Spatial-temporal variations of surface diffuse CO2 degassing at El Hierro volcano, Canary Islands

El Hierro (278 km²), the youngest, smallest and westernmost island of the Canarian archipelago, is a 5-km-high edifice constructed by rapid constructive and destructive processes in ~1.12 Ma, with a truncated trihedral shape and three convergent ridges of volcanic cones. It experienced a submarine eruption from 12 October, 2011 to 5 March 2012, off its southern coast that was the first one to be monitored from the beginning in the Canary Islands. As no visible emanations occur at the surface environment of El Hierro, diffuse degassing studies are a useful geochemical tool to monitor the volcanic activity in this volcanic island. Diffuse CO₂ emission surveys have been performed at El Hierro Island since 1998 in a yearly basis, with much higher frequency during the period 2011-2012. At each survey, about 600 sampling sites are selected to obtain a homogeneous distribution. Measurements of soil CO₂ efflux are performed in situ following the accumulation chamber method. During pre-eruptive and eruptive periods, the diffuse CO₂ emission released by the whole island experienced significant increases before the onset of the submarine eruption and the most energetic seismic events of the volcanic-seismic unrest (Melián et al., 2014. J. Geophys. Res. Solid Earth, 119, 6976–6991). The most recent diffuse CO₂ efflux survey was carried out in July 2019. Values ranged from non-detectable to 28.9 g m⁻² d⁻¹. Statistical-graphical analysis of the data shows two different geochemical populations; Background (B) and Peak (P) represented by 97.5% and 0.5% of the total data, respectively, with geometric means of 1.2 and 23.6 g m⁻² d⁻¹, respectively. Most of the area showed B values while the P values were mainly observed at the interception center of the three convergent ridges and the north-east of the island. To estimate the diffuse CO₂ emission for the 2019 survey, we ran about 100 sGs simulations. The estimated 2019 diffuse CO₂ output released to atmosphere by El Hierro was 214 ± 10 t d^-1, value lower than the background average of CO₂ emission estimated on 412 t d^-1 and slightly higher than the background range of 181 t d^-1 (−1σ) and 930 t d^-1 (+1σ) estimated at El Hierro volcano during the quiescence period 1998-2010 (Melián et al., 2014, JGR). Monitoring the diffuse CO₂ emission has proven to be a very effective tool to detect early warning signals of volcanic unrest at El Hierro.