SUBSURFACE THAW OF DISCONTINUOUS PERMAFROST IN NORTHWESTERN CANADA: EVIDENCE FROM GEOPHYSICAL IMAGING AND THERMAL CONDUCTION MODELING

Some of the most pronounced climate warming of recent times has been occurring in northwestern Canada, resulting in widespread permafrost thaw. Since permafrost within discontinuous permafrost zones has subsurface temperatures close to 0°C, small changes in the surface energy balance, resulting from rises in air temperature or landcover changes, can cause significant degradation of permafrost in the subsurface. By using electrical resistivity tomography (ERT), ground-penetrating radar (GPR) and thermal conduction modeling, we show how subsurface energy-transfer processes influence thawing of discontinuous permafrost in the wetland-dominated peatlands of the Hay River Lowland ecoregion of Canada. In this environment the permafrost bodies have thicknesses of ca. 10 m and occur under tree-covered peat plateaus that are elevated by 0.5-2 m from the local water table. By using interpretations of our ERT profiles, we show how disturbances within the active layer promote enhanced sites of long-term thaw in the underlying permafrost. In order to investigate how contemporary climatic conditions and landcover disturbances will influence future changes to the subsurface permafrost distribution, we construct two-dimensional thermal conduction models using constraints derived from one of our geophysics profiles and observed soil temperature data. Results show that thermal energy input is higher under permafrost-free bogs and channel fens than it is under the peat plateaus and that thaw-induced subsidence and flooding at the lateral margins of peat plateaus induces enhanced vertical energy transfer along the margins of peat plateaus. As a consequence of these processes the permafrost bodies are thawing faster laterally than they are vertically.