Landscape Evolution Triggered by Polycyclic Thermokarst on Herschel Island, Yukon Territory

Retrogressive thaw slumps are a common thermokarst landform in areas of ice-rich continuous permafrost characterized by large massive ground ice bodies, often several metres thick and hundreds of metres in extent. These features can retreat inland by as much as 15-20 metres annually (Figure 1) and are thus one of the most important carbon sources along Arctic coastlines. In locations like Herschel Island and the Yukon Coastal Plain, there are numerous active and stabilized thaw slumps (e.g. Lantuit and Pollard, 2008). In many cases, the new slumps form in the floor of a stabilized slump, leading to polycyclic thermokarst behaviour. Previous periods of thermokarst and retrogressive thaw slump activity can be identified morphologically and stratigraphically, as well as through changes in vegetation patterns (e.g. Cray and Pollard, 2015). Former slumps are usually marked by open vegetated depressions with a well-defined (low) head scarp that faces downslope. The headwall of an active slump provides natural permafrost exposures from which considerable cryostratigraphic information can be obtained. In the case of a polycyclic retrogressive thaw slump, the previous cycle of thermokarst is marked by a well-defined thaw unconformity and truncated structures (e.g. ice wedges) overlain by massive debris flow deposits containing blocks of organic material. However, the polycyclic nature of slumps and how one episode may impact another are not fully understood. The objectives of this project are to: 1) Visualize 3D landscape evolution changes related to polycyclic thermokarst for multiple slumps on Herschel Island from 2004 to 2013; and 2) Investigate and compare the polycyclic thermokarst behaviour between the slumps using a combination of cryostratigraphic, ground-penetrating radar, electrical resistivity, and biogeographic datasets of vegetation succession following disturbance (Cray and Pollard, 2015). The landscape evolution models are generated by comparing the annual headwall positions of the slumps (2004-2013) recorded using differential GPS to a LIDAR base image captured in 2013. The ground-penetrating radar and electrical resistivity surveys highlight the geomorphic details of a former slump’s transition from an active to stabilized state, as well as provide insight into the thickness and extent of the debris flow deposits and ground ice units for different episodes of polycyclic thermokarst