Soil CO2 Degassing and Temperature at Poás Volcano: Evidence From the 2025 Eruptive Phase

Abstract

Soil carbon dioxide (CO₂) degassing provides valuable insights into volcano–hydrothermal systems, especially during their active phases. Soil CO₂ flux and temperature surveys are particularly useful where direct access to vents is limited, providing information on subsurface magmatic dynamics. This multidisciplinary study presents new heat and soil degassing data from Poás Volcano, which entered an active phase in early 2025 with frequent phreatic and phreatomagmatic eruptions. Between March 9 and 15, 2025, we conducted soil CO₂ flux and temperature surveys along the northeastern part of the crater rim and the crater upper terrace to investigate the spatial extent of soil degassing beyond the main fumarolic fields. Despite limited access, we obtained 251 flux and temperature measurements, using three portable accumulation chambers, complemented by seven samples for carbon isotopic analyses. Soil CO₂ fluxes were generally low in the study area, with a mean value of 1.89 ± 0.05 g m⁻² day⁻¹ at the crater rim, reaching a maximum of 624.31 ± 4.65 g m⁻² day⁻¹ near fumaroles on the upper terrace. Interestingly, we also detected a localized thermal anomaly of approximately 2–5°C above background levels in the crater rim, possibly associated with fumarolic activity on the upper terrace. The soil CO₂ flux along the crater rim exhibits minor contributions from biogenic and magmatic CO₂ (≤3%). At the Poás crater rim, low CO₂ fluxes, near-atmospheric CO₂ concentrations, and δ¹³C–CO₂ isotopic fractionation patterns indicate that gas transport is dominated by diffusion in the very shallow subsurface, as confirmed by Fick’s model, with negligible advection due to the absence of measurable pressure gradients.