Soil CO2 degassing and diffuse thermal anomalies on El Hierro Island (Canary Islands, Spain)

Abstract

El Hierro is the westernmost, smallest, and youngest island of the Canary archipelago (Spain). The island is shaped by three rift zones, with the last eruption taking place in the submarine extension of the southern rift in October 2011. This study aimed to characterize volcanic activity on El Hierro Island. To this end, soil CO₂ degassing and temperature were determined at 182 sites covering the 268.7 km² surface of the island during a five-day field campaign in May 2025. Soil CO₂ fluxes were measured using the accumulation chamber method, and soil temperature was recorded at a depth of 10 cm. We also characterize the carbon isotopes composition of CO₂ (δ¹³C-CO₂) to infer its origin and the potential processes occurring during its accumulation within the soil and subsequently release to the open atmosphere. In addition, we analysed the spatiotemporal evolution of diffuse thermal anomalies, i.e., subtle (~1 °C), month-to-years, and large-scale (~km²) brightness temperature, over the past 20 years using MODIS spectroscopic products processed with the software SSTAR¹ (Subtle Surface Temperature Anomalies Recognizer). We found that diffuse thermal anomalies on El Hierro are temporally associated with periods of increased volcanic activity in the Canary archipelago and frequently occur above the three main rift-zones (west, northeast, and south). Soil CO₂ fluxes measured in May 2025 were slightly higher along the western and northeastern (~15 g m⁻² d⁻¹) compared to the rest of the island (~4 g m⁻² d⁻¹), consistent with background levels. The δ¹³C-CO₂ values suggests that the CO₂ emitted through the soil has mainly a biogenic origin, mixing in the accumulation chamber (for gas flux measurements) with the atmospheric component. The results also indicate the fractionation of the carbon isotopes during the molecular diffusion of CO₂. Despite CO₂ fluxes being biogenic and at low levels, their spatial distribution correlates with the location of the most prominent diffuse thermal anomalies recorded over the last two decades, suggesting interactions between volcanic and biological processes. Based on the geochemical survey and the spatiotemporal thermal analysis, we conclude that El Hierro Island is currently in a quiescent state.