Response of Permafrost to Anthropogenic Land Surface Disturbance near Fairbanks, Alaska

Permafrost near Fairbanks Alaska is relatively warm (measured between -1 and 0°C in this study), and is thus highly susceptible to thawing following surface disturbance by land clearing or fire. The surface moss layer and other vegetation are important insulators for near-surface permafrost in the summer months. The removal of this insulation causes the seasonally thawed (active layer) depth to increase and eventually results in formation of taliks (thawed ground below the seasonally frozen active layer). We have been investigating the response of permafrost seasonal thaw depths and rates in soils commonly found around Fairbanks, Alaska following anthropogenic disturbances such as trails, roads, and large clearings. This information is useful to predict the impact of future disturbances on the permafrost landscape and on local ecology and aids in modeling permafrost stability under land that has already been cleared of vegetation. We combined direct current resistivity, ground-penetrating radar (GPR), and borehole data to evaluate permafrost top-down thawing at multiple locations in the Fairbanks area: on Fort Wainwright north of the Chena River, south of the Chena River within Yukon Training Area (YTA), and at the Farmer's Loop Permafrost Research Site. These sites were cleared of vegetation in the past and were selected to represent time since disturbance. The trails north of the Chena River were cleared in 1994 and were surveyed with GPR in 1994-1995, the YTA site was cleared around 1965, and the Farmer's Loop site was cleared in 1946. These sites represent varying types of soil including alluvial soils (containing sandy gravel capped with sandy silt) on Fort Wainwright and thick loess at Farmer's Loop Road. The YTA site does not contain deep borings for detailed stratigraphic interpretation, but hand auguring confirmed this site also contains thick loess at the surface. Resistivity data were used to discern taliks from permafrost and were compared to the 1994-1995 GPR data at the Fort Wainwright site. Resistivity values at the loess sites were in the range of 200-300 ohm-m for "warm" frozen silt and generally over 3,000 ohm-m in frozen alluvium. Permafrost has thawed to depths greater than 9 m in portions of all the sites and patterned ground due to thawed ice wedges can be seen at the YTA site. Thermokarst wetlands are present along trails at the alluvial site and in depressions created by melting of ice wedges at the Yukon Training Area site. The results of this study provide long-term rates of top-down permafrost thaw at sites mechanically cleared of vegetation, relative subsidence measurements compared to surrounding undisturbed land, and examples of landscape change due to vegetation removal.