Transport of CH₄ through open-talik lakes in discontinuous permafrost aquifers

Abstract

As northern regions of the world experience warming climate, scientists look to permafrost, a crucial component of Arctic and subarctic ecosystems, as a source and sink of atmospheric carbon. It is well-known that the thawing of permafrost from above as a result of warming climate is a considerable source of greenhouse gases. However, few studies have considered the production of methane, a potent greenhouse gas, beneath the permafrost. A rugged permafrost bottom is proposed to favor the storage of gas in "pockets" that have been formed through permafrost thaw and degradation from below. Sub (below)-permafrost methane can migrate to reach the atmosphere when connections between the sub-permafrost and supra- permafrost open pathways from the pocket to the bottom of an open talik lake. We hypothesized that the migration of methane occurs through advection and diffusion as a dissolved gas and by movement as an immiscible fluid. Through measurement of environmental tracers in two thermokarst lakes in Goldstream Creek Basin, Fairbanks, Alaska, we found that advection was variable and was seasonally and climatically dependent demonstrating both upward and downward groundwater flow within our study lakes. Measurements of dissolved methane concentrations in the lakes demonstrated that diffusion of methane was not a significant transport mechanism in the groundwater-to-lake pathway due to the extreme temporal and spatial variability of methane concentrations. Immiscible flow of free-phase methane is likely the dominant transport mechanism but is dependent on the lake sediment composition and the formation of secondary pathways within the talik. Data obtained from this study allowed for a better understanding of methane transport and thermokarst lake dynamics.