[1] The fumarolic gas output has not been quantified for any of the currently deforming calderas worldwide, due to the lack of suitable gas flux sensing techniques. In view of resumption of ground uplift (since 2005) and the associated variations in gas chemistry, Campi Flegrei, in southern Italy, is one of the restless calderas where gas flux observations are especially necessary. Here we report the first ever obtained estimate of the Campi Flegrei fumarolic gas output, based on a set of MultiGAS surveys (performed in 2012 and 2013) with an ad-hoc-designed measurement setup. We estimate that the current Campi Flegrei fumarolic sulphur (S) flux is low, on the order of 1.5-2.2 tons/day, suggesting substantial scrubbing of magmatic S by the hydrothermal system. However, the fumarolic carbon dioxide (CO 2 ) output is $4606160 tons/day (mean6SD), which is surprisingly high for a dormant volcano in the hydrothermal stage of activity, and results in a combined (fumaroles þ soil) CO 2 output of $1560 tons/ day. Assuming magma to be the predominant source, we propose that the current CO 2 output can be supplied by either (i) a large (0.6-4.6 km 3 ), deeply stored (>7 km) magmatic source with low CO 2 contents (0.05-0.1 wt%) or (ii) by a small to medium-sized ($0.01-0.1 km 3 ) but CO 2 -rich (2 wt%) magma, possibly stored at pressures of $100 to 120 MPa. Independent geophysical evidence (e.g., inferred from geodetic and gravity data) is needed to distinguish between these two possibilities.

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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.