Yedomas in Alaska: Evolution of ice-rich landscapes in a changing climate

Yedomas (Ice complexes) have developed on lands that remained unglaciated during the Late-Pleistocene. Ground exposure to cold climate allowed large syngenetic ice wedges to form typically in fine-grained, organic-rich, and ice-rich enclosing sediments, resulting in particularly ice-rich and thick sequences. Changing climate since has triggered geomorphological changes of these ice-rich landscapes and now contemporary climate conditions generally favour their degradation. Yedoma remnants have been observed in areas of Alaska including in the northern part of Seward Peninsula and Iktilik River area where we studied their metrics, cryostratigraphy, soil properties, and their degradation processes. Understanding the dynamic of this particular periglacial landscape and determining its properties is essential for modeling its future evolution in a changing climate. At our three study sites, presence of typical geomorphological features and cryostratigraphic units revealed information on the landscape evolution since deposition of these ice-rich strata. A Yedoma deposit in the northern part of Seward Peninsula comprised ice wedges at least 36 m-deep. The enclosing sediment was characterized by an ice-rich cryofacies of coarse silt with microlenticular cryostructure and abundant fine rootlets. The intermediate layer, a typical extremely ice-rich layer located below the active layer, was observed above the Yedoma deposit in areas less affected by thermo-degradation. In the thermo-degraded areas characterized by an irregular terrain surface, the intermediate layer was replaced by the generally ice-poor taberal cryofacies which corresponds to a deposit that was formerly ice-rich, thawed, drained, and eventually refrozen. Yedoma remnants in their contemporary degrading state can be recognized with their abundant thermokarst lakes, drained lake basins, and drainage gullies. Thermokarst lakes can be particularly deep because of the considerable amount of ground ice that can melt. Catastrophic lake drainage often occurs along ice-wedge polygons connecting lakes. At our sites, baidjarakhs (thermokarst mounds) and taberal cryofacies was found over the remaining Yedoma deposit in the drained lake basin (alas) and in the drainage gullies. Coalescence of numerous drained lake basins eventually forms alas valleys and ultimately alas plains which represent the most advance stage of Yedoma degradation. Other processes of Yedoma degradation observed included thermo-erosion of massive ice in outcrops along river bank. At the Iktilik River site, the outcrops of large ice-wedges with baidjaraks at their toe were protected from thermo-erosion. However, areas of massive ice 30 m-high exposed to flowing water had niche formed at their toe where ice had melted, such as it is commonly observed along the Beaufort Sea coast, and sets of deep cracks of several meters developed within few days. The ice-wedge block collapsed in the river, melted and disappeared in less than 24h. It can be expected that the exposure protected by baidjarakhs will be exposed to such catastrophic degradation event once the baidjarakhs have been eroded by the river.