Magmatic Unrest and Gas Hazard at La Fossa Volcano (Italy): Insights from the 2021 Degassing Crisis

The La Fossa volcano on the Island of Vulcano, Italy, represents a critical case study for managing volcanic gas hazards in populated areas. Following the prolonged passive degassing phase subsequent to the 1888–1890 vulcanian eruption, the volcano exhibited signs of renewed energetic fumarolic–solfataric activity during 2021. This study characterizes some geochemical evidences and consequences of this "crisis", focusing on the anomalous degassing zones at the base of the volcanic cone (i.e., Palizzi, Faraglione, and Vulcano Porto), which exist in close proximity to inhabited settlements.

While the crater cone typically accounts for approximately 90% of emissions, diffuse degassing in the basal zones accounts for over 10% of total output, posing significant risks to indoor and outdoor air quality. During the 2021 unrest, we observed distinct variations in gas output. Soil CO₂ flux (φCO₂) in these anomalous zones increased from an average of 74 g m^-2 d^-1 in September 2021 to 370 g m^-2 d^-1 in November 2021. These values represent deviations of 27% and 538%, respectively, above the statistical background established since 1988 (φCO₂ ≈ 58 g m^-2 d^-1). To constrain the impact of these emissions on ambient air quality, we conducted five stable isotope surveys δ¹³C-CO₂ and δ¹⁸O-CO₂ of airborne CO₂ between August 2020 and November 2021, using a mobile laboratory equipped with a laser-based analyzer. By exploiting the distinct isotopic signature of volcanic CO₂ versus atmospheric background, we developed an isotopic mass balance model to partition the carbon sources. The results demonstrate that volcanic injections, modulated by local atmospheric circulation, significantly drove CO₂ concentration anomalies in the inhabited area of Vulcano Porto.

Using both φCO₂ and carbon isotope composition, we tracked a dramatic raise in total volcanic φCO₂ output, rising from 9.97 · 10⁴ kg d^-1 to 101.15 · 10⁴ kg d^-1. These estimates suggest that the instability of a deep magmatic body drove the transition from background activity to an unrest event. This escalation resulted in tangible hazards, necessitating the temporary displacement of the population from Vulcano Porto due to elevated gas concentrations. Our results demonstrate that synchronous monitoring of φCO₂ and outdoor air CO₂ concentration and stable isotopes δ¹³C-CO₂ and δ¹⁸O-CO₂ of airborne CO₂ are essential for the early detection of magmatic transients and the mitigation of gas exposure risks in the populated zones of Faraglione and Vulcano Porto.