Soil gas CO2 concentration, isotopic ratio and efflux measurements for geothermal exploration in La Palma, Canary Islands.

Abstract

Located in the northwest of Canary Islands, La Palma is one of the most volcanically active islands in the archipelago. The island experienced a volcanic eruption from 19 September to 13 December 2021, which had significant social, economic, and scientific impacts. This event serves as a reminder of the island's potential as a host for geothermal resources. Therefore, geochemical prospection of soil gases at Cumbre Vieja can provide valuable information for investigating the presence of permeable areas and potential upflow for degassing of geothermal systems at depth. This study was carried out between July and September 2023 and presents the results of a soil gas study located southwest of the 2021 lava flow. The survey aimed to identify permeable areas by conducting in-situ measurements of diffuse CO2 emissions and sampling and analyzing CO2 concentration and isotopic composition (δ13C-CO2). A total of 766 sampling sites were selected over an area of approximately 25 km2, with an average distance of 100 m between sites. Soil CO2 concentrations ranged from typical atmospheric values (≈ 400 ppm) up to 40,000 ppm. The average CO2 concentration measured was 1,700 ppm. The δ13C-CO2 isotopic composition revealed the presence of three distinct end-members: biogenic, atmospheric and deep-seated CO2, defined by isotopic compositions of 25‰>δ13C-CO2>-15‰, -8‰ and 2.1‰>δ13C-CO2>-8‰ and CO2 concentrations of 100%, 0.04% and 100%, respectively. Results show that, with a mean of -13.7‰, a minimum of -28.9‰ and a maximum of -4.8‰, CO2 at most sampling sites is composed of various mixtures of atmospheric and biogenic CO2, with some contributions from deep-seated CO2. The accumulation chamber method was used to measure soil CO2 efflux at each sampling site using a portable non-dispersive CO2 sensor, model LICOR-Li-820. The measured CO2 efflux values ranged from non-detectable to 160.3 g·m-2·d-1, with an average value of 4.7 g·m-2·d-1. For the estimation of the total diffuse CO2 emission from the study area, we calculated the average of 100 sequential Gaussian simulations. This gave a value of 100.8 ± 2.8 t·d-1, corresponding to a standardized emission rate of 4.1 t·km-2-d-1. The results show a significant correlation between the distribution of 222Rn gas activity anomalies and the highest CO2 efflux values in the eastern part of the study area. Soil gas measurements of CO2 concentration, isotopic ratio and efflux are a valuable and non-invasive technique for surface exploration, helping to define permeable areas and potential upflow zones of potential geothermal system structures and enabling an efficient subsequent subsurface exploration phase.