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 RayTM 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 DeltaRayTM 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.