Total (fumarolic + diffuse soil) CO₂ output from Furnas volcano

Furnas volcano, in São Miguel island (Azores), being the surface expression of rising hydrothermal steam, is the site of intense carbon dioxide (CO₂) release by diffuse degassing and fumaroles. While the diffusive CO₂ output has long (since the early 1990s) been characterized by soil CO₂ surveys, no information is presently available on the fumarolic CO₂ output. Here, we performed (in August 2014) a study in which soil CO₂ degassing survey was combined for the first time with the measurement of the fumarolic CO₂ flux. The results were achieved by using a GasFinder 2.0 tunable diode laser. Our measurements were performed in two degassing sites at Furnas volcano (Furnas Lake and Furnas Village), with the aim of quantifying the total (fumarolic + soil diffuse) CO₂ output. We show that, within the main degassing (fumarolic) areas, the soil CO₂ flux contribution (9.2 t day^-1) represents a minor (~15 %) fraction of the total CO₂ output (59 t day^-1), which is dominated by the fumaroles (~50 t day^-1). The same fumaroles contribute to ~0.25 t day^-1 of H₂S, based on a fumarole CO₂/H₂S ratio of 150 to 353 (measured with a portable Multi-GAS). However, we also find that the soil CO₂ contribution from a more distal wider degassing structure dominates the total Furnas volcano CO₂ budget, which we evaluate (summing up the CO₂ flux contributions for degassing soils, fumarolic emissions and springs) at ~1030 t day^-1.