Twenty five years of geochemical monitoring of the oceanic active volcanic island of El Hierro, Canary Islands

El Hierro (278 Km²), the youngest and westernmost island of the Canarian archipelago,  is settled on an ocean floor 3.5 km deep and reaches 1.5 km above sea level. The island was constructed by rapid constructive and destructive processes in ~ 1.12 Ma. A submarine eruption took place from October 2011 to March 2012 about 2 km south of the small village of La Restinga in the southernmost part of the island. The eruptive process was the second longest and the second largest volume discharged in the historical volcanic activity of the Canaries (in the last 500 years) and was the first one to be monitored from the beginning. Since visible volcanic emissions are absent at the surface of El Hierro, one of the most useful geochemical tools to monitor the volcanic activity of El Hierro is the diffuse degassing studies. Diffuse CO₂ emissions have been monitored at El Hierro Island since 1998 in a yearly basis, with higher frequency during the pre and eruptive period of 2011-2012. At each survey, 600 sampling sites are studied and measurements of soil CO₂ efflux are performed in situ following the accumulation chamber method. During pre-eruptive and eruptive period, 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. In the last survey, performed in the 2023 summer period, soil CO₂ efflux values ranged from non-detectable up to 39 g m⁻² d⁻¹. Statistical-graphical analysis of the data show three different geochemical populations, background (B), intermediate (I) and peak (P), represented by 97.7%, 1.6 % and 0.7% of the total data respectively, with geometric means of 1.2, 20 and 27 g m⁻² d⁻¹, respectively. To quantify the diffuse CO₂ emission for the 2023 survey, 100 sequential Gaussian simulations (sGs) were performed as interpolation method. The estimated 2023 diffuse CO₂ output released to atmosphere by El Hierro was 528 ± 22 t d^-1, value higher than the background average of CO₂ emission estimated in 410 t d^-1. The data presented here demonstrate that discrete surveys of diffuse CO₂ emission offer important information to optimize the early warning system in volcano monitoring programs.