The 2021-2022 unrest phase of Vulcano Island volcanic system (Aeolian islands): chemical and isotopic evolution of low-to-high temperature fluid discharges and waters from thermal and cold wells.

In summer 2021, a progressive increase of seismic activity and ground deformation was recorded at Vulcano (Aeolian Islands), followed by a strong change in fumarolic gas emission started in September 2021. New gas vents appeared on the crater rim and along the northern outer flank of the La Fossa crater with outlet temperatures up to 350 °C, and a general increase of CO₂ and SO₂ fluxes was measured.

In this study, we report the results of a geochemical monitoring, including gas and water samples collected during 5 field campaigns, carried out from November 2021 to October 2022. The main aim was to verify the evolution of the plumbing system feeding the fluid discharges of the Vulcano summit crater and those located in the Baia di Levante area. The geochemical dataset also includes the chemical and isotopic composition of thermal and cold wells located in the Vulcano Village. The analytical results have shown that in November 2021 and February 2022 the 9 selected crater fumaroles were affected by a strong increase, compared to the previous decade, of the (i) concentrations of acidic gases of marked magmatic origin (SO₂, HCl and HF) and temperature-dependent gases (H₂ and CO) (ii) gas/vapor and (iii) SO₂/H₂S ratios. In this period, the chemistry of waters and dissolved gases from the wells located at the foothill of the volcanic cone, and that of the gas discharges at Baia di Levante did not show significant anomalies.

Since June 2022, the SO₂/H₂S ratios as well as the concentrations of magmatic gases, H₂, and CO decreased, concurrent with a general decrease of the fumarolic flux. At Baia di Levante an opposite evolution was observed mainly consisting of a significant increase in H₂S, H₂, and CO accompanied by a decrease in CH4 concentrations. Such compositional changes were marked by the occurrence of seawater whitening events caused by enhanced emission of sulfur-rich fluids. In this period, the temperature, as well as the SO₄/Cl ratios and the concentrations of dissolved CO₂ in the thermal wells of the Vulcano Village also increased. The chemical-physical evolution of the crater fumaroles, culminated in February 2022, was likely related to a strong pulse of magmatic fluids occurred in summer 2021. The fluid reservoir feeding the discharges at the periphery of the magmatic fluid plumbing system, the latter being directly connected to the crater fumaroles, seems to have buffered the pulse until May 2022, when the heat and magmatic fluids fed by the deep source partially bypassed the hydrothermal aquifer. Further observations related to the continuation of the geochemical and geophysical monitoring of the Vulcano hydrothermal-magmatic system in the next months could provide fundamental insights to confirm the decline of the volcanic crisis as suggested by the recent evolution of the crater gas chemistry.