Evolution of incipient lowland thermokarst features in the Blackstone River Valley, Yukon

Widespread thermokarst initiation and the acceleration of thermokarst development in permafrost peatlands can result in ecological change, threaten the cultural integrity of northern indigenous people, change rates of carbon storage and release at high latitudes, and affect the geomorphology of Arctic and subarctic landscapes. Despite the multilayered significance of intensified thermokarst activity the processes controlling the early stages of thermokarst development and the factors differentiating features that will stabilize from those that will continue to expand are still poorly understood. Here we present preliminary observations of conditions associated with the stabilization or expansion of incipient thermokarst ponds in the Blackstone River valley, central Yukon. In this large valley located near the limit between continuous and discontinuous permafrost, the alluvial deposits of the river bed are surrounded with extensive moraines and outwash deposits. The vegetation cover is dominated by tundra, and ice wedge polygon networks are extensive. Signs of ice wedge degradation are widespread in several parts of the valley. However, comparison of recent remotely sensed imagery with historical aerial photographs indicates that several degradation features have been stable for at least 20 years, while others have expanded rapidly. This stabilization is associated with vegetation growth in the thaw features, causing partial terrestrialization of incipient ponds and cooling of the ground beneath. Mean annual temperatures near the ground surface beneath small ponds with dense Carex spp. or aquatic mosses ranged from - 1.5 to -2.7°C, whereas such temperatures ranged from 0.3 to 2.9°C beneath similarly shallow but unvegetated or sparsely vegetated water in expanding ponds. Expanding ponds exhibiting signs of aggressive shore erosion were often connected to thermo-erosion gullies, which visibly contributed to destabilising the shore bank. Such association with thermo-erosion gullies also enabled rapid degradation along some banks of seasonal ponds, preventing the stabilization that could result from prolonged loss of contact between the bank foot and water every summer.