Furnas Volcano is one of the dormant central volcanoes that form São Miguel Island. Currently low seismicity and several secondary manifestations of volcanism, such as fumaroles, thermal and cold CO2-rich springs, as well as diffuse degassing areas characterize its activity.
Furnas Lake fumarolic field, located in the northern side of Furnas Lake, comprises hydrothermal fumaroles and mud pools in a steaming ground. Despite the water vapour, the fumaroles are dominated by CO2 and H2S in the dry phase, with minor amounts of N2, Ar, He, H2, CO and CH4. Gas ratios (CO2/H2S, H2/CH4, CO2/CH4) measured in the last decade did not show major changes and geothermometers (H2-CO2-CO-CH4-H2O) applied to the gases released suggest equilibrium temperatures around 276 ºC. These temperatures are probably associated to a deeper reservoir (~500 m beneath the surface) comparing to the more surficial reservoir (~100 m) identified by geophysical studies. 13CCO2 and 3He/4He in the gases emitted from the fumaroles are, respectively, -4.2 ‰ and 5.23. The helium isotopic compositions measured at Furnas Volcano emissions suggest a recycled crustal component.
Soil CO2 flux measurements were also carried out in the fumarolic area through the accumulation chamber method. CO2 degassing maps elaborated based on the detailed surveys showed gas emissions varying between 2 and approximately 28 000 g m-2 d-1, highlighting the soil heterogeneities caused by significant differences in the permeability due to alteration (e.g., presence of clayey soils). Spatial distribution of the data shows anomalous alignments with clear association with the main tectonic structures recognized in the area. Total CO2 emitted in the fumarolic ground varied between 325 t km-2 d-1 and 1637 t km-2 d-1 .
A permanent soil CO2 flux station (named GFUR2) was also installed at about 50 m from the lake margin in October 2005. Gas fluxes in this monitoring site showed to be highly influenced by air and soil temperatures. Based on these interferences, seasonal variations were also detected at GFUR2 station with higher emissions recorded during wintertime comparing to the summer period. In addition, diurnal cycles are also observed in the CO2 flux time series, with lower gas fluxes at noontime.