The effects of soluble impurities of the flow and fabric of polycrystalline ice

Understanding the rate of flow of ice in Earth's large ice sheets is critically important for paleoclimate reconstruction and for predicting the fate of these ice sheets under climate change conditions. Soluble impurities have been shown to have dramatic effects on the mechanical properties of ice single crystals, including decreasing the strength, increasing the tensile ductility, and increasing the lifetime in secondary (tensile) creep, and they affect recrystallization behavior. In contrast, relatively little is known about the effects of soluble impurities on the flow of polycrystalline ice, of which ice sheets are composed.

The purpose of this project is to undertake a systematic examination of the effects of soluble impurities, particularly sulfuric acid, on the creep of polycrystalline ice as function of temperature, strain rate and impurity concentration. Our working hypothesis is that soluble impurities will increase the flow rate of polycrystalline ice compared to high-purity ice, that this effect will be temperature dependent and that the impurities by affecting the recrystallization and grain growth will change the fabric of the ice. To this end, we are producing both H2SO4-doped and high-purity polycrystalline ice by freezing sheets of ice, breaking them up, sieving the ice particles and then sintering them in a mold into fine-grained cylindrical specimens with at least ten grains across their diameter. The resulting microstructures (dislocation structure, grain size and shape, grain boundary character and microstructural location of the acid) are being characterized using a variety of techniques including: optical microscopy (OM); scanning electron microscopy (SEM) including secondary electron imaging, electron backscattered patterns (EBSPs), energy dispersive X-ray spectroscopy (EDS) and electron channeling contrast imaging (ECCI); and X-ray topography (XT). The ice's response to the creep deformation (grain boundary sliding, dislocation motion, recrystallization, grain boundary migration, impurity redistribution) is being studied using a combination of XT, OM and SEM including EDS, ECCI and EBSPs.

This work is funded by the National Science Foundation, Antarctic Glaciology Program.

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