Water isotope diffusion in the WAIS Divide ice core during the Holocene and last glacial

Abstract

We use a high-resolution water isotope record from the West Antarctic Ice Sheet Divide ice core (WDC) to evaluate the effects of water isotope diffusion for the last 29 ka B.P. Using spectral analysis of the data, we determine diffusion lengths in depth and time domains. The diffusion length quantifies the mean cumulative diffusive displacement of water molecules relative to their original location at time of deposition. We simulate the observed signal with models and find that our understanding of processes and conditions in the ice sheet is incomplete. With the effects of ice-deformational thinning removed, portions of the Holocene record show total diffusion lengths smaller than predicted for a lower limit case of diffusion through a single ice crystal. Such reduced diffusion is probably due to structural features such as crusts and tortuous porosity that inhibit vapor transport in the firn. In the late glacial portion of the record, diffusion lengths double between ~19.5 and 17 ka B.P. Known dependencies of diffusion on climatic variables do not account for this enhancement in models, and we hypothesize that it could arise from thermal gradients in the firn column, impurity-driven enhancement of solid ice diffusion, or changes in firn grain properties that alter vapor access to open pores. Despite model uncertainties, the WDC diffusion length chronology will be an essential input to future studies of high-frequency variability in the water isotope climate record, as it allows for the effects of diffusion to be removed.