CO2 flux distribution at Grindavík and Sundhnúkur craters prior to the August 2024 eruption at Reykjanes, SW Iceland

Diffuse volcanic degassing reflects subsurface magma and fluid transport, but its spatio-temporal relationship to seismicity and eruptions remains poorly constrained. Here, we present ground- and drone-based CO2 flux measurements acquired prior to the August 2024 eruption on the Reykjanes Peninsula, Iceland, together with seismic observations. The study focused on the highly fractured Grindavík and Sundhnúkur areas. A transect through Grindavík reveals a bimodal distribution of CO2 fluxes with values between 0.1 and 28 g·m−2·d−1, reflecting heterogeneous near-surface permeability and local correlations to fractures. Using a new drone-based gas flux detection system, measurements at the inaccessible Sundhnúkur fissure system were possible, identifying elevated CO2 fluxes, with up to 4400 g·m−2·d−1 close to the location where the next eruption initiated. A comparison with seismicity suggests (1) distributed pressurization with localized enhanced gas escape restricted to the Sundhnúkur area prior to the eruption, and (2) strongly focused seismicity in the Sundhnúkur area during the eruption.