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.

Key Points

• Exceptionally low deuterium excess values exist in surface snow in the McMurdo Dry Valleys and Allan Hills Blue Ice Area, East Antarctica

• To yield such low deuterium excess in Antarctic precipitation, unrealistic moisture contributions from high-latitude oceans are required

• Sublimation fractionation lowers deuterium excess in relatively dry, windy, and warm conditions

Plain Language Summary

Earth's past temperatures in the polar regions are often calculated from the relative abundances of heavy hydrogen or oxygen atoms (isotopes) in the polar ice. Implicit to this approach is the assumption that once the snow has fallen from the sky, its isotopic composition no longer changes. Yet, this conventional notion is incompatible with some observations. In and near the McMurdo Dry Valleys in Antarctica, for example, the surface snow and ice show an unusually large depletion in heavy hydrogen isotopes relative to heavy oxygen isotopes. Deficits of such a magnitude are rare in precipitation and therefore hint at sublimation causing the ice to change its isotopic composition (to “fractionate”) after deposition; if true, this process would challenge the conventional wisdom about snow and ice as paleoclimate indicators. Here, we investigate whether these unusual heavy-isotope deficits could originate from Antarctic precipitation. We determine that the moisture arriving at Antarctica today does not have these deficits. Sublimation of the snow that falls, however, can quantitatively explain the observed range of heavy-hydrogen depletions. We conclude that sublimation does fractionate isotopes in and near the Dry Valleys, and may affect the past temperatures reconstructed from the hydrogen and/or oxygen isotopes.