The impact of impurities and stress state on polycrystalline ice deformation

Despite the ubiquitous use of the constitutive relation for ice commonly referred to as “Glen’s Flow Law,” significant uncertainty exists particularly with regard to the role of impurities and the development of oriented fabrics. The aim of this project is to improve the constitutive relationship for ice by performing deformation tests and microstructural characterization of pure and sulfuric acid-doped ice. The research proposed here will focus on sulfuric acid’s impact on ice viscosity, fabric evolution, and diffusivity. Sulfuric acid can have both direct—by changing the dislocation velocity and/or density—and indirect—by changing the grain size and fabric—effects on the mechanical properties of polycrystalline ice. The complexity and interaction of these effects means that one cannot understand the effects of sulfuric acid by simply examining ice core specimens.

In this project, we will deform four types of ice:

  1. lab-grown ice samples doped with similar-to-natural concentrations of sulfuric acid,
  2. lab-grown high-purity ice,
  3. layered doped and pure ice, and
  4. natural ice from Antarctic ice cores.

Deformation will be performed in both uniaxial compression and simple shear. The addition of simple shear tests is critical for relating the laboratory-observed deformation behavior to the behavior of polar ice sheets where the shear strain dominates ice motion in basal ice. After deformation to strains from 5% up to 25%, the microstructural development will be assessed with state-of-the-art methods including a variety of scanning electron microscope techniques, Raman microscopy, synchrotron-based Nano-X-ray fluorescence and ion chromatography.

These analysis techniques will allow the determination of:

  1. the segregation and movement of impurities;
  2. the rate of grain boundary migration;
  3. the number of recrystallized grains; and
  4. the full orientation of the ice crystals.

The results will enable both microstructural modeling of the effects of H2SO4 and numerical modeling of diffusion in ice cores. The net result will be a better understanding of ice deformation that improves ice-core interpretation and ice-sheet modeling. The work will lead to the education of a PhD student at Dartmouth, introduce undergraduate students to research at both the University of Washington and Dartmouth, and bring together the microstructural expertise of Dartmouth with the ice-core measurement and modeling emphasis at the University of Washington.

Funded by the US National Science Foundation

Faculty contact: Ian Baker