Geochemical monitoring of the Tenerife North-East and North West Rift Zones by means of diffuse degassing surveys

Tenerife (2,034 km²), the largest island of the Canarian archipelago, is characterized by three volcanic rifts NW-SE, NE-SW and N-S oriented, with a central volcanic structure in the middle, Las Cañadas Caldera, hosting Teide-Pico Viejo volcanic complex. The North-West Rift-Zone (NWRZ) is one of the youngest and most active volcanic systems of the island, where three historical eruptions (Boca Cangrejo in 16^th Century, Arenas Negras in 1706 and Chinyero in 1909) have occurred, whereas the North-East Rift-Zone (NERZ) is more complex than the others due to the existence of Pedro Gil stratovolcano that broke the main NE-SW structure 0.8 Ma ago. The most recent eruptive activity along the NERZ took place during 1704 and 1705 across 13 km of fissural eruption in Siete Fuentes (Arafo-Fasnia). To monitor potential volcanic activity through a multidisciplinary approach, diffuse degassing studies have been carried out since 2000 at the NWRZ (72 km²) and since 2001 at the NERZ (210 km²) in a yearly basis. Long-term variations in the diffuse CO₂ output in the NWRZ have shown a temporal correlation with the onsets of seismic activity at Tenerife, supporting unrest of the volcanic system, as is also suggested by anomalous seismic activity recorded in the studied area during April, 2004 and October, 2016 (Hernández et al., 2017). In-situ measurements of CO₂ efflux from the surface environment were performed according to the accumulation chamber method using a portable non-dispersive infrared (NDIR) sensor. Soil CO₂ efflux values for the 2021 survey ranged between non-detectable values and 104 g·m^-2·d^-1, with an average value of 8 g·m^-2·d^-1 for NWRZ. For NERZ, soil CO₂ efflux values ranged between non-detectable values and 79 g·m²·d^-1, with an average value of 7 g·m^-2·d^-1. The probability plot technique applied to the data allowed to distinguish different geochemical populations. Background population represented 49.2% and 74.0% of the total data for NWRZ and NERZ, respectively, with a mean value (1.7 - 2.0 g·m^-2·d^-1) similar to the background values calculated for other volcanic systems in the Canary Islands with similar soils, vegetation and climate (Hernández et al. 2017). Peak population represented 0.9 and 0.7% for NWRZ and NERZ, respectively and with a mean value of 45 and 57 g·m^-2·d^-1. Soil CO₂ efflux contour maps were constructed to identify spatial-temporal anomalies and to quantify the total CO₂ emission using the sequential Gaussian simulation (sGs) interpolation method. Diffuse emission rate of 506 ± 22 t·d^-1 for NWRZ and 1,509 ± 58 t·d^-1 NERZ were obtained. The normalized CO₂ emission value by area was estimated in 7.03 t·d^-1·km^-1 for NWRZ and in 7.2 t·d^-1·km^-1 for NERZ. The monitorization of the diffuse CO₂ emission contributes to detect early warning signals of volcanic unrest, especially in areas where visible degassing is non-existent as in the Tenerife NWRZ and NERZ.

Hernández et al. (2017). Bull Volcanol, 79:30, DOI 10.1007/s00445-017-1109-9.