Geomorphic feedbacks enhance the stability of high-centered polygons

Pan-Arctic ice wedge degradation has accelerated abruptly in the past 30 years, driving widespread formation of high-centered polygons (HCPs) across tundra landscapes. This rapid geomorphic transformation alters near-surface hydrologic processes and influences the mobilization of soil organic carbon. However, the pathways by which ice wedges degrade are incompletely understood, causing high levels of uncertainty in projections of future landscape function. Here we use the Advanced Terrestrial Simulator, a physics-based modeling framework for surface and subsurface hydrologic and thermal processes in porous media, to explore the influence of geomorphic feedbacks during ice wedge degradation and re-stabilization. Our model simulates the thermal regime of the active layer in radially-symmetric polygons, using soil physical parameters inferred from field samples and validated using historic meteorological data and observed ground temperature. By varying microtopography and trough inundation among simulations, we isolate the influence of these variables on active layer development in conditions representing various stages of thermokarst. Our results indicate that ice wedge degradation is influenced by a mix of positive and negative feedbacks associated with trough subsidence and inundation, but that negative feedbacks predominate at almost all stages. Impoundment of water in deepening troughs modestly enhances thawing processes, but this effect is counteracted by the concurrent destruction of rims, which diminishes a conductive heat flux toward the ice wedge in summer. In most cases, rates of soil accumulation in the trough during thermokarst are sufficient not only to re-stabilize the ice wedge, but to increase the thickness of a protective layer of frozen soil atop it several fold compared to pre-thermokarst conditions. Overall, these results imply that currently observed development of surface water bodies in degrading ice wedge troughs is intrinsically limited. Because partially-degraded ice wedges become more stable than those unaffected by thermokarst, we forecast that HCPs are durable landscape features, which will exert a long-term influence on soil moisture, runoff generation, and rates of carbon export as CO₂, CH₄, and dissolved organic carbon in a warmer future.