The 2021-2022 unrest of Vulcano Island volcanic system (Aeolian islands): geochemical evidences from fumarolic gas discharges and well waters

Since September 2021, a gradual unrest at Vulcano (Aeolian Islands) has been recorded by monitoring systems. It was marked by a progressive variation of different geochemical, geophysical and ground deformation parameters. The fumaroles located on the crater rim and along the northern outer flank of the La Fossa crater were characterized by a general increase in temperature (up to 350°C) as well as CO2 and SO2 fluxes. New fractures and fumaroles and a fumarolic plume, up to >100 m high, also formed. Furthermore, an increasing number of VLP events was recorded by the seismic network, combined with an uplift of about 1 cm/month. In this work, we present the analytical results obtained during several sampling campaigns were carried out by DST-UNIFI from November 2021 to February 2023 in agreement with INGV-Palermo, to verify how the volcanic system was evolving. Nine fumaroles from the summit crater and five from the Baia di Levante area, together with 9 hot and cold wells located in Vulcano Porto, were sampled. The geochemical dataset revealed that the crater fumaroles had a significant increase in both the concentrations of acidic gases of marked magmatic origin (SO2, HCl and HF) and the gas/vapor and SO2/H2S ratios in November 2021, associated with significant chemical-physical variations of the waters and dissolved gases from the wells located at the foothill of the volcanic cone. On the contrary, the gas discharges at Baia di Levante did not show significant compositional variations when compared with the previous data. From February 2022, the SO2/H2S ratios as well as the concentrations of magmatic gases, H2, and CO from crater gas emissions decreased, concurrently with a general decrease of the fumarolic flux. Since June 2022, the Baia di Levante manifestations were characterized by a dramatic increase in the H2S, H2, and CO concentrations whereas those of CH4 decreased. Such compositional changes were marked by the occurrence by 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.