The Transition Zone of Upper Permafrost: The Frontline for Permafrost Changes across Climate and Landscape Gradients

Our project assessed the response of permafrost to climate change and disturbance across diverse landscapes by focusing on the dynamic properties and processes of the transition zone of upper permafrost. The transition zone consists of two layers: the transient layer (TL), which reflects interannual changes in thickness of the seasonally thawed active layer (AL) above permafrost, and the ice-rich intermediate layer (IL), which responds to long-term ecological and climate change. The main goal of the project was to evaluate the patterns and processes of formation, degradation, and recovery of the transition zone of the upper permafrost and its effects on permafrost vulnerability and resilience. To accomplish this goal, we studied the structure and properties of the IL during 2019-2023 at numerous sites across different climatic conditions and geomorphic settings in Alaska and Canada and summarized previous knowledge from other permafrost regions. Results of this study have led to a better understanding of the role of the IL in ecological transformations, its effects on trace-gas emissions and carbon balance, and its fundamental role in evolution of arctic and boreal landscapes.

The IL is sensitive to climatic and environmental changes that increase the thickness of the AL because it can be extremely ice-rich: its average excess-ice content (volume of ground ice that exceeds the pore volume of the unfrozen soil) exceeds 40% and often reaches 80% (Figure 1). Degradation of the IL is a major factor leading to thermal subsidence (Figure 2) that triggers development of thermokarst lakes, bogs, and fens, thermal erosion associated with gully formation and coastal erosion, and slope failure. For example, one of the widely occurring types of slope failure called the active-layer detachment slides (ALDSs) is the direct result of ice melting in degrading IL. Degradation of the IL is also a major factor in the damage to infrastructure from climate warming.

Wildfires and disturbance associated with development can have a devastating impact on the IL. In 2007, the huge Anaktuvuk River fire in northern Alaska burned 1,040 square kilometers of tundra with ice-rich permafrost, which triggered immediate degradation of the IL and underlying ice wedges and initiated extensive ALDs. Our studies showed that with the recovery of vegetation, the IL recovered on flat terrain (Figure 3), but permafrost continued to degrade on slopes.

Since the early 2000s, we have been documenting the abrupt increase in the degradation of ice wedges at numerous sites in northern Alaska. We have found that initially active thermokarst slows down with time, and the IL eventually reestablishes. Thus, the IL is the fundamental indicator of resilience and recovery of permafrost.

Structures such as roads and airfields are also sensitive to the degradation of the IL. It is extremely important to properly assess the thickness and ice content of the IL during geotechnical investigations. Our outreach has helped educate students and train personnel in government and industry dealing with various permafrost-related problems.

Our key achievements and major findings include:

• We have developed a major database "Alaska Permafrost Soils Inventory and Thermokarst Monitoring Database", which contains information on permafrost soils for 1883 sites compiled from 26 projects since 1977 (Figure 4). Summary data for each site can be accessed through the North Slope Science Initiative website data catalog (https://northslopescience.org/catalog/) via a link to the Permafrost Mapping and Soils Database under the "Web Maps" section.We have used data included in this database for more than a dozen publications related to this project.
• The IL is present in nearly all terrain types across a large climate gradient from the high Arctic to the mid-boreal.
• Permafrost vulnerability to thermokarst, thermal erosion, and detachment slides on slopes strongly depends on occurrence of the ice-rich intermediate layer.
• Degradation of ice wedges from climate warming and disturbance is widespread in the Arctic, but is limited by feedbacks from soil slumping and aquatic moss growth in flooded troughs that lead to aggradation of ground ice in a new ice-rich IL above ice wedges and stabilization of the system.
• Even under severe disturbance IL recovers and protects ice wedges. At the Anaktuvuk River Fire area, an ice-rich IL partially reformed above ice wedges that degraded after the 2007 tundra fire (Figure 3).
• Ground ice characterization of drained-lake basins of different age showed that the thickness of the ice-rich IL significantly increases in response to peat accumulation and thinning of the active-layer thickness after lake drainage.
• Knowledge gained from our studies and outreach to land and infrastructure managers have led to improved road design, an experimental shift in fire management strategies to protect the IL and underlying ice-rich permafrost, improved ecological monitoring approaches by federal agencies to incorporate permafrost dynamics, and a better understanding by climate system modelers of the importance of permafrost in global climate feedbacks.