Dynamics of sporadic mountain permafrost – case study of Mont Jacques-Cartier (1977-2014), Chic-Chocs Range, South-eastern Canada

English Abstract:

In southern Quebec, mountain permafrost is restricted to the highest bare ground flat summits of the Chic-Chocs Mountains (Northern Appalachians, Gaspé Peninsula). The existence of a contemporary permafrost body was reported on Mt Jacques-Cartier – the highest point in southern Quebec (1273 m a.s.l.) – in 1977 by way of a thermistors cable installed in a 29 m deep borehole. With a 36-year long record of ground temperature, this summit is one of the longest permafrost-monitoring sites in North America and is therefore of great interest to study the impact of climate change on sporadic mountain permafrost.

The rounded summit of Mt Jacques-Cartier is mantled by a 3m-thick regolith cover reworked into periglacial landforms. The soils on the plateau support an alpine tundra vegetation and isolated krummholz patches. To determine the factors controlling ground surface temperature (GST) on the summit, 20 sensors have been installed in the near-surface (-5 cm) in sites with different terrain parameters (e.g. bare ground surface, krummholz, snow bed).

The results showed that the spatio-temporal variability of the GST over the summit is mainly correlated with the snowpack distribution which is controlled by the interaction between the prevailing NW wind and the summit topography and the surface roughness resulting from periglacial landforms and krummholz patches. On the wind-exposed plateau, the snow is systematically blown away as surface roughness is low. The mean snowpack depth is about 35 cm which corresponds to the maximum snow-retaining capacity created by the microtopography of blockfields and sorted-polygons. The mean annual GST was on average about -1°C in 2013 over the summit area. In the krummholz patches and in the topographic depression on the leeward slope, drifted snow accumulation is significant (> 2 m thick) and the mean annual GST was thus warmer (3°C in 2013) than on the wind-exposed plateau.

From 1978 to present, the air temperature increased about 1°C at the summit. The resulting ground temperature warming was +0.4°C at 14 m through this period. From 2008 to 2013, the warming trend was more pronounced with rapid warm-side deviation of the temperature profile (e.g. +0.08°C/year at 14 m). The active layer depth increased by +0.5 m/year over the same period.