Ground CO2 monitoring at Timanfaya volcano (Lanzarote, Canary Islands) during the period 1999-2023

Lanzarote Island (795 km²) is a volcanic island located in the eastern part of the Canary Islands and approximately 100 km from the NW coast of Morocco. The largest historical eruption of the Canary Islands, Timanfaya, took place during 1730-36 in this island when long-term eruptions from a NE-SW-trending fissure formed the Montañas del Fuego. Tinguaton volcano, the last eruption at Lanzarote Island, occurred in 1824 and produced a much smaller lava flow that reached the SW coast. At present, one of the most prominent phenomena at Timanfaya volcanic field is the high maintained superficial temperatures occurring in the area since the 1730 volcanic eruption. The maximum temperatures recorded in this zone are 605ºC, measured in a slightly inclined well 13 m deep. Since fumarolic activity is absent at the surface environment of Lanzarote, to study the diffuse CO₂ emission becomes an ideal geochemical tool for monitoring its volcanic activity. We report herein the results of eight soil CO₂ efflux surveys performed from 2006 to 2023 at Timanfaya Volcanic Field (TVF) with the aim to evaluate the temporal variations of the diffuse CO₂ emission. Approximately 400 sampling sites were selected at each survey to obtain an even distribution of the sampling points over the study area. Soil CO₂ efflux was measured following the accumulation chamber method. Soil temperature at 40 cm depth and soil gas samples collected at each sampling site was also measured to evaluate the chemical and isotopic composition of soil gases. Diffuse CO₂ emission values have ranged between non detectable values to 34 g·m^-2·d^-1, with the highest values measured in September 2008. Conditional sequential Gaussian simulations (sGs) were applied to construct soil CO₂ efflux distribution maps and to estimate the total CO₂ output from the studied area at the TVF. Soil CO₂ efflux maps showed a high spatial and temporal variability. Most of the study area have shown relatively low values, around the detection limit of the instrument (~0.5 g·m^-2·d^-1). Higher soil CO₂ diffuse emission values were observed where thermal anomalies occur, indicating a convective mechanism transport of gas from depth at these areas. Diffuse CO₂ emission rates ranged between 41 and 519 t·d^-1 during the study period (57 t·d^-1 for 2023). Long-term temporal variation on total CO₂ diffuse emission shows a peak recorded on winter 2011, suggesting a seasonal control on the CO₂ emission. These observations along with the results from the eight soil gas surveys performed at TVF indicate that the short and long-term trends in the diffuse CO2 degassing are mainly controlled by environmental factors.