Estimated Amounts of Terrestrial Carbon Mobilized and Sequestered by the 1980 Mount St. Helens Eruption, Washington State, USA

Volcanic eruptions significantly influence the geologic carbon cycle by emitting CO2 and mobilizing terrestrial carbon pools. The 1980 Mount St. Helens (MSH) eruption offers a unique case study, triggering a massive landslide that displaced 2.7 km3 of sediment, buried 600 km2 of forest, and disrupted regional carbon dynamics. Pre-eruption terrestrial carbon stocks, including aboveground biomass (AGB), dead biomass (D), and soil organic carbon (S), averaged 209.5 tC/ha. Post-eruption, total carbon density fell to 118.7 tC/ha, driven mainly by a 51% reduction in AGB, modest declines in D, and redistribution of S. Over 36 years, ecosystem recovery increased AGB to 118.7 tC/ha by 2016, consistent within measurement uncertainty with the original levels. However, D and S pools remained lower, as inferred from modeling. Recovery rates of AGB varied by disturbance zone, with the Standing Dead Forest and Scorched Area zones showing the highest regrowth rates (8.72 and 4.40 tC/ha/year, respectively), which we infer to have benefited from residual structures and minimal soil disruption. Debris avalanche and pyroclastic flow zones recovered more slowly, likely due to sediment instability and soil sterilization. Despite full AGB recovery within error, persistent deficits in modeled D and S resulted in a net carbon loss of 25.2 tC/ha across the study area. Still, modeled fluxes indicated that all zones functioned as net carbon sinks during recovery. This research illustrates volcanic landscapes' potential transition from initial carbon sources to long-term sinks under suitable conditions. It emphasizes accounting for all carbon pools in post-disturbance assessments and highlights site-specific ecological processes critical for enhancing global carbon models and informing climate mitigation strategies.