CLIMATIC AND ECOLOGICAL CONTROLS ON ICE SEGREGATION AND THERMOKARST -THE CASE-HISTORY OF A PERMAFROST PLATEAU IN NORTHERN QUEBEC

A reconstruction of the historical evolution of a permafrost plateau t ypical of the discontinuous permafrost zone was carried out at a site selected along the shore of Hudson Bay. The attempt was made by using data from four thermistor cables, cryostratigraphic data from 25 boreh oles, repeated snow surveys, and dendrochronological analysis on 209 s pruce trees over the flanks and the top of the plateau. Four cryostrat igraphic layers make up the complete permafrost stratigraphy. Layer I is the active layer. Layer II is an ice-enriched layer composed mainly of aggradational ice near the top of the permafrost; annual thermal a mplitude at that level is 8.5 degrees C. Layer III contains a low volu me of reticulated ice; thermal amplitude is considerably damped in thi s layer. And layer IV (350 cm to permafrost base) has a very high ice content made of lenses that get thicker with depth. Historic interpret ation is as follows. Starting about 1830, ice-poor layer III formed du ring the first few years of frost penetration when thermal gradients w ere too strong to allow ice lensing. Thereafter, layer IV progressivel y formed with permafrost deepening as the thermal gradient decreased. Meanwhile, layer II slowly formed, fed by downward migration of water following repeated thermal gradient inversions near the surface. Since the 1940s, thermokarst dominates around the edges of the plateau as f orest expands on slopes and depressions. A degradation sequence takes place: early colonization leads to thicker snow cover which warms up t he ground; ice-rich layer II melts within a few years, initiating sett lement. The shallow depressions so formed provide still better shelter for continued vegetation growth, leading to increased ground warming and slope wasting. However, final thermokarst subsidence is delayed by the latent heat effect of the deeper layer IV. Actually (1990-94), so me areas of the plateau are aggrading while others are degrading under the contemporary climate.