bodies. The soil CO₂ fluxes ranged from 1 to 26,000 g m⁻² d⁻¹, whereas lower fluxes <29 g m⁻² d⁻¹ were determined as the degassing baseline. A total CO₂ output of 492 t d⁻¹ km⁻¹ was estimated using an integrated SGS-GSA approach, where the highest total soil CO₂ fluxes were obtained for Alcaparrosa (299 t d⁻¹ km⁻²) compared with Los Azufres (164 t d⁻¹ km⁻²), and Surroundings (29 t d⁻¹ km⁻²). Such results agree well with those values measured in other worldwide volcanic and active geothermal ecosystems. The range of CO₂ isotopic composition values from −28.83 ‰ to −3.11 ‰, together with their statistical distribution, suggests multiple CO₂ production sources feeding soil degassing. The combined interpretation of flux and isotopic data allowed us to identify two distinct gas sources: endogenous and biogenic. The present study highlights the importance of using soil CO₂ monitoring to determine baseline emissions at the early exploration stage of geothermal systems.