Continuous measurement of carbon isotopic composition in soil gases at Cumbre Vieja volcano: a new frontier in volcano monitoring

In October 2017, two remarkable seismic swarms interrupted a 46-year seismic silence in Cumbre Vieja volcanic system, La Palma, Canary Islands, Spain. As a response to this seismic unrest episode, INVOLCAN strengthened the volcano monitoring in the island with the installation of a new automatic geochemical station in the municipality of Fuencaliente (LPG08) in the southern part of the island, which included a Delta Ray^TM Isotope Ratio Infrared Spectrometer (Thermo Fisher Scientific), to measure the content and isotopic composition (δ¹³C-CO₂) of the soil gas CO₂ using a PVC trap buried in the soil at 40 cm depth and transporting the gas through a polyamide pipe. After different seismic swarms occurred in the following years, a volcanic eruption started in Cumbre Vieja on September 19, 2021, lasting 85 days and 8 hours, the longest historical eruption in the island. On September 22, 2021, INVOLCAN installed an additional automatic geochemical station in the municipality of Los Llanos de Aridane (LPG10, around 5 km far from the eruption site) in the western part of the island, including another DeltaRay^TM analyzer. In this work, we show the results from August 2020 to December 2021 measured at LPG08, and from September 2021 to January 2021 measured at LPG10. LPG08 data showed a range of δ¹³C-CO₂ from -24.3 to -17.9‰ vs. VPDB (this last value just before the eruption started), with an average value of -20.9‰, during the study period. A clearly increasing trend to less negative values of δ¹³C-CO₂ was detected from the beginning of 2021 to the moment when the eruption started, showing an increasing magmatic component in the soil CO₂ measured, which was corroborated by plotting δ¹³C-CO₂ vs. 1/[CO₂] mean monthly values. During and after the eruptive period, the values showed a decreasing trend. Regarding LPG10, the values ranged from -18.8 to -7.3‰ vs. VPDB, with a mean value of -13.4‰. In this case, a general decrease trend of the δ¹³C-CO₂ values to more negative values was observed after the eruption finished, while mean monthly values in the δ¹³C-CO₂ vs. 1/[CO₂] plot showed a shift from values ​​with a higher contribution of deep-seated CO₂ at the beginning of the eruption to values ​​with a lower contribution at its end. This data demonstrates that the continuous measuring of carbon isotopic composition in soil gases before, during and after a volcanic eruption constitutes a powerful new tool for volcano monitoring.