Abstract
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.Electronic supplementary materialThe online version of this article (doi:10.1186/s40623-015-0345-5) contains supplementary material, which is available to authorized users.
Highlights
Volcano-hosted hydrothermal systems are the source of sizeable carbon dioxide (CO2) emissions, either vented by hydrothermal steam vents (Chiodini et al 1998) or diffusively released by degassing soils (Chiodini et al 1999; Rogie et al 2001; Werner et al 2008)
tunable diode laser (TDL)-based CO2 distribution maps Figure 4 is a distribution map of atmospheric CO2 concentrations, calculated for a horizontal air cross-section taken at a 1.20-m height above the degassing soil and fumaroles of Furnas Lake
In the most vigorously degassing areas, the soil CO2 flux contribution represents a minor (~18 %) contribution to the total CO2 output, which is dominated by the fumaroles
Summary
Volcano-hosted hydrothermal systems are the source of sizeable carbon dioxide (CO2) emissions, either vented by hydrothermal steam vents (Chiodini et al 1998) or diffusively released by degassing soils (Chiodini et al 1999; Rogie et al 2001; Werner et al 2008). Hydrothermal activity is widespread on the island and includes soil diffuse degassing areas (Ferreira et al 2005; Viveiros et al 2010), steaming ground, thermal springs, cold CO2-rich springs, and lowtemperature fumaroles (95–100 °C), mostly concentrated. CO2 concentrations were obtained in each of the measurement days/sites by pointing the laser beam toward a mirror, positioned upwind of the fumarolic area. These background values were subtracted in the calculation of integrated column amount (ICA) (see below). The GasFinder was left to acquire data along each single GasFinder-retroreflector path for ~3–5 min, before being rotated to measure along the successive path (an additional documentation file shows more details of acquisition-paths (see A1 in Additional file 1).
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