Climate change in Arctic landscapes may increase freeze-thaw frequency within the active layer as well as newly thawed permafrost. Freeze-thaw is a highly disruptive process that can deform soil pores and alter the architecture of the soil pore network with varied impacts to water transport and retention, redox conditions, and microbial activity. Our objective was to investigate how freeze-thaw cycles impacted the pore network of newly thawed permafrost aggregates to improve understanding of what type of transformations can be expected from warming Arctic landscapes. We measured the impact of freeze-thaw on pore morphology, pore throat diameter distribution, and pore connectivity with X-ray computed tomography (XCT) using six permafrost aggregates with sizes of 2.5 cm3 from a mineral soil horizon (Bw; 28-50 cm depths) in Toolik, Alaska. Freeze-thaw cycles were performed using a laboratory incubation consisting of five freeze-thaw cycles (-10 C to 20 C) over five weeks. Our findings indicated decreasing spatial connectivity of the pore network across all aggregates with higher frequencies of singly connected pores following freeze-thaw. Water-filled pores that were connected to the pore network decreased in volume while the overall connected pore volumetric fraction was not affected. Shifts in the pore throat diameter distribution were mostly observed in pore throats ranges of 100 mu m or less with no corresponding changes to the pore shape factor of pore throats. Responses of the pore network to freeze-thaw varied by aggregate, suggesting that initial pore morphology may play a role in driving freeze-thaw response. Our research suggests that freeze-thaw alters the microenvironment of permafrost aggregates during the incipient stage of deformation following permafrost thaw, impacting soil properties and function in Arctic landscapes undergoing transition.

We present a method to characterize soil moisture freeze-thaw events and freezing/melting point depression using permittivity and temperature measurements, readily available from in situ sources. In cold regions soil freeze-thaw processes play a critical role in the surface energy and water balance, with implications ranging from agricultural yields to natural disasters. Although monitoring of the soil moisture phase state is of critical importance, there is an inability to interpret soil moisture instrumentation in frozen conditions. To address this gap, we investigated the freeze-thaw response of a widely used soil moisture probe, the HydraProbe, in the laboratory. Soil freezing curves (SFCs) and soil thawing curves (STCs) were identified using the relationship between soil permittivity and temperature. The permittivity SFC/STC was fit using a logistic growth model to estimate the freezing/melting point depression (T-f/m) and its spread (s). Laboratory results showed that the fitting routine requires permittivity changes greater than 3.8 to provide robust estimates and suggested that a temperature bias is inherent in horizontally placed HydraProbes. We tested the method using field measurements collected over the last 7 years from the Environment and Climate Change Canada and the University of Guelph's Kenaston Soil Moisture Network in Saskatchewan, Canada. By dividing the time series into freeze-thaw events and then into individual transitions, the permittivity SFC/STC was identified. The freezing and melting point depression for the network was estimated as T-f/m = - 0.35 +/- 0.2,with T-f = - 0.41 +/- 0.22 degrees C and T-m = - 0.29 +/- 0.16 degrees C, respectively.

Climate change might increase the frequency of events such as heat waves, freeze-thaw cycles (FTC), and flooding, and more specifically in permafrost rich regions. These climate hazards are expected to have an impact on railway track performance. There is little publicly available data on their quantitative impacts on railway operations. Such quantitative data is essential for determining when, where, and to what extent climate adaptation measures are needed. Freeze and thaw cycle results in frost heave and thaw softening in track foundation (substructure). Both frost heave and thaw softening may lead to unsafe operating conditions especially for rail transit and passenger rail systems as their high operating speed makes them much less tolerant to deviations in track geometry parameters. In order to investigate the effects of a freeze-thaw cycles on an active railway, a structural and geotechnical monitoring system was designed and installed on a of VIA's track in Ontario. The instruments measure various track parameters such as pore water pressure, heave, and deformation at different depth within track foundation, track temperature, strain in the rail, and track surface deformation during freeze-thaw cycles. The data logging system relays static data and high speed data that are triggered by train passages. We show that the selection of instruments and design of the data logging system provide relevant geotechnical data in a manner that could be applied to northern regions and introduce recommendations for future installations. Moreover, we discuss the installation methods appropriate for cold climates because some instruments are temperature-sensitive. Since such systems typically need to be self-sufficient special considerations have to be taken to account for the relatively high power requirements of dynamic monitoring. The suggested system is shown to be useful for track monitoring projects in permafrost-rich regions where freeze-thaw cycles are a concern.