Dataset: Dissolved methane (CH₄) concentrations from the U.S. GEOTRACES Arctic cruise (HLY1502; GN01) on USCGC Healy from August to October 2015

Samples were collected from the ODF rosette rather than GEOTRACES carousel. The GEOTRACES Cookbook does not specifically address sample collection for methane; however, this rosette system was used during the cruise for sampling various ancillary parameters (e.g., oxygen, nutrients) and thus sampling overall followed Chpt. III (HYDROGRAPHY AND ANCILLARY PARAMETERS) of the Cookbook. Sampling methodology (from the Niskin bottles) followed that of Roberts and Shiller (2015). The dissolved methane samples were obtained from the Niskin bottles immediately after other dissolved gases were sampled.

Dissolved CH₄ was determined cavity ringdown spectroscopy (CRDS) following headspace equilibration following the method of Roberts and Shiller (2015). Samples were collected from the Niskin bottles immediately after the other dissolved gas samples, equilibrated with methane-free zero-air (approximately 70 milliliters (mL) water and 70 mL zero air) immediately after samples were collected. Equilibrated head-space methane was determined using a Picarro G2301 Greenhouse Gas Analyzer. Calibration was performed using both methane-free zero air and a 5 ppmv CH₄ gas standard (NIST traceable). Variability of dissolved methane in deep ocean samples from the GP15 section suggests a standard deviation of better than 0.06 nanomoles per kilogram (nmol/kg) on samples of ∼0.6 nmol/kg, similar to the results of Roberts and Shiller (2015).