UV camera-based SO2 flux observations during the Etna’s 2021 paroxysmal sequences

Volcanic SO2 flux observations are relevant to understanding the mechanisms governing transition from open-vent, quiescent degassing to paroxysmal explosive activity, as repeatedly observed on Mt. Etna since the early 2000s. Here, we report on a SO2 flux time-series derived from a permanent UV camera system during a ~1 year-long temporal interval encompassing the two most recent paroxysmal sequences of Etna’s South-East Crater (SEC) in February/April 2021 and May/October 2021. This fully automated UV camera system, housed in the Montagnola INGV-OE hut, is designed to spatially resolve SO2 emissions from the southern portion (SEC + Central Craters) of the summit craters’ terrace, and is thus complementary to distal bulk plume observations with scanning spectrometers. This gas record is interpreted by integration with independent volcanological, seismic tremor and (ground- and satellite-based) thermal output records. Our results identify a clear acceleration in SO2 degassing during the two 2021 paroxysmal sequences (time-averaged SO2 flux ≥ 3500 t/d), relative to non-eruptive (quiescent) periods (time-averaged SO2 flux ≤ 1750 t/d). This accelerating degassing parallels a notable escalation in seismic tremor amplitude and thermal output, which exhibit a peaky behaviour with prominent peaks associated with individual paroxysmal episodes. From this evidence, we propose that initiation of a paroxysmal sequence is controlled by a factor ~2 increase in the rate of magma transport (and degassing) in the shallow Etna’s plumbing system, e.g., at pressures above the S exsolution level (~100 MPa). Our results are therefore corroborative of recent geophysical and geochemical evidence showing Etna’s paroxysmal sequences are driven by over-pressure development in a ~3 km deep magma storage zone below the summit region, caused by escalating supply of fresh volatile-rich magma rising from deeper in the pluming system.