Seasonal patterns of coupled flow in the active layer at three sites in northwest North America
Vertical arrays of temperature and electric-potential probes were installed in the upper soil at sites along the Mackenzie River valley and on the North Slope of Alaska. Time series were obtained at subdiurnal frequencies throughout the year in the active layer and upper permafrost. If the data acquisition system is properly configured, the time series can be used to infer soil physiochemical processes. The electric potential develops primarily in response to soil water solute concentration gradients in the soil column, and is a crude surrogate of the soil water electrolytic conductivity. Summer precipitation can cause rapid penetration of the thaw front when percolating rainwater, warmed at the ground surface, carries sensible heat downward to the thawing front. Rates of warming at depth occur significantly faster than those typical of conductive heat transfer. In early winter, as the freezing front penetrates downward toward the permafrost table, ions are excluded from the ice and concentrated in the intermediate unfrozen zone. Nearly instantaneous warming of the active layer is triggered by spring snowmelt. At Happy Valley in northern Alaska, temperatures at the 29 cm depth rise from −7 to −3 °C in 1 h. For several hours during this event, the temperature at 29 cm is warmer than that at regions both above and below, producing a strong thermal inversion. Time series of electric potential, or a surrogate derived from electric potential, suggest rapid transport of meltwater from the snowpack to depth, probably through soil cracks. Serial events hasten active-layer warming by 1–2 weeks.