Dissolved methane and CO2 concentrations and stable carbon isotopes from the coastal Arctic landscape of the Greiner Lake watershed, Nunavut, Canada in June 2022 and June-July 2023

Discrete water samples were collected from the study area during two field campaigns. The first between June 17-29, 2022, and the second from June 20 to July 4th, 2023. Station replicates between seasons were collected from the same water body, with proximity determined by ice cover and other local conditions. Sampling protocol followed that outlined in Pohlman and others, 2021. Samples were collected by filling 1-Liter (L) plastic syringes fitted with 3-way stopcocks with 880 milli-Liters (mL) of natural water and 120 mL of Zero Air. A headspace extraction was performed by shaking the syringes for two minutes and collecting the headspace into 250 cc multi-layer foil bags (Cali-5 Bond). These samples were transported back to the lab, stored at room temperature, and processed for measurements within two months of collection. Additional measurements were made with the CTD (RBR Concerto conductivity, temperature, and depth probe.)

Sample analysis was performed at the U.S. Geological Survey (USGS) Woods Hole Coastal and Marine Science Center, utilizing the USGS Automated Sample Introduction Module (AutoSIM) interfaced to a Picarro G2201-i CRDS that simultaneously measures CH4, CO2, and stable carbon isotope ratios reported in the d-notation relative to Vienna Pee Dee Belemnite (VPDB) referenced gas standards (δ13C-CO2, δ13C-CH4). Check standards for concentration and stable isotopic composition were run approximately every 4-5 samples and corrected as described by Pohlman and others (2021). Concentrations are reported with a relative standard deviation of 0.4% and isotopic ratios are reported with an analytical precision of 0.6‰ based on the precision reported in Pohlman and others (2021).
For CO2 concentrations below 300 parts per million (ppm), the analytical precision of the isotopic ratio (d13C_co2) decreases exponentially and was characterized by laboratory measurements. This decrease is be described by the equation: Error = 2.2501*exp(-0.003*pCO2) Where pCO2 is the concentration of the headspace in ppm (co2_ppm). For samples with a concentration under 300 ppm, the reported precision is calculated using this formula.
Sample analyses were performed by Shawnee Traylor and Michael Casso under the supervision of John Pohlman. Sample processing occurred in August 2022 and November 2023.

Headspace concentrations were converted to dissolved concentrations using the method of Magen and others, 2014.
Explicitly, the following equations were used to calculate dissolved concentrations: ((pCO2 * 10^-6 * 0.120) / (R * T_extraction) + (0.880 * pCO2 * 10^-6 * Sol_CO2)) / 0.880 * 10^6 = [CO2]
Where pCO2 is the headspace concentration of carbon dioxide in ppm, 0.120 is the volume of gas sample in mL, R is the gas constant in L-atm/mol-K, T_extraction is the extraction temperature in Kelvin, 0.880 is the volume of water sample that was extracted in mL, Sol_CO2 is the solubility of carbon dioxide in mol/L-atm (Weiss, 1974), and [CO2] is the dissolved concentration of carbon dioxide in the original natural water sample, in micromoles per liter.
((pCH4 * 10^-6 * 0.120) / (R * T_extraction) + (0.880 * pCO2 * 10^-6 * Sol_CH4)) / 0.880 * 10^9 = [CH4]
Where pCH4 is the headspace concentration of methane in ppm, 0.120 is the volume of gas sample in mL, R is the gas constant in L-atm/mol-K, T_extraction is the extraction temperature in Kelvin, 0.880 is the volume of water sample that was extracted in mL, Sol_CH4 is the solubility of methane in mol/L-atm (Wiesenburg & Guinasso, 1979; Yamamoto et al., 1976), and [CH4] is the dissolved concentration of methane in the original natural water sample, in nanomoles per liter.
The data were organized into a Microsoft Excel spreadsheet (Microsoft Excel for Mac) and are available in both XLSX and CSV formats.