Abstract
The isotopic effects of sublimating ice is poorly understood and
disagreement from diverging results from studies spans decades. The core
question is whether sublimation occurs layer-by-layer with no
fractionation or whether diffusion within the ice and vapor-ice exchange
generate fractionation. Here, small ice spheres were suspended in an
unsaturated atmosphere and a Rayleigh distillation model was used to
estimate fractionation of the spheres. A small, yet statistically
significant and repeatable, fractionation (103lna18O of
~ -0.6‰ (ɑ = 0.999) and 103lna2H -3 to -6‰ (ɑ = 0.994 to
0.997 ) was observed, smaller than predicted for equilibrium
fractionation at this temperature and humidity. Assuming a modest
porosity of 0.0005%, porosity could sufficiently increase diffusivity
to explain the observed fractionation. The results help reconcile how
sublimation varies between experimental and observational studies where
uncontrolled porosity varies substantially across a continuum from
porous firn layers to low porosity ice deep in glaciers.