The Impact of Variable Saturation on the Subsurface Thermal Regime: Implications for Ground Freezing and Talik Dynamics at a Degrading Permafrost Mound near Umiujaq (Nunavik, Canada)

Previous work has shown how complex interactions between topography, vegetation and seasonal snow cover can influence the subsurface thermal and moisture regimes in cold regions. Yet because of the large degree of spatial heterogeneity and inter-seasonal variability of these surficial conditions in the discontinuous permafrost zone, the extent of the interactions between surficial conditions and seasonally- and perennially-frozen ground is not well understood. Indeed, there is only a small amount of coupled field and modelling work that has examined these interactions, as well as their implications for permafrost extent and talik dynamics, in variably-saturated, mineral soil-type environments. As a result, there is a need for a more critical analysis of the role of saturation in permafrost environments, specifically at locations outside of saturated environments with organic-rich soils, such as peatlands or wetlands, where unsaturated conditions are highly improbable or of limited depth. This study focuses on evaluating the impacts of different surface and subsurface processes on local seasonal frost, talik and permafrost dynamics at a degrading permafrost mound located in the discontinuous permafrost zone near Umiujaq, Nunavik, Canada. Data indicate that the surface energy balance is significantly influenced by the large degree of spatial heterogeneity and seasonal inter-variability of surface conditions, including topography, vegetation and snow cover, over the small scale of the study site. Data further indicate that the formation and closure of an isolated supra-permafrost talik at the study site are controlled primarily by the degree of saturation. As such, a cryohydrogeological conceptual model was developed to provide insight into the processes that govern local permafrost dynamics at the studied permafrost mound. The SHAW numerical model was then used to investigate the mechanisms that control the surface energy balance and the thermal and moisture regimes at the top and the side of the permafrost mound, as well as at the location of the observed ephemeral supra-permafrost talik. Improved knowledge of the site-specific thermo-hydraulic processes will be employed to better constrain permafrost and talik dynamics in future large-scale models.