Cyclic Development of Permafrost in the Peatlands of Northwestern Alberta, Canada

Vegetation in thermally subsided peatlands (collapse scars) grades from the wettest part near the collapsing peat bank with Sphagnum riparium to successively drier conditions with S. angustifolium and S. fuscum until the peat plateau surface is reached, covered with Picea mariana-lichen woodlands. The same sequence can be found in the peat stratigraphy, where the charred surface of the treed peat plateaus (sylvic peat) is overlain by a sequence of Sphagnum riparium, S. angustifolium, and S. fuscum, capped by sylvic peat. Often several such sequences occur in the peat stratigraphy, indicating periodic permafrost degradation, triggered by fires, and regeneration. Radiocarbon dates show that such cycles can be as short as 600 yr. The earliest incidence of permafrost in the study area was dated at 3700 yr BP, indicating the end of the mid-Holocene warm period and the onset of the current climatic regime.