Methane (CH4) release to the atmosphere from thawing permafrost contributes significantly to global CH4emissions. However, constraining the effects of thaw that control the production and emission of CH4is needed to anticipate future Arctic emissions. Here are presented robust rate measurements of CH4production 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 (CH4accumulation) and short term (14C 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 CH4production despite heterogeneous lithology in the surface ~1 m of sediment. To estimate the total CH4production, the CH4production 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 CH4in the first meter was produced further below. Interestingly, labeled14C incubations with 2‐14C acetate and14C CO2indicate that variations in the pathway used by microbes to produce CH4depends 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 CH4produced by microbial breakdown of organic matter in actively expanding thermokarst is emitted to the atmosphere, and that the majority of this CH4is produced in the deep sediment.
The ongoing global temperature rise enhances permafrost thaw in the Arctic, allowing Pleistocene‐aged frozen soil organic matter to become available for microbial degradation and production of greenhouse gases, particularly methane. Here, we examined the extent and mechanism of anaerobic oxidation of methane (AOM) in the sediments of four interior Alaska thermokarst lakes, which formed and continue to expand as a result of ice‐rich permafrost thaw. In cores of surface (~ 1 m) lake sediments we quantified methane production (methanogenesis) and AOM rates using anaerobic incubation experiments in low (4°C) and high (16°C) temperatures. Methanogenesis rates were measured by the accumulation of methane over ~ 90 d, whereas AOM rates were measured by adding labeled‐13CH4and measuring the produced dissolved inorganic13C. Our results demonstrate that while methanogenesis was vigorous in these anoxic sediments, AOM was lower by two orders of magnitude. In almost all sediment depths and temperatures, AOM rates remained less than 2% of the methanogenesis rates. Experimental evidence indicates that the AOM is strongly related to methanogens, as the addition of a methanogens' inhibitor prevented AOM. Variety of electron acceptor additions did not stimulate AOM, and methanotrophs were scarcely detected. These observations suggest that the AOM signals in the incubation experiments might be a result of enzymatic reversibility (“back‐flux”) during CH4production, rather than thermodynamically favorable AOM. Regardless of the mechanism, the quantitative results show that near surface (< 1‐m) thermokarst sediments in interior Alaska have little to no buffer mechanisms capable of attenuating methane production in a warming climate.
more » « less- Award ID(s):
- 1936752
- NSF-PAR ID:
- 10418950
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 68
- Issue:
- 6
- ISSN:
- 0024-3590
- Page Range / eLocation ID:
- p. 1316-1330
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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