Isotopic Stratification and Non-Equilibrium Processes in a Sub-Arctic Snowpack

Abstract. Water vapor transport is a primary driver of snowpack metamorphism – a process happening on the microscale which affects important macroscale properties of the snowpack, important for the mass and energy balance of the snowpack. However, vapor transport and vapor-ice interactions are difficult to observe directly, which necessitates alternative, indirect methods of analysis. Stable water isotopes are an excellent tool to study the vapor-ice continuum in a snowpack because they act as natural tracers of thermodynamics in a snowpack, yet the depth-resolved isotopic signature of pore-space vapor has never been measured in a field setting, leaving a key process gap in snow physics. Here we present the first continuous, multi-level winter record of water-vapor isotopes (δ¹⁸O, δ²H and d-excess) in a snowpack pore space. These data were collected within a coastal, mid-latitude sub-Arctic snowpack and combined with parallel measurements of overlying air. Coupled with high-resolution meteorological observations and repeated snow-core profiles we explored the temporal and spatial variability of water vapor within the snowpack-atmosphere continuum. Our measurements showed that the pore-space vapor was rarely in isotopic equilibrium with the surrounding ice and that the magnitude and sign of the disequilibrium varied systematically with depth and season. The disequilibrium shifted from diffusion-dominated exchange under cold, strongly stratified conditions to wind-pumping ventilation during warmer, drier late winter. Late-winter warming, unsaturated atmosphere and stronger winds produced large diurnal swings and midday peaks in water vapor concentration, δ¹⁸O, δ²H and especially d-excess in the snowpack pore space and ambient air, which were indicative of enhanced sublimation and rapid advective mixing. These observations demonstrated that non-equilibrium fractionation by diffusion and wind ventilation reshaped snow and vapor isotopes on hourly to seasonal timescales. This research provides essential insights into how variable environmental conditions rapidly influenced isotopic variability in the snow-atmosphere continuum, implying that isotope-enabled snow models must capture these rapidly changing processes to avoid biasing hydrological and paleoclimate reconstructions.