Diffuse CO2 emission from Timanfaya volcano (Lanzarote, Canary Islands) during the period 1999-2022

Lanzarote (795 km²) is a volcanic island with 58 km long and 21 km wide located in the eastern part of the Canary Islands and it is approximately 130 km away from the African coast. The island has experienced the longest historical eruption occurred in the Canarian archipelago, Timanfaya eruption, from 1730 to 1736. It created more than 30 volcanic cinder cones along several NE – SW fissures and produced voluminous amounts of lava flows that covered an area of 200 km², which corresponds to almost a quarter of the island. Nowadays, several thermal anomalies remain active in the Islote de Roque Hilario. The latest eruption recorded in the island was in 1824, known as Tinguatón eruption, along an ENE – WSW fracture.

Since there are not visible gas emanations at the surface environment of Timanfaya Volcanic Field (TVF), diffuse degassing surveys became a useful tool to monitor the volcanic activity. Among degassing phenomena, soil CO₂ efflux is important because of the characteristics of CO₂: it is the major gas species after water vapor in both volcanic fluids and magmas and it is an effective tracer of subsurface magma degassing due to its low solubility in silicate melts at low to moderate pressures (Gerlach and Graeber, 1985). To do so, since 1999, diffuse CO₂ emission surveys have been yearly undertaken in and around the TVF. Between September and October 2022, a new survey was performed with 410 sampling sites, covering an area of 252 km² in order to have a homogeneous distribution. Soil CO₂ efflux was measured following the accumulation chamber method. Soil temperature was determined by inserting a thermocouple at each sampling site at a depth of 40 cm. Soil CO₂ efflux values ranged from non-detectable (<0.5 g·m^-2·d^-1) to 43.1 g·m^-2·d^-1, with an average of 2.0 g·m^-2·d^-1), while soil temperature ranged from 16.1 to 125 ºC. Statistical-graphical analysis of the data showed different geochemical populations; background (B) and peak (P), represented by 85.2 and 0.8 % of the total data, respectively, with geometric means of 0.32 and 26.2 g·m^-2·d^-1, respectively. Higher CO₂ efflux values were measured at the north and south sectors of the TVF. Other relative high values were measured at the central part of TVF, where thermal anomalies occur. During this campaign, a period of rains took place, which may have influenced the measurement of higher values. Sequential Gaussian simulations (sGs) were applied to construct soil CO₂ efflux and soil temperature distribution maps and to estimate the diffuse CO₂ emission from the studied area, 519 ± 42 t·d^-1. This value is of the same order as the maximum value of the time series, and registered in February 2011 (winter). This type of studies demonstrate the great utility of using diffuse CO₂ degassing as a useful geochemical method to contribute to volcanic monitoring programs in systems where there are no visible geothermal surface manifestations.