Hg and CO2 emissions from soil diffuse degassing and fumaroles at Furnas Volcano (São Miguel Island, Azores): Gas flux and thermal energy output

Gaseous elemental mercury (Hg0 g or GEM) and CO2 are emitted from active hydrothermal systems in volcanic areas mostly through diffuse degassing. Here, data from about 400 simultaneous measurements of soil GEM and CO2 flux performed within the caldera of Furnas Volcano, São Miguel Island (Azores) are discussed for the first time. This survey aimed at providing a new insight into the origin and magnitude of GEM in the investigated hydrothermal/volcanic environment and its relation with CO2 release. The distribution of GEM and CO2 emissions over an area of about 0.04km2 are correlated with soil temperature, and measurements have provided total CO2 and GEM output of 39td−1 and 1.8×10−6td−1, respectively. These results are similar to the emission from currently active volcanic/hydrothermal areas elsewhere, as well as from important non-volcanic areas, such as Sulphur Bank Mercury Mine (California, USA) and Idrija Mercury Mine (Slovenia, EU), pointing out the relevance of diffuse degassing processes at Furnas Volcano. Atmospheric spot measurements in the most vigorous fumarole vents of the geothermal field have shown that the fumarolic GEM contribution (9.2 ×10−7td−1) represents a minor fraction of the total (fumarolic+diffusive) GEM output (2.7 ×10−6td−1) for the study area of this volcano. Basing upon the integration of the hydrothermal CO2 released and the H2O/CO2 ratio in the fumarolic gas, we have also estimated the thermal energy release associated with diffuse degassing at Furnas Volcano. Our estimates are ∼7.8MW and 9.8MW, respectively for Furnas Lake and Furnas Village fumaroles.