Long-term variations of diffuse CO2 at Cumbre Vieja volcano, La Palma, Canary Islands

On September 19, 2021, a volcanic eruption began at the west flank of Cumbre Vieja, La Palma, the most northwestern of the Canary Islands. The lava flows caused the evacuation of thousands of residents living in the vicinity of the volcano, and 1,219 hectares were covered by lava flows. After 85 days of activity, the eruption ended on December 13, 2021. Since visible volcanic gas emissions (fumaroles, hot springs, etc.) do not occur at the surface environment of Cumbre Vieja, the geochemical program for the volcanic surveillance has been focused mainly on diffuse (non-visible) degassing studies. Since 2001, diffuse CO₂ emission surveys have been yearly performed in summer periods to minimize the influence of meteorological variations. Measurements of soil CO₂ efflux have been performed following the accumulation chamber method in about 600 sites and spatial distribution maps have been constructed following the sequential Gaussian simulation (sGs) procedure to quantify the diffuse CO₂ emission from the studied area. In the period 2001-2016, the diffuse CO₂ output released to the atmosphere from Cumbre Vieja volcano ranged between 320 to 1,544 t·d^-1. During pre-eruptive period (2016-2021), time series of the diffuse CO₂ emission showed a change with an increasing trend from 788 t·d^-1 up to 1,870 t·d^-1, coinciding with the beginning of the seismic swarms. This increase of diffuse CO₂ emission is interpreted as a geochemical precursory signal of volcanic eruption of Cumbre Vieja, on September 19, 2021. The observed increase on the diffuse CO₂ emission during this time window suggests that in October 2017 a process of magma ascent began from the upper mantle to depths between 35-25 km, at which the seismic swarms were recorded for four years. During eruption period, diffuse CO₂ emission showed strong temporal variations with a minimum value of the diffuse CO₂ emission in October 21, followed by an increase trend of up to 4,435 t·d^-1 on December 14, the highest of time series and coinciding with the end of the eruption. During the post-eruptive period, the diffuse CO₂ emission has shown a descending trend. Our results demonstrate that periodic surveys of diffuse CO₂ emission are extremely important in the volcanic surveillance tools of Cumbre Vieja to improve the detection of early warning signals of future volcanic unrest episodes.