Sublimation Origin of Negative Deuterium Excess Observed in Snow and Ice Samples From McMurdo Dry Valleys and Allan Hills Blue Ice Areas, East Antarctica

Abstract

The oxygen and hydrogen isotopic composition in snow and ice have long been utilized to reconstruct past temperatures of polar regions, under the assumption that post‐depositional processes such as sublimation do not fractionate snow. In low‐accumulation (<0.01 m yr−1) areas near the McMurdo Dry Valleys in Antarctica, surface snow and ice samples have exceptionally low deuterium excess values (d‐excess ≡ δD – 8*δ18O)—sometimes as negative as −5‰—an uncommon phenomenon that is not fully understood. Here we use both an isotope‐enabled general circulation model and an ice physics model and establish that such exceptionally low d‐excess values can only arise from precipitation if the majority of the moisture is sourced from the Southern Ocean (south of 55°S). However, the model results show that moisture sourced from oceans north of 55°S contributes significantly (>50%) to precipitation in Antarctica today. We thus propose that sublimation must have occurred to yield the low d‐excess values in snow observed in and near the Dry Valleys, and that solid‐phase‐diffusion in ice grains is sufficiently fast to allow Rayleigh‐like isotopic fractionation in similar environments. We calculate that under present‐day conditions at the Allan Hills outside the Dry Valleys, 3%–24% of the surface snow is lost due to sublimation. Because the magnitude of sublimation may be nonstationary (i.e., it could vary in time) during past cold periods, we suggest that sublimation‐induced fractionation can alter the relationship between the snow isotopic composition and polar temperatures.

Atmosphere‐Snow Exchange Explains Surface Snow Isotope Variability

“…One way to improve model fidelity might be the introduction of a temperature‐dependency which impacts the magnitude of fractionation. It has been suggested that temperature controls whether the diffusion within the solid snow matrix is effective enough to allow homogenization of the remaining snow crystal and thus controls the imprint of the fractionation signal (Hu et al., 2022 ). In other words, temperature can control snow metamorphism and thus impacts the fractionation strength.…”

Contrasting Seasonal Isotopic Signatures of Near‐Surface Atmospheric Water Vapor in the Central Arctic During the MOSAiC Campaign

“…However, such very low negative d-excess values have not only been measured in vapor by the Picarro CRDS, but also in snow samples taken in the vicinity of POL during the MOSAiC expedition (Mellat et al, 2022). Furthermore, negative d-excess values in Arctic water vapor have been reported before: Bastrikov et al (2014) (Hu et al, 2022;Hughes et al, 2021) reported negative d-excess values in snow as a result of non-equilibrium exchange processes with the atmosphere. Yet, quantitative transfer functions linking snow and vapor are still missing and there is no consensus on the physical mechanisms that could result in such negative d-excess values in the atmospheric water vapor.…”

Post-depositional modication on seasonal and 1 interannual timescales resets the deuterium excess 2 signals in summer snow layers in Greenland

“…Isotope-enabled models (IEMs) of regional-to-global extent are now employed to probe complex relationships between water isotopes, including evaporative processes at the source, mixing and cloud physics processes along the way, and nal precipitation physics (e.g., Blossey et al, 2010;Dee et al, 2015;Dütsch et al, 2019;Hu et al, 2022;Werner et al, 2011). Some focus is still on water-isotope-temperature relationships like γ t (e.g., Werner et al, 2018).…”

“…IEMs of a range of complexity have opened up nuanced, integrated interpretation of isotope concentration derivatives like d, advancing modeled hydrologic processes and interpretation of ice cores (e.g. Merlivat & Jouzel, 1979;Jouzel & Merlivat, 1984;Blossey et al, 2010;Dütsch et al, 2019;Hu et al, 2022).…”