Damm, Ellen; Sellmaier, Amanda; Björk, Göran; Abrahamsson, Katarina: Measurements of CH4 concentration and 𝜹13C-CH4 in seawater three months after the Nord Stream 2 detonation in January 2023 [dataset]. PANGAEA, https://doi.pangaea.de/10.1594/PANGAEA.997021 (dataset in review)

Our sampling took place on the research vessel RV Skagerak on 9–12 January 2023. During the cruise, 20 stations northeast of the island Bornholm were sampled comprising stations influenced by the Nord Stream pipeline leak and background stations representing unaffected conditions typical of the central Baltic Sea. Profiles of salinity and temperature in the water column were recorded by a Sea-Bird SBE911, a conductivity–temperature–depth (CTD) system. Water samples for the analysis of concentrations of dissolved methane and δ13C-CH4 values were taken from up to 10 discrete water depths across the entire water with Niskin bottles mounted on a rosette sampler. The deepest bottles were closed at about 1–2 m above the sea floor. The selection of water sampling depths was determined taking the thermocline and halocline depths into consideration. Sampling procedure was performed as follows: Seawater from the rosette was filled in 500 ml glass vials, sealed with rubber stoppers, and crimped with aluminium caps. Care was taken to avoid any bubbles. A head- space was created by injecting 20 ml hydrocarbon free synthetic air while at the same time water was pushed out via a second needle to balance the pressure. When equilibrated, a subsample was removed with a gas tight syringe and injected to the Small Sample Isotope Module (SSIM) coupled to a Picarro G2132-i cavity ring-down spectrometer. For calibration, we used standard gas mixtures with different concentrations (Fa. Linde), and standard gas mixtures with isotopic ratios of − 25‰ vs. VPDB, − 45‰ vs. VPDB and 69‰ vs. VPDB (Fa Airgas). Considering the complete sampling procedure, the overall total uncertainty was < 5% estimated on duplicates. The calculation of the dissolved methane concentration followed Wiesenburg and Guinasso (1979).