The Evolution of Methane Emissions in Thermokarst Lakes

Thermokarst lakes, formed by permafrost thaw in the Arctic, are ubiquitous with abrupt permafrost thaw and large atmospheric source "hotspots" of methane (CH₄) and carbon dioxide (CO₂) emissions, which are expected to double permafrost carbon emissions by the end of the century. While the implications of ongoing permafrost thaw on CH₄ dynamics within these lakes have been modeled, here we provide empirical data on CH₄ production dynamics as lakes evolve from young recently formed lakes to older lakes that have been present for hundreds of years. Sediment cores were collected from the centers and thermokarst margins of a new thermokarst lake and from an older thermokarst lake from the same interior Alaskan watershed. The highest CH₄ production rates were observed in the uppermost sediments near the sediment-water interface at the thermokarst margins of both lakes, with a steep decrease with sediment depth into the talik. The young lake exhibited elevated CH₄ production rates, correlated with higher carbon lability. The integrated sediment-column CH₄ production rates were similar, primarily due to the thinner talik at the young lake. Our data support the predictions that formation and expansion of thermokarst lakes over the next centuries will increase CH₄ production in newly thawed Yedoma permafrost sediments, while CH₄ production will decrease as taliks mature and labile organic carbon is used up. Our results also suggest important controls of methane production and oxidation in the sediments.