Pre-eruptive CO2 loss during shallow magma storage and its impact on eruption volumes at Reykjanes

Magmatic volatiles shape eruption dynamics, but a substantial fraction of magmatic CO₂ can outgas into crustal hydrothermal systems during shallow storage. Here we combine time-resolved measurements of eruptive gas compositions from the 2021–2025 Reykjanes eruption sequence with chemical monitoring of geothermal fluids at Svartsengi to track CO₂ transfer from magma into the upper crust. While the 2021 Fagradalsfjall eruption initially emitted CO₂-rich gas, subsequent eruptions at Fagradalsfjall in 2022–2023 and Sundhnjúkur in 2023–2025 emitted persistently CO₂-poor gases from the onset of activity. Although the Sundhnjúkur eruptions took place in close proximity to the nearby Svartsengi geothermal power plant, elevated CO₂ in geothermal steam only emerged several months later, indicating extensive pre-eruptive degassing of stored magma and a delayed, time-integrated hydrothermal response. Across the Sundhnjúkur eruptions, erupted volumes scale with the CO₂ content of eruptive gas, with the smallest eruptions associated with the most CO₂-depleted magmas. These observations are consistent with pre-eruptive CO₂ loss modulating eruption magnitude and highlight the role of shallow crustal magma reservoirs as dynamic filters that redistribute magmatic carbon from magma to hydrothermal systems during basaltic rifting episodes.