Monitoring diffuse CO2 emission: a geochemical surveillance tool for Cumbre Vieja volcano, La Palma, Canary Islands, Spain

La Palma Island (708 km²) is located in the northwestern part of the Canarian Archipelago and represents one of its youngest volcanic structures, with an estimated geological age of around 2.0 million years. On September 19, 2021, a significant volcanic eruption occurred within the Cumbre Vieja volcanic system situated in the southern region of the island. This event, lasting 85 days and 8 hours, is recorded as the longest volcanic episode in La Palma's documented history. The eruption resulted in extensive lava flows that covered an area of approximately 1,219 hectares, causing substantial geological and social impact. 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. This study presents the findings from annual diffuse carbon dioxide (CO₂) emission surveys conducted since 2001, with increased monitoring frequency between 2017 and 2024 to optimize the early warning system for future volcanic eruptions at La Palma island.

The measurement of soil CO₂ efflux was performed following the accumulation chamber method across approximately 600 sampling sites distributed throughout the volcanic system. The long-term time series data reveal distinct periods of diffuse CO₂ emissions that provide valuable insights into the system's volcanic activity: (1) A baseline period (2001-2016), when diffuse CO₂ emissions fluctuated between 320 and 1,544 t/d, establishing a reference range for background degassing levels.; (2) A pre-eruptive period (2016-2021), when a marked increase in CO₂ emissions was observed, with values rising from 788 t·d⁻¹ to a peak of 1,870 t·d⁻¹. This last period coincided with the onset of seismic swarm activity, highlighting a clear correlation between increased degassing and evolving magmatic processes beneath the surface; (3) The eruptive period (2021). During the eruption, CO₂ emissions exhibited significant temporal variations. A minimum emission rate was recorded on October 21, followed by a sharp increase that peaked at 4,435 t·d⁻¹ on December 14, aligning with the conclusion of the eruptive phase. This maximum emission rate represents the highest value observed in the entire monitoring serie; and (4) the post-eruptive period (2022-2024), when diffuse CO₂ emissions showed a decreasing trend, stabilizing around 760 t·d⁻¹, reflecting a gradual return to lower degassing levels.

These findings underscore the critical importance of continuous diffuse CO₂ monitoring as a key component of volcanic surveillance at Cumbre Vieja. Regular measurements of diffuse gas emissions provide essential early warning indicators of potential volcanic unrest, allowing for improved risk assessment and hazard mitigation strategies. The integration of geochemical monitoring with other geophysical and geological tools enhances the comprehensive understanding of the dynamic behavior of volcanic systems.