An Atmospheric CH4 Sink in the High Arctic and its Implication for Global Warming

Arctic permafrost underlies about 16% of the Earth's surface and contains ~500 Pg of C down to one meter. Organic-rich peatlands (averaging ~4 wt% SOC) comprise 19% of this area, whereas the remaining 81% is permafrost-affected mineral cryosols (0.5-1.5 wt% SOC). Temperatures in the Arctic are predicted to increase ~6°C over the next 100 years which increases the depth of the active layer, the seasonally thawed soil above the permafrost. Thawing permafrost peat deposits (e.g. Stordalen Mire, Sweden) are currently CH4 sources. Field measurements, intact core studies and microcosm experiments performed by us, however, over the past few years on mineral cryosols associated with ice wedge polygons from Axel Heiberg Island (AHI) in the Canadian High Arctic consistently indicate that they are sinks for atmospheric CH4 as well as the CH4 emitted from the underlying permafrost. After 1.5 years of thawing at 4°C, 1 m long intact cores of the active layer and underlying permafrost mineral cryosols collected from ice wedge polygons at AHI continue to exhibit uptake of atmospheric CH4 even for water saturated cores. The measured core fluxes are consistent with flux measurements performed in the field over the past two years, which range from 0.005 to 0.89 mg CH4-C/m2/hr and which have revealed significant differences in the atmospheric CH4 consumption fluxes between the polygon interiors and the polygon troughs. Metagenome and metaproteome analyses of these mineral cryosols indicates that the pmoA genes and proteins are most similar to an uncultured methanotroph type known as USC-α, which is recognized as a high affinity, atmospheric CH4 oxidizer. Our microcosm studies have yielded atmospheric CH4 uptake rates that are consistent with those of published results from high latitude organic-rich soils and temperate forest soils and indicate a temperature dependency for the cellular rate of CH4 oxidization that is approximately twice that reported for methanogenesis. This temperature dependency when combined with annual temperature records from nearby Eureka weather station suggests that these High Arctic ice wedge polygons are significant annual sinks for atmospheric CH4. Because the maximum atmospheric CH4 uptake rate coincides with the summer time dips in the recorded atmospheric CH4 and peaks in δ13C, we propose that seasonal variations in the high latitude atmospheric CH4 are partially modulated by the activity of atmospheric CH4 oxidizers. We also suggest that this sink will increase with rising Arctic temperatures and may lessen the interannual increases in atmospheric CH4.