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The Krafla geothermal system is located within a volcanic center that periodically erupts basaltic lavas, and has recently attracted an economic interest due to supercritical fluids forming near a shallow magma intrusion (∼ 2 km depth). Here, we discuss new soil CO 2 flux and stable isotope data of the CO 2 efflux (δ13C) and hydrothermal calcites (δ13C, δ18O) of drill cuttings to estimate both the current magmatic outgassing from soils and the thermal flows in the geothermal system. Soil CO 2 emission is controlled by tectonics, following the NNE-SSW fissure swarm direction and a WSW-ENE trend, and accounts for ∼62.5 t d−1. While the δ18O of the H 2 O in equilibrium with deep calcites is predominantly meteoric, both the δ13C of the soil CO 2 efflux and of the fluids from which calcite precipitated have a clear magmatic origin, overlapping the δ13C estimated for the Icelandic mantle (−2.5 ± 1.1 ‰). Estimates based on the soil CO 2 emission from the southern part of the system show that these fluxes might be sustained by the ascent and depressurization of supercritical fluids with a thermal energy of ∼800 MW. Such significant amount of energy might reach 1.5 GW if supercritical conditions extended below the whole investigated area. Finally, we report an increase in the soil CO 2 emission of about 3 times with respect to 14 years ago, likely due to recent changes in the fluid extracted for power production or magmatic activity. Pairing the soil CO 2 emission with stable isotopes of the efflux and calcite samples has important implications for both volcano monitoring and geothermal exploration, as it can help us to track magmatic fluid upflows and the associated thermal energy. • Krafla geothermal field naturally emits 62.5 t d−1 of CO 2 from soils into the atmosphere. • δ13C of both CO 2 and hydrothermal calcites reveals a magmatic origin. • Supercritical fluids with a significant thermal energy (up to 1.5 GW) might feed the CO 2 emission. • The soil CO 2 emission has increased about 3 times in the last 14 years. • Such increase is ascribed to changes in geothermal production or magmatic activity. [ABSTRACT FROM AUTHOR]

The 2014-2015 Holuhraun eruption was the largest fissure eruption in Iceland in the last 200 years. This flood basalt eruption produced ~ 1.6 km of lava, forming a lava flow field covering an area of ~ 84 km. Over the 6-month course of the eruption, ~ 11 Mt of SO were released from the eruptive vents as well as from the cooling lava flow field. This work examines the post-eruption SO flux emitted by the Holuhraun lava flow field, providing the first study of the extent and relative importance of the outgassing of a lava flow field after emplacement. We use data from a scanning differential optical absorption spectroscopy (DOAS) instrument installed at the eruption site to monitor the flux of SO. In this study, we propose a new method to estimate the SO emissions from the lava flow field, based on the characteristic shape of the scanned column density distribution of a homogenous source close to the ground. Post-eruption outgassing of the lava flow field continued for at least 3 months after the end of the eruption, with SO flux between < 1 and 9 kg/s. The lava flow field post-eruption emissions were not a significant contributor to the total SO released during the eruption; however, the lava flow field was still an important polluter and caused high concentrations of SO at ground level after lava effusion ceased. [ABSTRACT FROM AUTHOR]