STABLE ISOTOPE RECORD DEVELOPMENT IN GROUND ICE ALONG ALPINE TUNDRA SLOPES IN THE OGILVIE MOUNTAINS, YUKON TERRITORY

Stable isotope (δ¹⁸O, δ²H) signatures of permafrost ground ice provide a proxy for past hydroclimate. This relationship arises from the strong temperature dependence of δ¹⁸O and δ²H in precipitation. Precipitation that infiltrates the active layer – the horizon which undergoes annual thaw in permafrost-affected soils – becomes incorporated into the upper permafrost ice as the surface aggrades. However, mechanisms which potentially modify the original δ18O and δ2H values of meteoric waters during their residence within the active layer, and during the freezing process, are not well understood. This study investigates how δ18O and δ2H are modified in the active layer prior to long-term incorporation as ground ice by addressing three questions: 1) how do δ18O and δ2H in active layer waters vary across topography and hydrological regime, 2) what mechanisms give rise to these variations, and 3) what is the relationship between δ18O and δ2H values of active layer waters and the upper permafrost aggradational ice? To test these questions, ground ice samples were cored in June 2019, and active layer waters were collected in September 2019 along two catenas within the Ogilvie Mountains, Yukon Territory. The isotopic values of active layer waters and upper permafrost ice co-varied with respect to topographic position, likely arising from compounding influences of evapotranspiration and thawed ground ice waters during downslope migration. Furthermore, although upper permafrost ice (sourced from lower active layer waters) was consistently more enriched than September active layer waters, this enrichment was significantly less than predicted by ideal Rayleigh-style cryofractionation, which is consistent with findings from previous studies. Ultimately, this research improves our understanding of stable isotopic development in pore ice, enhancing their potential as quantitative paleotemperature proxies.