Potential of the Thermokarst Development at the Council Research Area
Authors
Alexander Kholodov1* (alkholodov@alaska.edu), Nicholas Hasson1, Colleen Iversen2
Institutions
1University of Alaska–Fairbanks, AK; 2Oak Ridge National Laboratory, Oak Ridge, TN
URLs
Abstract
As a climatically driven phenomenon, permafrost is undergoing degradation following global trends of air warming. Depending on the ground ice content, this might be realized in two ways: (1) formation of taliks (perennial unfrozen horizon between seasonally frozen layer and permafrost) when ice content does not exceed soil porosity and (2) formation of thermokarst, i.e., subsidence and collapsing of the ground surface due to melting of excessive ground ice. This case study estimated the potential for thermokarst development in the tundra at Council on the Seward Peninsula, Alaska. A deep, frozen core 6.7 meters long was collected in June 2017. The core was subsampled and analyzed for cryostratigraphy and basic soil properties such as total and dry bulk soil density and volumetric ice content.
A borehole was instrumented for long-term geothermal measurements. In June 2022 the team conducted a very-low-frequency geophysical survey combined with greenhouse gas flux measurements along a transect spanning an undisturbed peat plateau, a disturbed drilling site, and a natural thermokarst hollow. The core consists of the modern peat horizon about 70 cm thick, composing the active layer, and is underlain with mineral soil changing in texture from silt to sandy loam and then to sand and gravel. Volumetric ice content in this horizon of permafrost gradually decreased downward from ~70% to ~40%. Sustained increasing of permafrost temperature at a rate of about 0.05°C per year was recorded. This led to the formation of talik ~3 meters deep, which had been refrozen during the winter of 2019 to 2020 but thawed again in 2021 and kept developing. As a result, a local depression ~50 cm deep had formed. Vegetation had changed from tundra (moss, lichens, evergreen, and deciduous shrubs) to sedge wetland (cotton grass). Based on existing data about ice content, ground surface subsidence can be estimated at as much as 25% of the thaw depth increment. High methane flux recorded at the drilling site confirms that development of the thermokarst process converted the peat plateau from carbon sinking to greenhouse gas release. Methane production took place permanently within the 3-meter deep talik during entire year. Ongoing thermokarst processes can provide an excellent natural experiment that models consequences of further permafrost degradation in terms of greenhouse gas release. Two sets of core subsamples are available for biogeochemical, microbiological, and hydrochemical analyses.