As part of the field season of the 2007 International Polar Year, the Zackenberg research station was kept open two months longer than normal to host the project: Influence of Snow and Ice on the Winter Functioning and Annual Carbon Balance of a High-Arctic Ecosystem (ISICaB). This opening gave us a chance to observe autumn and early-winter carbon fluxes. Six automated chambers provided methane flux measurements once per hour, day and night in a typical fen area. At the end of the growing season, emissions decreased during September until they reached the presumed low winter emission level. However, at the onset of soil freeze-in, a substantial increase in emissions was observed and was sustained for several weeks, corresponding to the time required for a complete freeze-in of the entire active layer. Peak emissions during the freeze-in period in individual chambers reached levels of 112.5 mg CH4 m-2 h-1, which to our knowledge are the highest rates reported from tundra ecosystems (excluding hotspot emissions from thermokarst lakes), and they appear at a time when previous assumptions would put tundra emissions at a negligible level. The integral of emissions during the freeze-in period is approximately equal to the amount of methane emitted during the entire summer season.
A late-autumn shoulder is consistently observed in the seasonal cycles of atmospheric methane at high-latitude sites, but the sources responsible for these increased methane concentrations have remained uncertain. However, three-dimensional atmospheric model simulations of global atmospheric methane concentrations indicate that the observed early winter emission burst improves the agreement between the simulated seasonal cycle and atmospheric data from the polar areas. Our findings suggest that permafrost-associated freeze-in bursts of methane emissions from tundra regions could be an important and so far unrecognized component of the seasonal distribution of methane emissions from high latitudes.
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The automatic chambers that monitor the methane flux from the fen area in the Zackenberg Valley. The muskoxen in the background are frequent visitors in the fen areas. Photo by C.Sigsgaard from 17 October 2007. 
The equipment is checked frequently to avoid problems before the extended season. Photo by M.P.Tamstorf from 31 July 2007. 
Low sun in mid-October at the methane monitoring site. Photo by C.Sigsgaard from 18 October 2007. http://www.nature.com/doifinder/10.1038/nature07464 