Holocene dynamics of an inland palsa peatland at Wiyâshâkimî Lake (Nunavik, Canada)

This study examines the Holocene evolution of an inland subarctic permafrost peatland located on the north bank of Wiyâshâkimî Lake (Nunavik, northeastern Canada). The analysis of plant macrofossils allowed us to reconstruct the succession of the trophic conditions of a palsa and a filled thermokarst pond. The accumulation of organic matter began at around 6290 cal. y BP. The evolution of the site then followed three stages: a pond (6290–5790 cal. y BP), a minerotrophic peatland (5790–4350 cal. y BP) and an ombrotrophic peatland (from 4350 cal. y BP). The establishment of permafrost caused a palsa to form at around 170 cal. y BP, which corresponds to the coldest period of the Little Ice Age in northeastern Canada. A subsequent degradation of the palsa and the formation of a thermokarst pond were induced by the climate warming that began at the turn of the 20th century. The analysis of plant macrofossils from an adjacent filled thermokarst pond indicated three phases of development over a short 450-year period: subaquatic, minerotrophic, and ombrotrophic phases. When combined with previous studies of filled thermokarst ponds in northern Québec, this result indicates that ponds are rapidly filling in with vegetation and acting as carbon sinks.