Methane production controls in a young thermokarst lake formed by abrupt permafrost thaw.

Methane (CH ) release to the atmosphere from thawing permafrost contributes significantly to global CH emissions. However, constraining the effects of thaw that control the production and emission of CH is needed to anticipate future Arctic emissions. Here are presented robust rate measurements of CH 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 CH  production, long term (CH accumulation) and short term ( C tracer), produced similar average rates of 11 ± 3.5 and 9 ± 3.6 nmol cm  d , respectively. The rates had small variations between the different lithological units, indicating homogeneous CH production despite heterogeneous lithology in the surface ~1 m of sediment. To estimate the total CH production, the CH production rates were multiplied through the 10-15 m deep talik (thaw bulb). This estimate suggests that CH  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 CH in the first meter was produced further below. Interestingly, labeled C incubations with 2- C acetate and C CO indicate that variations in the pathway used by microbes to produce CH depends on the age and type of organic matter in the sediment, but did not appear to influence the rates at which CH  was produced. This study demonstrates that at least half of the CH produced by microbial breakdown of organic matter in actively expanding thermokarst is emitted to the atmosphere, and that the majority of this CH is produced in the deep sediment.