Soil H2 degassing studies: a useful geochemical tool for monitoring Cumbre Vieja volcano, La Palma, Canary Islands

Hydrogen (H₂) is one of the most abundant trace species in volcano-hydrothermal systems and is a key participant in many redox reactions occurring in the hydrothermal reservoir gas. Although H₂ can be produced in soils by N₂-fixing and fertilizing bacteria, soils are considered nowadays as sinks of molecular hydrogen. Because of its chemical and physical characteristics, H₂ generated within the crust moves rapidly and escapes to the atmosphere.These characteristics make H₂ one of the best geochemical indicators of magmatic and geothermal activity at depth. Regular surface geochemical studies have been conducted focusing on hydrogen (H₂) emissions along Cumbre Vieja volcano (La Palma, Canary Islands) since 2001, encompassing the analysis of soil H2 content using a micro-gas chromatograph (Agilent 490 microGC) in samples collected at a depth of approximately 40 cm across 600 sites during each survey. Spatial distribution maps have been generated using sequential Gaussian simulation (sGs) techniques to quantify the diffuse H₂ emissions from Cumbre Vieja volcano. The time series data of the diffuse H₂ emissions indicate significant increases during the occurrence of seismic swarms observed between 2017 and 2021, reaching the maximum value of the series (36 kg·d^-1) in June 2017, 4 month before the seismic swarms. During the eruptive phase, substantial spikes in the diffuse H₂ emissions were observed, closely correlating with the volcanic tremor escalation. During 2024, the soil H₂ emission ranged between 7 and 16 kg·d^-1, values that can be considered within the background range. The absence of visible volcanic gas emissions before the eruption, such as fumaroles or hot springs, on the surface of Cumbre Vieja underscores the importance of such studies in serving as a critical tool for continuous volcanic surveillance and monitoring purposes. This update represents ongoing efforts to comprehensively study and understand the behavior of hydrogen emissions within the volcanic system, providing essential insights into volcanic activity and potential precursor signals for enhanced monitoring and risk assessment.