Sixteen years monitoring of Cumbre Vieja volcano (La Palma, Canary Islands) by means of diffuse CO2 degassing surveys

La Palma Island, the fifth longest (706 km2) and second highest (2,423 m asl) of the Canary Islands, is located at the northwestern end of the archipelago. Subaerial volcanic activity on La Palma started 2.0 My ago and has taken place exclusively at the southern part of the island during the last 123 ka, where Cumbre Vieja volcano, the most active basaltic volcano in the Canaries, has been constructed. Major volcano-structural and geomorphological features of Cumbre Vieja volcano are a north-south rift zone 20 km long, with vents located also at the northwest and northeast, and up to 1,950 m in elevation covering an área of 220 km2. Nowadays, there are no visible gas emissions from fumaroles or hot springs at Cumbre Vieja; therefore, diffuse CO2 degassing monitoring is important geochemical tool for its volcanic surveillance. Recent studies have shown that enhanced endogenous contributions of deep-seated CO2 might have been responsible for higher diffuse CO2 efflux values (Padrón et al., 2015). We report here the latest results of the diffuse CO2 emission survey at Cumbre Vieja volcano. The surface CO2 efflux measurements were taken using the accumulation chamber method in the period 1997- 2016 to evaluate their spatial distribution on this 220 km2 volcano and the diffuse CO2 emission rate from Cumbre Vieja volcano. Surface CO2 efflux values ranged from non-detectable up to 94 g m-2 d-1 in the last survey. Spatial distribution maps were constructed following the sequential Gaussian simulation (sGs) procedure. The spatial distribution of diffuse CO2 emission values did not seem to be controlled by the main structural features of the volcano since the highest values were measured in the southern part. The diffuse CO2 emission for the 2016 survey has been estimated about 739 ± 30 t d-1. The 2016 emission rate is slightly higher than the estimated average for Cumbre Vieja volcano (493 t d-1), but within the observed background range for this volcanic system over the past 20 years (1,219 to 134 t d-1). On the contrary, the 2011 and 2013 surveys showed diffuse CO2 emission rate values relatively higher than the background range, 1,544 ± 81 and 1,538 ± 160 t d-1. Could these diffuse CO2 emission pulses be a long term geochemical precursory signal of a potential volcanic unrest at Cumbre Vieja volcano?