Soil carbon dioxide efflux weekly monitoring network for the volcanic surveillance of Tenerife, Canary Islands

Carbon dioxide (CO₂) is one of the first gases to escape from the magmatic environment due to its low solubility in basaltic magmas at low pressures. Monitoring of volcanic gases in Tenerife Island (2,304 km²) has been focused mainly on diffuse CO₂ degassing and other volatiles due to the absence of visible gas manifestations except fumaroles at the summit of Teide volcano. An inexpensive method to determine CO₂ fluxes based in the absorption of CO₂ through an alkaline medium followed by titration analysis has been used with the aim of contributing to the volcanic surveillance of Tenerife. During summer 2016, a network of 31 closed alkaline traps was deployed along the three volcanic rifts of Tenerife (NE, NW and NS) and at Cañadas Caldera. To do so, an aliquot of 50 mL of 0.1N KOH solution is placed inside the chamber at each station to absorb the CO₂ released from the soil. The solution is replaced in a weekly basis and the trapped CO₂ is later analyzed at the laboratory by titration. Values are expressed as weekly integrated CO₂ efflux. We present herein the results of one year CO₂ efflux estimated by closed alkaline traps. The CO₂ efflux values ranged from 1.0 to 14.5 g·m^-2·d^-1, with average values of 8.5 g·m^-2·d^-1 for the NE rift-zone, 5.2 g·m^-2·d^-1 for Cañadas Caldera, 6.4 g·m^-2·d^-1 for NW rift-zone and 6.1 g·m^-2·d^-1 for NS rift-zone. The estimated CO₂ efflux values were of the same order than the observed ones in 2016. Relatively high CO₂ efflux values were observed at the NE rift-zone, where maximum values were measured. The temporal evolution of CO₂ efflux estimated by closed alkaline traps did not show significant variations during 2019. However, small seasonal variations are observed during the period 2016 – 2019. To investigate the origin of the soil CO₂, soil gas samples were weekly sampled on the head space of the closed chambers. Chemical and isotopic composition of C in the CO₂ were analysed in the gas samples. The concentration of CO₂ on the head space of the closed chambers showed a range of 355-50,464 ppm, with an average value of 1,850 ppmV, while the isotopic composition expressed as d¹³C-CO₂ showed a range from -5.03 to -30.44 ‰, with an average value of -15.9 ‰. The heaviest values of d¹³C-CO₂ are in the NW rift-zone. The systematics of closed static chambers alkaline traps can be a simple and economical tool with volcanic surveillance purposes in system where visible volcanic gases manifestations are absence.