The Holocene evolution of permafrost near the tree line, on the eastern coast of Hudson Bay (northern Quebec)

Abstract

Permafrost evolution in postglacial marine silts near the tree line was reconstructed using landform analysis, 14C dating, and palynostratigraphic analysis of peat sections. In the forest–tundra, below the tree line, four sites in peat plateaus have a stratigraphic sequence indicating an alluvial plain environment from 6000 to 4800 BP followed by a wetland supporting trees and shrubs with deep snow accumulation and without permafrost. Ground heave occurred between 1900 and 1200 BP as peat plateaus and palsas were formed. In the shrub–tundra, above the tree line, three permafrost sites with buried peat beds suggest that climatic conditions were cold enough for discontinuous permafrost in the surrounding landscape starting from land emergence, about 5800 BP; however, fen expansion and sedge peat accumulation continued over unfrozen ground until 2300, 1560, and 1400 BP. At these dates, the sites were buried with silt, probably as a result of mass wasting on nearby permafrost mounds and then permafrost aggraded under the sites. Generally, the palynostratigraphic data reflect a marked cooling of climate starting by 3200–2700 BP and culminating in a major period of permafrost aggradation between 1900 and 1200 BP. Permafrost degradation has been dominant since then despite other possible cold intervals. Nowadays, the permafrost in marine silts is twice as thick and three times more widespread in the shrub–tundra than in the forest–tundra.

Reconstruction of changes in vegetation and trophic conditions of a palsa in a permafrost peatland, subarctic Québec, Canada

“…1), recorded a low lake level between 5300 and 4600 cal yr BP (Payette and Filion, 1993). Allard and Séguin (1987a) also documented this warm period in relation to the permafrost degradation near the tree line on the eastern coast of Hudson Bay between 4000 and 2500 cal yr BP. Synchronous warming was also recorded in lake sediment elsewhere in the northern hemisphere, e.g.…”

“…Ombrotrophication of peatland 6 km south of Lake Kaapumticumac was also linked to these cold conditions (Bhiry and Robert, 2006). Such conditions favored permafrost aggradation in subarctic Québec between 2500 and 2000 cal yr BP (Allard and Séguin, 1987a) in addition to alluvial fan activities in the Guillaume-Delisle area (200 km north of this site) (Lafortune et al, 2006). Despite the fact that it was difficult to pinpoint this event by examining lake sediments at the study site, natural lacustrine banks were observed in some areas around the lake.…”

Development and decay of a lithalsa in Northern Québec: A geomorphological history

“…The same studies concluded, through dating of the peat/lichen (water logged/dry) transition on top of many palsas that the permafrost started to form after 3200 BP and that particular periods (e.g. 1550-1000 BP and 500-100 BP or roughly the Little Ice Age) had a colder climate conducive to palsa formation (Heim, 1976;Allard et al, 1987a;Lavoie and Payette, 1995;Allard and Rousseau, 1999). Thermokarst that probably occurred during warmer intervening periods did not completely destroy all the pre-existing palsas.…”

“…Lithalsas can be encountered in discontinuous permafrost regions (Zoltai and Tarnocai, 1975;Washburn, 1983;Zuidhoff, 2002). A large concentration of such landforms exists along the eastern coast of Hudson Bay, where these features have developed during the Late Holocene period in emerged post-glacial marine silts (Dionne, 1978;Seguin and Allard, 1984;Allard et al, 1987a;Allard and Rousseau, 1999;Payette et al, 2004).…”

Paleolimnology of thermokarst lakes: a window into permafrost landscape evolution

“…Permafrost inception started in the mid-to late Holocene, culminating during the Little Ice Age. Palsas and lithalsas, formed by ground ice aggradation and related surface heaving, are widespread in the region and typical of the discontinuous permafrost landforms in subarctic Québec (Allard and Seguin 1987). Recent increases in air/ground temperatures and snow cover along the eastern shore of Hudson Bay have contributed to widespread reduction of permafrost extent, resulting in increasing surface areas occupied by subarctic thermokarst ponds (Payette et al 2004;Vallée and Payette 2007;Jolivel and Allard 2013).…”