Methane, Methanogens and Permafrost

Permafrost_thaw_ponds_in_Hudson_Bay_Canada_near_Greenland

10th November 2014.

It has been known for some time that permafrost stores a significant proportion of global soil carbon, and that thawing permafrost acts as an atmospheric source of methane and carbon dioxide, thereby creating a positive feedback to climate change.  Earlier in 2014, an international team of researchers discovered a new methanogen in the class Methanomicrobia.  They named the new species Methanoflorens stordalenmirensis and proposed a new family for it: Methanoflorentaceae.

New research by the same team, at the same Swedish field site, has shed more light on the activities of M. stordalenmirensis.  The team used a natural gradient of permafrost thaw to determine how thaw-induced changes in vegetation and hydrology affect methane dynamics.  They found that average methane fluxes were zero at an intact permafrost site, but increased to 1.46 mg CH4 m−2 h−1 at a thawing Sphagnum site, and increased further to 8.75 mg CH4 m−2 h−1 at a fully thawed site dominated by Eriophorum.  There was also a significant difference in the isotopic signature of the produced methane: average δ13C of emitted methane was −79.6 ‰ at the Sphagnum site and −66.3 ‰ at the Eriophorum site.  A similar pattern was observed for porewater CH4 isotopes between the Sphagnum and Eriophorum site.

These changes in methane dynamics were accompanied by changes in the microbial communities of the sites.  For the intact site there was a low abundance of methanogens, whilst the communities at the Eriophorum site and the deeper (anaerobic) layers of the Sphagnum site contained 20-30 % methanogens.  The Sphagnum site was dominated by hydrogenotrophic methanogens, including species similar to M. stordalenmirensis, whilst the Eriophorum site featured a high abundance of acetoclastic methanogens in the genus Methanosaeta.

Further analysis suggested that M. stordalenmirensis was the best single-variable predictor of isotopic patterns, although a multi-variable model also highlighted the importance of organic matter chemistry.  Altogether, these results show how ecosystem-scale methane dynamics are driven by changes in the composition of microbial communities.  Such research can be incorporated into future models of permafrost thaw and climate change methane feedbacks.

Lead author Carmody McCalley at the University of New Hampshire said: “By taking microbial ecology into account, we can accurately set up climate models to identify how much methane comes from thawing permafrost versus other sources such as fossil-fuel burning.”

References:

Discovery of a novel methanogen prevalent in thawing permafrost. 2014. Mondav, R., Woodcroft, B.J., Kim, E-H., et al. Nature Communications, 5, doi:10.1038/ncomms4212.

Methane dynamics regulated by microbial community response to permafrost thaw. 2014. McCalley, C.K., Woodcroft, B.J., Hodgkins, S.B., et al. Nature, 514, doi:10.1038/nature13798.

Photo: Permafrost thaw ponds, Hudson Bay, by Steve Jurvetson. CC BY 2.0

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Emissions Likely to Increase Significantly

F6 frost polygons_0

26th January 2011.

A major new study published in Reviews of Geophysics* in December 2010 puts the spotlight on methane emissions from natural sources and asks the question, how might they both affect and be affected by future climate change.

The authors conclude, based on a comprehensive review of recent literature, that ‘significant increases in methane emissions are likely, and catastrophic emissions cannot be ruled out’.

Major uncertainties surround the impact and timescale of several important feedback processes in the global methane cycle. How will natural methane emissions from wetlands, permafrost areas and methane hydrate deposits respond to climate change? At the most extreme, one can envisage a scenario in which permafrost melts, the carbon-rich wetland areas increase and the methanogens up their rate of production in response to greater warmth, which in turn promotes release of biogenic volatile organic compounds, these additional BVOCs reduce the potential for atmospheric oxidation of methane by competing for OH radicals, and finally, under further warming, hydrates become unstable and release their vast store of methane, as may have happened at the Palaeocene-Eocene Thermal Maximum.

In reality, the authors acknowledge that this ‘perfect storm’ of methane emissions is likely to be countered by negative feedback processes such as the drying out of some wetlands and reduced BVOC emission efficiency.  But the point remains that the strength and relative timing of both positive and negative feedbacks are currently poorly understood and the potential for significant increases in methane emissions exists.

When asked which gaps in current knowledge he considered the most important, co-author Olivier Boucher of the Met Office listed “…wetland processes and their sensitivity to climate change, permafrost dynamics and its interactions with vegetation, fire and hydrological processes,  the transient aspects of marine hydrate destabilisation and the fate of methane emitted in the ocean”.

*O’Connor, F.M., Boucher, O., Gedney, N., Jones, C.D., Folberth, G.A., Coppell, R., Friedlingstein, P., Collins, W.J., Chappellaz, J., Ridley, J. & Johnson, C.E. 2010, “Possible role of wetlands, permafrost, and methane hydrates in the methane cycle under future climate change: A review”, Reviews of Geophysics, vol. 48, no. 4.

Supersaturated Siberian Seas

Winter ice

19th April 2010.

Covered in ice for 265 days of the year, and bordered by the frozen wastes of the Siberian tundra, it is hardly surprising that the shallow seas of East Siberian Arctic Shelf (ESAS) have not before now been subject to extensive monitoring for methane emissions.  However, due to the efforts of an international collaboration between researchers based in Alaska, Vladivostock and Stockholm, a comprehensive survey of methane concentrations in these waters has now been conducted.  The results, reported in Science (Shakhova et al. 2010) make compelling reading and raise new questions about future Arctic methane fluxes.

Six summer field campaigns were conducted between 2003 and 2008, and methane concentrations were measured in 5100 seawater samples taken from 1080 stations.  The researchers found that over 50% of the surface waters were supersaturated with methane. In hotspot areas, the median supersaturation was 8300%.  One over-ice winter expedition and one helicopter survey provided additional data with which to constrain estimates of the total methane flux from these Arctic seas.  Based on all the observations, the team calculated annual atmospheric methane flux from the ESAS at 7.98 Tg C-CH4. To put this in context, previous research has estimated the global methane flux from oceans as 4 – 15 Tg C-CH4 y-1 (IPCC, 2007).

The worldwide ocean methane flux figures would remain relatively small compared to terrestrial sources such as wetlands and rice paddies, even if revised upwards based on the ESAS study. The new data are however interesting  in that they demonstrate the potential for methane emissions from flooded areas underlain by large pools of carbon-rich vulnerable permafrost. The authors note that the question of whether the amount of methane being released from the ESAS sediments has changed in response to the general warming of the Arctic region is not answered by their study, and suggest that this issue merits further attention.

Natalia Shakhova, Igor Semiletov, Anatoly Salyuk, Vladimir Yusupov, Denis Kosmach, and Örjan Gustafsson. 2010. Science 1246-1250.