The evolution of methane production rates from young to mature thermokarst 1 lakes

Thermokarst lakes, formed by permafrost thaw in the Arctic, are hotspots for methane (CH4) and carbon dioxide (CO2) emissions, and are expected to double permafrost carbon emissions by the end of the century. While the implications of ongoing permafrost thaw on methane dynamics within these lakes have been modeled, here we provide empirical data on methane production dynamics as lakes evolve from young recently formed lakes to older lakes that have been present for hundreds of years. Sediment cores (up to 4 m long) were collected from the centers and thermokarst margins of a new thermokarst lake [Big Trail Lake (BTL), <70 years] and from an older thermokarst lake [Goldstream Lake (GSL), ~900 years] from the same interior Alaskan watershed. Highest methane 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. BTL exhibited elevated methane production rates, correlated with higher carbon lability for thermal induced reactions measured by Rock Eval analyses, and suggesting its potential use as a proxy for organics susceptibility for methanogenesis. In contrast, GSL displayed lower methane production rates, likely due to a longer period of organic matter degradation and reduced carbon lability. The integrated sediment-column methane production rates were similar (around 7-10 mol m-2 year-1), primarily due to the thinner talik at BTL. Our data support the predictions that formation and expansion of thermokarst lakes over the next centuries will increase methane production in newly thawed Yedoma permafrost sediments, while methane production will decrease as taliks mature and labile organic matter is used up. The positive warming effect of yedoma lake methane emissions may weaken over longer periods as the organics becomes mainly refractory, and the landscape can no longer support significant lake formation and expansion.