Slowing down permafrost degradation: Quantifying the thermal buffer effect of the transient layer from novel large-scale laboratory experiments

Permafrost is receding and warming globally in polar and high-altitude regions due to ongoing climate change. To better represent the role of ground ice on the long-term stability of permafrost soils, previous studies have introduced a transition zone between the seasonally freezing and thawing active layer and the permafrost layer. The transient layer, an ice-rich layer within the transition zone, is proposed to have the potential to temporarily slow down the climate change-related permafrost thaw. The ice-water phase transition absorbs heat and temporarily buffers the downward heat propagation. However, so far this layer has not received much scientific attention and its formation and degradation processes and their associated timescales remain largely unconstrained.

To determine the physical degradation processes of the transient layer as well as the involved time frame, we present a novel experimental approach to replicate the transient layer degradation under controlled laboratory conditions. The experimental setup at the GEOPS cold chamber facility consists of an acrylic glass container (approximately 80 × 40 × 40 cm, H × W × L). It is filled with fully saturated sand (d50 = 0.2 mm) or polycarbonate analog (d50 = 0.6 mm) material. The lateral boundaries of the container are insulated to minimize horizontal heat exchange, while the base of the container is kept at a constant temperature with a cryostat to represent the underlying permafrost. By cyclically varying the air temperature in the cold chamber between -30 °C and +30 °C we forced repetitive freeze-thaw cycles on the surface of the volume to simulate a permafrost system. To simulate the transient layer we have added different artificial cryostructures (ice lenses, ice veins, and dispersed ice) at the active-layer and permafrost layer interface. We varied the cryostructure type between experimental runs but kept the total ice-mass to keep the latent heat capacity constant. Then, to degrade the transient layer we increased the temperature forcing by shifting the temperature cycle upward by 3 °C. We monitored the transient layer degradation with an array of temperature sensors, a ground-penetrating radar, and photographic observations through the transparent side walls of the experimental container.

In this initial work, we show how different ice contents, spatial distributions, and cryostructure types within the transient layer protect the underlying permafrost beyond the latent-heat buffering of the ice-water phase transition alone. We highlight the importance of expanding the future use of these analog experiments to better understand and isolate the physical transient layer formation and degradation processes. This is essential in determining the transient layer evolution and its long-term implications for permafrost retreat and destabilization.