A chemical and thermal modelling study of the active layer on Herschel Island, Yukon Territory

An essential requirement in evaluating the potential impacts of climate warming in Polar Regions, is a greater understanding of the nature and behavior of the active layer and the permafrost system. Numerous studies have documented the inherent capacity of permafrost to limit significant degradation from seasonal thawing because of the ability of overlying soil layers to dissipate influxes of heat. However, information on the degree to which these layers are able to act as buffers to permafrost degradation is limited, and thus further research on this system is needed.The research presented in this thesis in the form of two manuscripts. The first manuscript (Chapter 3) discusses the role of chemical processes in the soil weathering regime on Herschel Island. Three distinct cryostratigraphic layers were observed in the soil profiles, including; (1) the modern active-layer located immediately below the ground surface (0 – 39 cm); (2) a paleo-active (transition layer), extending from the bottom of the modern active layer to the top of an undisturbed permafrost layer (39 – 192 cm); and (3) the underlying permafrost. These layers are marked by two thaw unconformities that separate the modern active layer from the transition layer and the transition layer from the permafrost layer, respectively. The two thaw unconformities were distinguished by abrupt breaks in the cryogenic texture and peaks in the concentrations of several chemical components. The upper thaw unconformity, located at the base of the modern active layer, coincided with a doubling in the concentration of major soluble cations and a tripling in the organic matter content compared to those of the overlying soil horizons. The lower thaw unconformity is marked by the highest concentration of Al2O3 and K2O in the entire profile and the second highest concentration of Na2O. Between these two unconformities, (i.e. in the transition zone), soluble cation concentrations varied considerably, the bulk soil SiO2 concentration increased significantly and there was a noticeable decrease in the concentrations of Al2O3, Fe2O3, CaO, MgO and K2O in the depth interval of 82-107 cm. Chemical weathering indices recorded an overall decrease in the intensity of chemical weathering with depth; the soil layers immediately above the first thaw unconformity were the most weathered and the deeper permafrost layer was the least weathered.The second manuscript (Chapter 4) employs a heat conduction algorithm (Stefan equation) based on the composition and thermal properties of the soils discussed in Chapter 3, and measured active layer depth values, to evaluate heat transfer from the ground surface to the underlying permafrost. Thermal constants (the square root of the ratio of twice the thermal conductivity of the unfrozen soil to the latent heat of fusion of ice) were calculated using the De Vries method and were compared to empirical thermal constants derived from measurements of active layer depths and the corresponding thawing degree days using the Stefan equation. The theoretical constants are approximately 38% lower than those derived empirically, a difference that is likely due to the fact that the Stefan equation does not consider advective heat transfer and that the active layer depths were probably overestimated because of the method of measurement (resistance to penetration of a metal probe). Third generation Canadian Global Coupled Models (CGCM 3.1/T63) from the Canadian Centre for Climate Modelling and Analysis (CCCma), based on the IPCC SRES A2 and B1 scenarios, were used in conjunction with the theoretically and empirically derived thermal constants to predict active layer depths for 2050 and 2100. These calculations suggest that the active layer will thicken by up to 41.76 % before the end of this century. This thesis provides new insights into the important role that the active and transition layers play in controlling the response of permafrost systems to climate change.

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