Polygonal peatlands and treed peat plateau in Northwest Hudson Bay Lowlands, Canada: Holocene development and permafrost dynamics

Since the retreat of the Laurentide Ice Sheet, peatland initiation in the Hudson Bay Lowlands (HBL) has been mainly driven by postglacial isostatic rebound and climate. We studied the timing of permafrost aggradation and the successional trajectories of low Arctic and subarctic polygonal peatlands and a northern boreal treed peat plateau in Wapusk National Park (WNP) in northwestern HBL. We also explored the timing of peatland initiation and long-term carbon accumulation between peatland types. Peatland initiation in WNP dates to at least 6000 cal BP, and peat thickness and carbon mass were higher in treed peat plateaus (mean: 71.7 ± 21.4 (SE) kg C m −2 ) and intermediate-centred polygonal peatlands (63.2 ± 7.14 (SE) kg C m −2 ) than in non-permafrost fens (20.4 ± 1.75 (SE) kg C m −2 ). In the subarctic and northern boreal forest zones, permafrost peatlands were preceded by treed Sphagnum bogs with Picea mariana and P. glauca . Abrupt declines in peat and carbon accumulation in subarctic sites ∼1500–1050 cal BP reflected cooling from the Dark Ages Cold Period to the Little Ice Age (LIA). Low peat accumulation, spruce disappearance ∼1330–780 cal BP, and the shift toward vegetation dominated by Dicranum spp. mosses and Cladonia lichens suggest that permafrost aggradation occurred across the study region during the LIA. These results further suggest that in the subarctic tundra, polygons formed in Sphagnum -dominated peat plateaus, rather than evolving from low-centred polygons. The low Arctic polygon was characterized by wet fen vegetation until permafrost developed ∼190 cal BP, but the exact successional pathway remains uncertain.