Ground ice degradation and thermokarst terrain formation in Canada over the past 16 000 years

Abstract

Thermokarst results from thawing of excess ground ice in permafrost sediments. Thermokarst processes and landforms are controlled by ground ice type, amount and distribution, as well as the patterns of ground ice loss over time. Recent acceleration of varied thermokarst processes across diverse Canadian permafrost terrains make for a challenging task in predicting landscape-scale thaw trajectories. Using existing ground ice models, we examined the modelled amounts and spatial extent of ground ice loss relative to ground ice maxima in the last ca. 16 ka BP for relict, segregated and wedge ice. We relate observed thermokarst features to the nature of ground ice development and loss in different environments (cold continuous permafrost, discontinuous permafrost, and no current permafrost). In cold, continuous permafrost areas where ground ice loss has been limited over the last 16 ka BP, thermokarst processes include active layer detachments and slumps in segregated and relict ice, gullying and ponding in ice wedge troughs, and the cyclical development of shallow thermokarst ponds in segregated ice. With ground ice loss in discontinuous permafrost, thermokarst processes are wide-ranging. Slumps, subsidence, and collapse of lithalsas, palsas and peat plateaus occur from thawing of segregated ice, thermokarst ponds from melting wedge and segregated ice, and involuted terrain from melting and creep of relict or segregated ice. In former permafrost terrain, evidence of thermokarst includes former ice wedge polygons, collapsed lithalsas, and irregular hummocky terrain. The relations between modelled ground ice loss and observed thermokarst landscapes assist in understanding present-day processes and in predicting future thermokarst landform evolution with a changing climate.