Research Progress and Prospect of Transition Zone in Permafrost

As one of the critical climatic elements of the Earth system, the permafrost is approaching its climatic tipping point, highlighting the limitations of its two-layered structure consisting of active layer and perennially frozen layer. Therefore, it is necessary to consider the transition zone situated between the active layer and perennially frozen layer, which has specific properties, as a separate layer. The transition zone is the ice-rich upper part of permafrost, which thaws over sub-decadal to centennial time scales, particularly during extremely warm and wet summers, becoming part of the active layer. It comprises an ice-rich transient layer and an icier intermediate layer. The cryostructures, thermophysical properties, and mechanical structures of the transition zone are distinct from both overlying active layer and underlying permafrost, below which is the "authentic" permafrost. Under the combined influence of global climate warming and anthropogenic activities, the degree and extent of permafrost degradation are related to external forcing factors such as climate, environment, basic properties of watershed, and human activities, as well as internal properties like the thickness and position of the transition zone, cryogenic structures, ground ice, and organic matter content, displaying strong spatiotemporal heterogeneity. Research shows that transition zone is widely distributed in silty-clay and parts of frost-susceptible weathered bedrocks with fine-grained pores. It is the main distribution zone of intrasedimental ice and excess ice, with ground ice mainly existing as segregated ice, vein ice, and massive ice. The cryostructures are primarily lenticular, layered, reticular, and ataxitic, and their changes are closely related to phenomena such as thermal subsidence, thermokarst slumping, solifluctions, and active layer detachment. The rich organic matter and humus contained within are often associated with permafrost aggradation and repeated segregation ice formation processes, serving as reliable proxies for reconstructing the climate and environment of permafrost formation. Due to substantial latent heat effects of phase changes in ground ice, the transition zone can slow down or even resist permafrost degradation. However, once it melts, it triggers a tipping point effect, accelerating permafrost degradation, increasing thermokarst phenomena, and leading to the instability of overlying engineering structures. Therefore, it is urgent to carry out research on climate and environmental reconstruction, eco-hydrological effects, mechanical structure evolution, and precise permafrost modeling that includes the transition zone.