Continuous measurement of carbon isotopic composition in soil gases at Cumbre Vieja volcano: a new frontier in volcano monitoring
Authors: Asensio-Ramos María , Padrón Eleazar , Barrancos José , Hernández Pedro A. , Melián Gladys V. , Rodríguez Fátima , Padilla Germán D. , Pérez Nemesio M. 
Published: March 2022

&lt;p&gt;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&lt;sup&gt;TM&lt;/sup&gt; Isotope Ratio Infrared Spectrometer (Thermo Fisher Scientific), to measure the content and isotopic composition (&amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CO&lt;sub&gt;2&lt;/sub&gt;) of the soil gas CO&lt;sub&gt;2&lt;/sub&gt; 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&lt;sup&gt;TM&lt;/sup&gt; 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 &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CO&lt;sub&gt;2&lt;/sub&gt; from -24.3 to -17.9&amp;#8240; vs. VPDB (this last value just before the eruption started), with an average value of -20.9&amp;#8240;, during the study period. A clearly increasing trend to less negative values of &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CO&lt;sub&gt;2&lt;/sub&gt; was detected from the beginning of 2021 to the moment when the eruption started, showing an increasing magmatic component in the soil CO&lt;sub&gt;2&lt;/sub&gt; measured, which was corroborated by plotting &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CO&lt;sub&gt;2&lt;/sub&gt; vs. 1/[CO&lt;sub&gt;2&lt;/sub&gt;] 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&amp;#8240; vs. VPDB, with a mean value of -13.4&amp;#8240;. In this case, a general decrease trend of the &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CO&lt;sub&gt;2&lt;/sub&gt; values to more negative values was observed after the eruption finished, while mean monthly values in the &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CO&lt;sub&gt;2&lt;/sub&gt; vs. 1/[CO&lt;sub&gt;2&lt;/sub&gt;] plot showed a shift from values &amp;#8203;&amp;#8203;with a higher contribution of deep-seated CO&lt;sub&gt;2&lt;/sub&gt; at the beginning of the eruption to values &amp;#8203;&amp;#8203;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.&lt;/p&gt;