Pre- and post-depositional processes affecting isotopic composition of seasonal alpine snowpacks – Austrian Alps

Abstract. Studying the formation and evolution of the seasonal Alpine snow pack is essential to obtain a clear picture of hydrological catchment processes during the snow season and the following melt period. In this study, three seasonal snow packs in one cirque in the Austrian alps were monitored on a weekly interval, the snow packs were allocated in line with each other at three different elevations (1500, 1600 and 1700 m a.s.l.). We analysed the establishment of isotopic signals using atmospheric observations and a back-trajectory model. In the post-depositional analysis, we tracked individual layers over time using Dynamic Time Warping to quantify the isotopic changes and relate these changes to the layer specific temperatures, temperature gradients and isotopic gradient individually, and also a multi-linear regression approach with the combination of temperature gradient and isotopic gradients. We also tried to relate the effects of global and net radiation on the upper snow layer isotopic metamorphism. We identified that meteoric first-order isotope signals correlate strongly with cloud conditions during snow accumulation events. The second-order stable water isotope interpreter, Deuterium excess (dxs), correlates strongest to moisture origin conditions. In the post-deposition analysis we concluded that during the stable period in the snow season trends in isotopic change are characterized by erratic week-to-week variations which always recover to the decreasing layer average. The magnitude of isotopic change increased with normalized profile depth, where the biggest fluctuations occurred at the snow-atmosphere boundary. In our analysis, week-to-week changes of the first isotopes cannot be explained by the snow temperature, nor the temperature gradient. The temperature gradient significantly affects layer dxs changes, driven by the preferential mobilization of δ¹⁸O rather than δ²H, contrary to classical sublimation-based expectations. The strong correlation between the isotopic gradient and isotopic layer metamorphism highlights diffusive homogenization as a dominant process, while temperature gradient induced vapour transport drives isotopologue differentiation. Upper layer snow isotopy is not affected by air temperature, but is affected by global radiation and net radiation. This study consolidates the idea that meteoric isotopy is shaped locally and non-locally for first- and second order isotopes, respectively. Furthermore, we highlight the coinciding processes of temperature gradient induced heterogenization of the snowpack and isotopic gradients induced homogenization.