Dating ice-wedge growth in subarctic peatlands following deforestation

Ice wedges are one of the most prominent periglacial features in the zone of continuous permafrost, and constitute significant climatic and palaeoclimatic markers 1–4 . Active ice wedges are found in areas with a mean annual air temperature below −6 °C (ref. 5), although recent data 6 suggest that they can form at −3.5 °C. Most of the reports 7–9 dealing with ice wedges have referred to Arctic and Antarctic conditions, whereas subarctic ice wedges from glaciated areas are poorly known. Ice-wedge inception and spatial development as orthogonal and polygonal nets were inferred generally from theoretical analysis 10,11 and sparse detailed field work 7,8 . Because ice wedges occur also in subarctic permafrost peat lands 12–15 , the combined use of macrofossil peat stratigraphy and radiocarbon dating may yield useful information concerning their development. Here we present the first account of such a framework, derived from the analysis of marginal ice wedges developed in the eastern Canadian Subarctic. We show that the inception and spatial development of epigenetic ice wedges in some subarctic peatlands occurs after the natural removal of coniferous vegetation, which induces snow-free conditions, permafrost aggradation and, ultimately, deep frost cracking. We have dated ice wedges indirectly, using combined curves of rates of spruce removal and ice-wedge growth.