Thermokarst lake expansion and carbon mobilization in polygonal tundra of Old Crow Flats, northern Yukon

Roy-Levillee P, Burn CR. (2018) Thermokarst lake expansion and carbon mobilization in polygonal tundra of Old Crow Flats, northern Yukon. In ArcticNet Scientific Meeting 2018, Ottawa, ON, December 10th to 14th 2018. doi: 10.6084/m9.figshare.7476098

The expansion and drainage of thermokarst lakes respond to climatic trends and are an important part of the permafrost carbon feedback. The effects of climate on total lake area in thermokarst lowlands vary from region to region and may be monitored via remote-sensing, but it is difficult to interpret changes in lake area in terms of volumes of permafrost thawed and organic carbon released from permafrost. This is because the rates of subaerial and sublacustrine permafrost degradation associated with lake expansion, as well as the distribution of organics in the sediment profile, vary across Arctic lowlands due differences in environmental conditions.

This research links changes in lake area with tridimensional estimates of permafrost degradation, and assesses associated changes in permafrost carbon storage at the landscape scale. The study area was a zone of polygonal tundra within Old Crow Flats (OCF), YT, a 5600 km² Arctic peatland located in an inland basin separated from the Arctic Coast by mountains. The research objectives are: 1) to use remotely sensed imagery to assess rates of lake expansion and characterize the relation between lake size and shore erosion rates in the study area; 2) to use modelling in combination with ground temperature measurements and observations of talik geometry to estimate volumetric rates of permafrost degradation beneath expanding lakes; 3) to use field measurements of shore bank height and samples of permafrost to estimate organic carbon content where permafrost degradation is imminent.

Results indicate that, between 1951 and 2011, lake expansion encroached on the surrounding tundra at an average rate of 0.27 km² a^-1. The total lake expansion during this period is approximately equal to the total lake area lost to catastrophic drainages in the 1060 km² study area. Permafrost thaw occurred beneath the areas that became part of the lakes, and this loss of permafrost was compensated by the aggradation of permafrost in drained basins. However, lake expansion occurred via the erosion of organic-rich permafrost banks varying in height between 0.5 and 4 m, representing an additional loss of permafrost. Due to bank erosion alone, approximately 430 000 m³ a^-1 of sediment fell in the lakes of the study area annually. This represents an input of organic carbon into the lakes of 0.22 Tg C a^-1, of which 0.15 Tg C a^-1 was stored in permafrost prior to being thawed during bank erosion. Comparatively, a doubling of active layer depth over the entire study area in the next 20 years would lead to the thawing of 0.01 Tg C a^-1 of organic carbon previously stored in permafrost, less than one tenth of what would be released via bank erosion if current erosion rates are sustained. While current climate models with carbon budgets focus on active layer deepening as the main mechanism of permafrost degradation associated with carbon mobilisation, these research results highlight the importance of considering thermokarst lake expansion as a tridimensional process when quantifying the permafrost carbon feedback in Arctic lowlands.