Our sampling took place on the RV Skagerak on 3-5 October 2022 . In total, 20 stations Northeast of the island Bornholm were covered. 16 sites were located within the methane plume formed by the intense methane bubbling in the week after the explosion, but outside the 7 NM wide security circle around the leaks. In addition, we included 4 sites on the track to the explosion site to detect natural background concentrations.
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 from 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 were filled in 500 ml glass vials, sealed with rubber stoppers and crimped with aluminum caps. Care was taken to avoid any bubbles. A headspace 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 SSIM (Small Sample Isotope Module) 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).
