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

Abstract Gaseous elemental mercury (Hg 0 g or GEM) and CO 2 are emitted from active hydrothermal systems in volcanic areas mostly through diffuse degassing. Here, data from about 400 simultaneous measurements of soil GEM and CO 2 flux performed within the caldera of Furnas Volcano, Sao 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 CO 2 release. The distribution of GEM and CO 2 emissions over an area of about 0.04 km 2 are correlated with soil temperature, and measurements have provided total CO 2 and GEM output of 39 t d −1 and 1.8 × 10 −6  t d −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 −7  t d −1 ) represents a minor fraction of the total (fumarolic + diffusive) GEM output (2.7 × 10 −6  t d −1 ) for the study area of this volcano. Basing upon the integration of the hydrothermal CO 2 released and the H 2 O/CO 2 ratio in the fumarolic gas, we have also estimated the thermal energy release associated with diffuse degassing at Furnas Volcano. Our estimates are ∼7.8 MW and 9.8 MW, respectively for Furnas Lake and Furnas Village fumaroles.