Fate and Transport of Methane Formed in the Active Layer of Alaskan Permafrost

Abstract:

Over the past 2 years a series of tracer tests designed to estimate rates of methane formation via acetoclastic methanogenesis in the active layer of permafrost soils were conducted at the Barrow Environmental Observatory (BEO) in northernmost Alaska. The tracer tests consisted of extracting 0.5 to 1.0 liters of soil water in gas-tight bags from different features of polygons at the BEO, followed by addition of a tracer cocktail including acetate with a ¹³C-labeled methyl group and D₂O (as a conservative tracer) into the soil water and injection of the mixture back into the original extraction site. Samples were then taken at depths of 30 cm (just above the bottom of the active layer), 20 cm, 10 cm and surface flux to determine the fate of the ¹³C-labeled acetate.

During 2014 (2015 results are pending) water, soil gas, and flux gas were sampled for 60 days following injection of the tracer solution. Those samples were analyzed for concentrations and isotopic compositions of CH₄, DIC/CO₂ and water. At one site (the trough of a low-centered polygon) the ¹³C acetate was completely converted to ¹³CH₄ within the first 2 days. The signal persisted for throughout the entire monitoring period at the injection depth with little evidence of transport or oxidation in any of the other sampling depths. In the saturated center of the same polygon, the acetate was also rapidly converted to ¹³CH₄, but water turnover caused the signal to rapidly dissipate. High δ¹³C CO₂ in flux samples from the polygon center indicate oxidation of the ¹³CH₄ in near-surface waters. Conversely, CH₄ production in the center of an unsaturated, flat-centered polygon was relatively small ¹³CH₄ and dissipated rapidly without any evidence of either ¹³CH₄ transport to shallower levels or oxidation. At another site in the edge of that polygon no ¹³CH₄ was produced, but significant ¹³CO₂/DIC was observed indicating direct aerobic oxidation of the acetate was occurring at this site. These results suggest that a longer thaw season resulting from a warming climate may increase the net CH₄ flux from saturated polygonal features, but that effect may be minimized by increased drainage due to degradation of low-centered centered polygons.