Methane concentration and stable isotope ratios in seawater and sea ice, Utqiagvik shelf, Alaska

Sea ice was collected at station IMB on 11 and 15 April 2016 in the Beaufort Sea (71.372° N, -156.540° W) using a Kovacs Mark II ice corer (Kovacs, Roseburg, Oregon, USA). The ice cores were sectioned into 15 cm and split lengthwise. The core sections were sealed into custom-made gas-tight tubes for determination of methane concentration and isotope ratios.

In the laboratory, the gas-tight tubes were flushed with ultrapure nitrogen for several gas volumes. Due to technical limitations, ice core 1 (IC1) was melted within a week at 5°C, while ice core 2 (IC2) was melted within a day, while frequently being mixed, at room temperature. Seawater temperature and salinity were recorded with an YSI Professional Plus probe (YSI, Ohio, USA) and a YSI 600 OMS V2 sonde (YSI, Ohio, USA).

Water was collected using either a peristaltic pump (Masterflex Environmental Sampler, Cole Parmer, Illinois, USA) or submersible pump (Cyclone, Proactive Environmental Products, Florida, USA) from different water depths. Methane concentration and isotope ratios were determined with a Picarro G2201-i cavity ring-down spectrometer (Picarro, Santa Clara, California, USA) coupled to a Small Sample Isotope Module (SSIM) (Picarro, Santa Clara, California, USA), in a headspace approach.

Nutrient concentrations were determined using a QuickChem QC8500 automated ion analyzer (Lachat, Loveland, Colorado, USA). The total number of prokaryotic cells was counted on a BD Influx^TM^ flow cytometer with BD FACS^TM^ software. Formol-fixed samples were stained with SYBR Green I (Invitrogen, Molecular Probes, Eugene, Oregon, USA) before analysis.