Palaeoecology of a palsa and a filled thermokarst pond in a permafrost peatland, subarctic Quebec, Canada

Radiocarbon dating and macrofossil data from a palsa and a filled thermokarst pond within a subarctic permafrost peatland were used to reconstruct their evolution and to distinguish between allogenic and autogenic processes that had been involved in their development. Peat began to accumulate in the peatland basin shortly after 6000 cal. BP. The initial stage was a shallow bay or a salty marsh, followed by a wet marsh triggered by a relatively rapid drop in sea level related to a rapid isostatic uplift of land. By 5640 cal. BP, the site had transformed into a rich fen. A high rate of peat accumulation led to the establishment of a short-lived intermediate fen by 4610 cal. BP and to a poor fen from 4200 cal. BP until 1760 cal. BP. Low water-tables associated with decreased precipitation occurred between 5170 and 4610 cal. BP, and 3100 and 1760 cal. BP. Between 1760 and the Little Ice Age, there was ombrotrophication of the site largely as a result of a thick peat accumulation. During this period, autogenic processes had controlled the general hydrosere, while allogenic processes, mainly precipitation, had influenced species composition. Permafrost established during the Little Ice Age leading to palsa formation and it has been melting in response to recent climate warming and precipitation increases. Macrofossil results from the filled thermokarst pond show that plant succession used the followed hydrosere: Calliergon giganteum and Sphagnum riparium when the pond's depth was at a maximum, S. riparium and C. giganteum when the pond was partly filled in, and S. lindbergii and S. riparium since the pond has been completely filled in.