Remote monitoring of freeze-thaw transitions in arctic soils using the complex resistivity method

Our ability to monitor freeze-thaw transitions is critical to developing a predictive understanding of biogeochemical transitions and carbon dynamics in high latitude environments. In this study, we conducted laboratory column experiments to explore the potential of the complex resistivity method for monitoring the freeze-thaw transiti ons of the arctic permafrost soils. Samples for the experiment were collected from the upper active layer of Gelisol soils at the Barrow Environmental Observatory (BEO) in Barrow, Alaska. Freeze-thaw transitions were induced through exposing the soil column to controlled temperature environments at 4 and-20°C. Complex resisti vity and temperature measurements were collected regularly during the freeze-thaw transiti ons using electrodes and temperature sensors installed along the column. During the experiments, over two orders of magnitude of resistivity variations were observed when the temperature was increased or decreased between-20 and 0°C. Smaller resistivity variations were also observed during the isothermal thawing or freezing processes that occurred near 0°C. Single frequency electrical phase response and imaginary conductivity at 1 Hz were found to be exclusively related to the unfrozen water in the soil matrix, suggesting that these geophysical attributes can be used as a proxy for the monitoring of the onset and progression of the freeze-thaw transitions. Spectral electrical responses and fitted Cole-Cole parameters contained additional information about the freeze-thaw transition affected by the soil grain size distribution. Specifically, a shift of the observed spectral response to lower frequency was observed during the isothermal thawing process, which we interpret to be due to sequential thawing, first from fine particles and then to coarse particles within the soil matrix. Our study demonstrates the potential of the complex resistivity method for remote monitoring of freeze-thaw transitions in arctic soils. Although conducted at the laboratory scale, this study provides the foundation for exploring the potential of the complex resistivity signals for monitoring spa-tiotemporal variations of freeze-thaw transitions over field-relevant scales.