Evidence of CO2 recharge and release at Lago Albano (Colli Albani volcano, Rome, Italy) following the August 2020 seismic swarm: implications for present and future gas hazard

This study reports on the continuation of the 30-year surveillance of the CO 2 content stored in Lago Albano for the most recent period from September 2020 to July 2023, in the aftermath of the last, minor seismic swarm in August 2020. We scrutinize whether the gradual decrease of the CO 2 content (as dissolved CO 2 , HCO 3 − , and CO 3 2− ) in the lake waters through partial lake winter overturn events that followed the previous, major seismic events in 1989–1990 also occurred after the 2020 swarm, when approximately 450 ± 54 tons of total CO 2 were injected into the bottom waters of Lago Albano. Despite this CO 2 recharge and the consequent degassing of low magnitude, especially compared to the 1989–2019 period (154,000 ± 13,200 tons of CO 2 recharge and steady yearly release), we found that the 2020 seismic swarm not only caused an increase in total CO 2 content but also instigated a degassing above long-term background due to winter lake overturn. During the first post-seismic winter overturn (January 2021) up to 500 ± 60 tons of CO 2 were estimated to have been released from the lake surface. As such, Lago Albano currently results less gas-rich than if the seismic swarm had not occurred. Nevertheless, climate change is expected to jeopardize winter overturn and, consequently, CO 2 degassing as surface water temperature may not cool enough in the future; this will keep the lake permanently stratified (i.e., a higher degree of meromicticity) becoming a more efficient “gas storer”. Based on a total CO 2 budget approach, this study highlights that dissolved CO 2 is not necessarily fully degassed but it can be redistributed as HCO 3 − , CO 3 2− and calcite, in addition to isotopic exchange between C-species and CH 4 along the vertical profile of the lake, depending on pH-T conditions. Although this C-speciation can buffer the degassing capacity of Lago Albano, arguably lowering related gas hazards, it could primarily destabilize the biological activity (e.g., algal blooms) and its interaction with the changing atmosphere (i.e., more CO 2 -rich atmosphere) in the future.