ASSESSING THERMOKARST LAKE HISTORY IN THE OLD CROW BASIN, NORTHERN YUKON, USING CRYOSTRATIGRAPHY AND STABLE ISOTOPE RECORDS

Thermokarst lakes form following the thaw of ice-rich permafrost and typically drain after 100s to 1000s of years followed by the accumulation of peat, which may lead to the re-aggradation of ice-rich permafrost and a subsequent generation of thaw lakes should disturbance occur. In this study, we investigate the thaw lake cycle through drilling and recovery of permafrost cores from drained thermokarst lake basins in the Old Crow Flats, northern Yukon. Thermokarst lakes and drained thermokarst lake basins are ubiquitous (>3000) in the Old Crow area, but little is known about the environmental dynamics of these basins over Holocene timescales, and whether these lakes follow a “thaw lake cycle”. Here, we assess the history of several thermokarst lakes through sampling and analysis of permafrost cores recovered from six drained thaw lakes in the Old Crow Flats. We establish the hydrology and drainage history of these lakes on the basis of pore-ice stable isotope analyses and radiocarbon dates of associated organic materials. The results indicate that only 1 out of the 6 lakes shows evidence of multiple thermokarst cycles. Radiocarbon ages from woody macrofossils show that post-drainage peat accumulation rates are initially rapid, but slow down thereafter. Pore-ice isotopic analyses suggest differing hydrological conditions following lake drainage and peatland development. Specifically, four of our lakes show depletion in δ¹⁸O over time, reflecting increased contributions from winter precipitation, while one lake shows δ¹⁸O enrichment over time, indicating predominantly evaporative conditions. The sixth lake shows a relatively constant isotopic history due to largely unchanged near-surface hydrology. These findings suggest variable post-drainage isotopic histories between thermokarst lake basins as a result of spatially varying ecological succession and hydrologic conditions.