Collapse of permafrost mounds along a subarctic river over the last 100 years (northern Québec)

Abstract

Widespread permafrost decay is currently occurring in the northern hemisphere. In subarctic Québec, most permafrost mounds are located in peatlands in the form of palsas and peat plateaus. The formation and degradation of these periglacial landforms are influenced by several regional and local factors including air temperature, depth of snow cover, and peat insulation. Mineral palsas are another type of permafrost landforms found along river shores in subarctic Canada. Due to their peculiar position near or in the river floodplain, the dynamics of these palsas are influenced by water level fluctuations. This study examines palsa dynamics along a subarctic river, the Boniface River, in northern Québec. Mapping of palsas and thermokarst ponds over a 44-year period, i.e., from 1957 (from aerial photographs) to 2001 (from field surveys) was used to evaluate changes in the distribution and area covered by permafrost landforms. Also the decay of 14 palsas was assessed using the mortality dates of black spruce trees as determined by tree-ring analysis. Between 1957 and 2001 the area occupied by palsas decreased by 23% whereas 76% of the present-day thermokarst area formed since 1957. No new palsas developed during this period. For the 14 palsas studied, degradation began at the end of the 19th century and accelerated during the 20th century. Palsa degradation was closely related to distance from the river channel. Palsas located in the river floodplain were the most affected by thawing and showed a 48% reduction in area. Degradation was less severe for palsas located 1 to 15 m from the river margin, which experienced a 19% reduction in area. The spatiotemporal distribution of palsas suggests that changes in water level are among the most important factors influencing the dynamics of riparian palsas, particularly for those palsas directly in contact with the river water.