Sampling and Analytical Methodology:
Discrete groundwater, lake, and river water samples were collected from Toolik Lake in September 2013 and July 2014. Groundwater samples were collected in all locations from freshly dug pits or temporary PVC well points of variable depth depending on the depth of the water table. All water samples were collected by submersible pump. Sample collection followed previously established methods into glass wheaton bottles, as did measurement of methane concentration. Dissolved CH₄concentrations in water samples were measured using a headspace equilibration technique. Water samples were collected by direct filling of 125 mL or 160 mL glass serum bottles. The serum bottles were sealed without headspace using blue butyl stoppers, and saturated HgCl₂ solution (0.3 mL) was added immediately after sample collection to halt biological activity. Before analysis, 10 % of total water sample volume was removed; replaced by the same volume of helium gas as the headspace. Sample vials were shaken vigorously for 3 minutes and placed on a shaker for 30 min at room temperature (25 ºC).

CH₄ concentrations for all samples were measured on an SRI 310 Gas Chromatograph (GC) equipped with a flame ionization detector and an Alltech Haysep S 100/120 column (6’ x 1/8” x 0.085”). 0.25 mL of gas was removed from the headspace with a syringe for analysis, and the same volume of Milli-Q water was injected to replace the volume of the gas removed. Helium was used as the carrier gas at a flow rate of 15 mL min^-1, and the column and detector temperatures were maintained at 50 °C and 150 °C, respectively. Peak integration was performed using Peak Simple NT software. Gas mixtures used for GC calibration and standard curves were made using successive dilutions of 1000 ppm CH₄. Total [CH₄] in the water samples was calculated by adding the measured headspace [CH₄] and the amount of CH₄ remaining in the water sample after headspace equilibration, calculated from the solubility equation of (Yamamoto et al. 1976). The average combined standard error of sampling and analysis was 3.6 % (n= 27). After methane concentration analysis, the remaining headspace gas samples were split into two 10-mL exetainers for δ¹³C-CH₄ and δD-CH₄ analysis. δD-CH₄ was analyzed at the UC Davis Stable Isotope Facility on a ThermoScientific PreCon concentration system interfaced to a ThermoScientific Delta V Plus isotope ratio mass spectrometer (ThermoScientific, Bremen, DE). δ¹³C-CH₄ was analyzed at the Lawrence Livermore National Laboratory using the standard TraceGas preconcentration system interfaced with an IsoPrime isotope ratio mass spectrometer (IsoPrime Ltd, UK) as described by Fisher et al. (2006). The mass requirements for δ¹³C-CH₄ and for δD-CH₄analyses was 10 nmoles and 2 nmoles respectively.

Isotopes_Methane.csv