CYCLIC DEVELOPMENT OF PERMAFROST IN THE PEATLANDS OF NORTHWESTERN ALBERTA, CANADA

Vegetation in thermally subsided peatlands (collapse scars) grades fro m the wettest part near the collapsing peat bank with Sphagnum ripariu m 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 stratigr aphy, where the charred surface of the treed peat plateaus (sylvic pea t) is overlain by a sequence of Sphagnum riparium, S. angustifolium, a nd S. fuscum, capped by sylvic peat. Often several such sequences occu r in the peat stratigraphy, indicating periodic permafrost degradation , triggered by fires, and regeneration. Radiocarbon dates show that su ch cycles can be as short as 600 yr. The earliest incidence of permafr ost in the study area was dated at 3700 yr BP, indicating the end of t he mid-Holocene warm period and the onset of the current climatic regi me.