Diffuse CO2 degassing monitoring of the Tenerife North-South Rift Zone (NSRZ) volcano, Canary Islands

Tenerife (2,034 km²), the largest and one of the most active islands of the Canarian volcanic archipelago, has registered six volcanic eruptions in the last 500 years. One of the main volcano-structural and geomorphological features of Tenerife is the triple junction-shaped rift system, as a result of inflation produced by the concentration of emission vents and dykes at 120º one to another. The oriented North South Rift Zone (NSRZ; 325 km²) is one of the three active volcanic rift-zones of the island and is characterized mainly by effusive activity of basaltic lavas forming spatter and cinder cones and comprising 139 monogenetic cones representing the most common eruptive activity occurred on the island during the last 1My. The main structural characteristic of the NSRZ is the apparent absence of a distinct ridge and a fan shaped distribution of these monogenetic cones. Since no visible degassing at Tenerife NSRZ surface occurs, a geochemical monitoring program at Tenerife NSRZ was established mainly consisting on performing diffuse CO₂ emission surveys to evaluate the temporal and spatial variations of soil CO₂ efflux values and the diffuse CO₂ emission rate. Ten diffuse CO₂ degassing surveys have been carried out at NSRZ of Tenerife since 2002, the last one in the summer period of 2023. Measurements of soil CO₂ efflux were performed in situ by means of a portable non-dispersive infrared sensor following the accumulation chamber method at about selected 600 sampling sites to obtain a homogeneous distribution after taking into consideration the local geology, structure and accessibility. During the 2023 survey, soil CO₂ efflux values ranged from non-detectable up to 55.5 g m⁻² d⁻¹. Statistical-graphical analysis of the 2023 data show three different geochemical populations; background (B), intermediate (I) and peak (P) represented by 93.9%, 5.4% and 0.7% of the total data, respectively. The geometric means of the B, I and P populations are 1.6, 11.4 and 36.6 g m⁻² d⁻¹, respectively. Most of the area showed B values while the P values were observed as multiple isolated anomalies in the study area. 100 equiprobable sequential Gaussian simulations were performed to construct the spatial interpolation map and to estimate the diffuse CO2 emission in tons per day released from Tenerife NSRZ in the 2023 survey. The diffuse CO₂ output released to atmosphere by the NSRZ of Tenerife estimated in the 2023 survey was 884 ± 27 t d^-1. This value overcomes the estimated background range (201 - 760 t d^-1), and confirms the clear relationship  between the temporal evolution of the CO₂ output released by the NSRZ and the seismic activity in and around Tenerife island. These geochemical observations are clear evidence of changes of processes operating deep in the hydrothermal-magmatic system of Tenerife. Monitoring the diffuse CO₂ emission contributes to detect early warning signals in the activity of the Tenerife North-South Rift-Zone volcanic system.