Location and distribution of micro-inclusions in the EDML and NEEM ice cores using optical microscopy and in-situ Raman spectroscopy

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Eichler, Jan ; Kleitz, Ina ; Bayer, Maddalena ; Jansen, Daniela ; Kipfstuhl, Sepp ; Shigeyama, Wataru ; Weikusat, Christian ; Weikusat, Ilka (2016)

Impurities control a variety of physical properties of polar ice. Their impact can be observed at all scales &ndash; from the microstructure (e.g., grain size and orientation) to the ice sheet flow behavior (e.g., borehole tilting and closure). Most impurities are likely to form micrometer-sized second phase inclusions. It has been suggested that these particles control the grain size of polycrystalline ice by the pinning of grain boundaries (Zener pinning), which should be reflected in the distribution of the inclusions in relation to the grain boundary network. We used an optical microscope to generate high-resolution large-scale maps (3&thinsp;μm&thinsp;pix<sup>&minus;1</sup>, 8&thinsp;×&thinsp;2&thinsp;cm<sup>2</sup>) of the distribution of micro-inclusions in four samples from the EDML (Antarctica) and NEEM (Greenland) polar ice cores. The in-situ positions of more than 5000&thinsp;μ-inclusions have been determined. A Raman microscope was used to confirm the extrinsic nature of a sample proportion of the mapped inclusions. A superposition of the 2D grain boundary network and μ-inclusion distributions show no significant correlations between grain boundaries and μ-inclusions. In particular, no signs could be found of grain boundaries harvesting μ-inclusions, no evidence of μ-inclusions inhibiting grain boundary migration by slow mode pinning could be detected. Consequences for our understanding of the impurity effect on ice microstructure and rheology are discussed.
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