Microstructural analysis of Greenland ice using a cryogenic scanning electron microscope equipped with an electron backscatter diffraction detector

Abstract

Mass loss from ice sheets contributes to global sea level rise, and accelerated ice flow to the oceans is one of the major causes of rapid ice sheet mass loss. To understand flow dynamics of polar ice sheets, we need to understand deformation mechanisms of the polycrystalline ice in ice sheets. Laboratory experiments have shown that deformation of polycrystalline ice occurs largely by dislocation glide, which mainly depends on crystal orientation distribution. Grain size and impurities are also important factors that determine ice deformation mechanisms. Compared with ice formed during interglacial periods, ice formed during glacial periods, especially ice that forms cloudy bands, exhibits finer grain sizes and higher impurity concentrations. A previous report suggests the deformation rate of ice containing cloudy bands is higher than that of ice without cloudy bands. To examine the microstructures and deformation histories of ice in cloudy bands, we applied the electron backscatter diffraction (EBSD) technique to samples from the Greenland Ice Sheet using an environmental scanning electron microscope (ESEM) equipped with cold stages. Prior to the EBSD analysis, we optimised our ESEM/EBSD system and performed angular error assessment using artificial ice. In terms of c- and a-axis orientation distributions and grain orientation spread, we found little difference between samples taken from a cloudy band and those taken from an adjacent layer of clear ice. However, subgrain boundary density and orientation gradients were higher in the cloudy band, suggesting that there are more dislocations in the cloudy band than in the clear ice layer.