Fumarolic CO2 emissions from weakly degassing, hydrothermal volcanoes: a case from Fogo volcano, São Miguel, Azores

Current global catalogues of hydrothermal CO2 emissions from quiescent volcanoes are manifestly incomplete, primarily because of the shortage of available flux measurements. Fogo, a quiescent stratovolcano volcano in the central part of São Miguel island (Azores, North Atlantic Ocean), is an emblematic example of such hydrothermal systems that lack assessment of fumarolic gas fluxes. Here, for the first time, we characterize the CO2 flux emissions from the main hydrothermal manifestations (steaming grounds, boiling pools and fumaroles) of this volcano. For this, we use the results of a ground-based CO2 profiling campaign, carried out using a portable Multi-GAS unit. We find these hydrothermal manifestations to be very CO2-rich and H2S–H2-poor, with CO2/H2S and H2/CO2 ratios of respectively 1600–80,700 and 2⋅10−4 to 2⋅10−3. We additionally show that these emissions, although very modest at visual observation, still emit CO2 at levels of 1.3 to 95 tons/day, for a cumulative fumarolic CO2 flux of 232 ± 140 tons/day for the volcano. This is ∼6 times higher than the CO2 flux from diffuse soil degassing in the Caldeiras da Ribeira Grande area, and of the same order of the CO2 flux sustained by crater plume degassing at a typical medium-sized arc volcano. The contribution of weak hydrothermal manifestations (steaming grounds, boiling pools and fumaroles) to the global volcanic CO2 budget is undetermined, but may have been overlooked so far.