RECENT investigations on Mount Etna (Sicily)1-3 have revealed that volcanoes may release abundant carbon dioxide not only from their active craters, but also from their flanks, as diffuse soil emanations. Here we present analyses of soil gases and air in water wells on Vulcano Island which provide further evidence of such lateral degassing. Nearly pure carbon dioxide, enriched in helium and radon, escapes from the slopes of the Fossa active cone, adding a total output of 30 tonnes per day to the fumarolic crater discharge (̃180 tonnes CO2 per day). This emanation has similar He/CO2 and 13C/12C ratios to those of the crater fumaroles (300%ndash;500°C) and therefore a similar volcanic origin. Gases rich in carbon dioxide also escape at sea level along the isthmus between the Fossa and Vulcanello volcanic cones, but their depletion in both He and 13C suggests a distinct source. Diffuse volcanic gas emanations, once their genetic link with central fumarole degassing has been demonstrated, can be used for continuous volcano monitoring, at safe distances from active craters. Such monitoring has been initiated at Vulcano, where soil and well emanations of nearly pure CO2 themselves represent a threat to the local population. © 1990 Nature Publishing Group.
