Evolution of a Thermokarst Landscape: A History of Late-Quaternary Permafrost Degradation and Stabilization in Interior Alaska Illustrates the Importance of Multiple Environmental Controls

Abstract:

The course of permafrost degradation depends on climate, vegetation, disturbance, and excess ground-ice content and distribution, which vary over time. Our understanding of possible future landscape trajectories under climate warming can be improved by observing past responses to changes in these critical drivers. Using a novel combination of lake-sediment records, field observations and LiDAR imagery, we reconstructed the late-Quaternary history of the marginal upland of the Yukon Flats, interior Alaska, a loess-mantled region with massive ground ice and numerous thermokarst lakes. Two lakes formed ~11,000-12,000 cal yr BP through inferred thermokarst processes. Charcoal in basal sediments indicates fire may have influenced lake initiation. In a third, older basin, major input of terrigenous silt occurred before or during this time. At all studied lakes, sedimentation has been stable through much of the Holocene. Meso-scale topographic features that are obscured by forest are revealed by LiDAR images to include widespread linear corrugations cutting across the uplands, deep gullies, and other features resembling lake drainage channels. These imply past dramatic surface-sediment mobilization. Lakes intersect the corrugations, indicating that the mobile phase occurred no later than the earliest Holocene. Several interacting factors may have been critical in first enhancing, then slowing, this activity. High summer insolation, increasing moisture, initially sparse vegetation, then development of woody cover and increase in fire disturbance likely combined to generate rapid and deep thaw of glacial-age permafrost. Initial lake lowering and generation of steep local topography favoring drying of uplands, a summer water deficit, and early-Holocene establishment of evergreen forest cover and consequent surface insulation likely combined to shift the system to a near-quiescent state (the system is not entirely inactive today: a partial lake drainage event was observed in 2103). The history and current state of this thermokarst-affected landscape differ from other regions. It has undergone non-linear shifts in process rates and developmental trajectory. Overall resilience to current and future environmental change here and in other regions may be equally individualistic.