Methane (CH₄ ) release to the atmosphere from thawing permafrost contributes significantly to global CH₄ emissions. However, constraining the effects of thaw that control the production and emission of CH₄ is needed to anticipate future Arctic emissions. Here are presented robust rate measurements of CH₄ production and cycling in a region of rapidly degrading permafrost. Big Trail Lake, located in central Alaska, is a young, actively expanding thermokarst lake. The lake was investigated by taking two 1 m cores of sediment from different regions. Two independent methods of measuring microbial CH₄ production, long term (CH₄ accumulation) and short term (¹⁴ C tracer), produced similar average rates of 11 ± 3.5 and 9 ± 3.6 nmol cm^-3 d^-1 , respectively. The rates had small variations between the different lithological units, indicating homogeneous CH₄ production despite heterogeneous lithology in the surface ~1 m of sediment. To estimate the total CH₄ production, the CH₄ production rates were multiplied through the 10-15 m deep talik (thaw bulb). This estimate suggests that CH₄ production is higher than emission by a maximum factor of ~2, which is less than previous estimates. Stable and radioactive carbon isotope measurements showed that 50% of dissolved CH₄ in the first meter was produced further below. Interestingly, labeled ¹⁴ C incubations with 2-¹⁴ C acetate and ¹⁴ C CO₂ indicate that variations in the pathway used by microbes to produce CH₄ depends on the age and type of organic matter in the sediment, but did not appear to influence the rates at which CH₄ was produced. This study demonstrates that at least half of the CH₄ produced by microbial breakdown of organic matter in actively expanding thermokarst is emitted to the atmosphere, and that the majority of this CH₄ is produced in the deep sediment.

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