First observations of the fumarolic gas output from a restless caldera: implications for the current period of unrest (2005–2013) at Campi Flegrei

The fumarolic gas output has not been quantified for any of the currently deforming calderasworldwide, 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 therestless calderas where gas flux observations are especially necessary. Here we report the first everobtained 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 currentCampi Flegrei fumarolic sulphur (S) flux is low, on the order of 1.5–2.2 tons/day, suggesting substantialscrubbing of magmatic S by the hydrothermal system. However, the fumarolic carbon dioxide (CO2)output is 4606160 tons/day (mean6SD), which is surprisingly high for a dormant volcano in thehydrothermal stage of activity, and results in a combined (fumarolesþsoil) CO2 output of 1560 tons/day. Assuming magma to be the predominant source, we propose that the current CO2 output can besupplied by either (i) a large (0.6–4.6 km3), deeply stored (>7 km) magmatic source with low CO2contents (0.05–0.1 wt%) or (ii) by a small to medium-sized ( 0.01–0.1 km3) but CO2-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.