Isotopic Fractionation during Sublimation of Low Porosity Ice

The magnitude of isotopic fractionation during sublimation of ice remains poorly constrained. Field and laboratory studies that span decades have persistently shown conflicting results. A better understanding of fractionation during sublimation is needed to improve interpretation of alpine hydrology, glaciology, paleoclimate and planetary histories that rely on stable isotopic records stored in icy reservoirs. At the core of the problem is the question of whether sublimation occurs as a layer-by-layer process with no fractionation or whether diffusion within the ice and vapor-ice exchange generate fractionation. We present results from an experiment where we suspended ice spheres in temperatures ranging from -25 to -10 °C in an unsaturated atmosphere and used aRayleigh distillation model to estimate fractionation of the spheres. A small, yet statistically significant and repeatable, isotope fractionation (10 3 lnα 18 O of ∼ −0.6 ‰ and 10 3 lnα 2 H between −3.0 and − 4.0 ‰ or α = 0.999, α = 0.9940 to 0.9970, respectively) was found during the sublimation of ice. The results indicate the presence of fractionation yet the values are an order of magnitude smaller than would be predicted for equilibrium fractionation at this temperature. By assuming a porosity for the ice of 0.0005 % - which is typical for ice frozen under these conditions, we estimate an increase in diffusivity relative to solid ice consistent with the observed fractionation values. The results help to reconcile how fractionation estimates during sublimation could vary between experimental and observational studies where porosity is not controlled for and can vary substantially across a continuum from porous firn layers to low porosity ice deep in glaciers. • Closed porosity within ice can increase the diffusivity generating a small but observable isotope fractionation. • Secondary processes such as ice compaction can affect porosity, thus the magnitude of isotope fractionation may vary depending on ice type. • Isotope fractionation during sublimation is much lower than equilibrium fractionation predictions, yet statistically significant.