Methane plume detection after the 2022 Nord Stream pipeline explosion in the Baltic Sea

On September 26th, 2022, the detonations at the gas pipelines Nord Stream 1 and 2 resulted in some of the largest non-natural releases of methane known. The distribution of methane in the surrounding seawater and the possible effects were not apparent. To trace the pathways of methane we recorded CH 4 concentrations and the isotopic signal (δ 13 C-CH 4 ) in seawater, and air. A week post-explosion, we detected methane concentrations up to 4 orders of magnitude above the natural Baltic Sea background. The released fossil methane created a distinct plume with δ 13 C-CH 4 ratios differing from natural background values. The strong water stratification preserved the distribution pattern initiated by the explosion, shown by the laterally strong concentration gradient within the plume. Our analysis encompasses three stages of the explosion's impact; the initial sea-air methane release, measurements taken during our research expedition one week later, and a third stage triggered by the shift from summer to winter conditions as an outlook on how winter mixing and microbial activity will influence the plume.