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

Methane (CH 4 ) release to the atmosphere from thawing permafrost contributes significantly to global CH 4 emissions. However, constraining the effects of thaw that control the production and emission of CH 4 is needed to anticipate future Arctic emissions. Here are presented robust rate measurements of CH 4 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 4  production, long term (CH 4 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 CH 4 production despite heterogeneous lithology in the surface ~1 m of sediment. To estimate the total CH 4 production, the CH 4 production rates were multiplied through the 10-15 m deep talik (thaw bulb). This estimate suggests that CH 4  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 4 in the first meter was produced further below. Interestingly, labeled 14 C incubations with 2- 14 C acetate and 14 C CO 2 indicate that variations in the pathway used by microbes to produce CH 4 depends on the age and type of organic matter in the sediment, but did not appear to influence the rates at which CH 4  was produced. This study demonstrates that at least half of the CH 4 produced by microbial breakdown of organic matter in actively expanding thermokarst is emitted to the atmosphere, and that the majority of this CH 4 is produced in the deep sediment.