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The Cryosphere An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/tc-2020-2
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2020-2
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 22 Jan 2020

Submitted as: research article | 22 Jan 2020

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This preprint is currently under review for the journal TC.

Temperature and strain controls on ice deformation mechanisms: insights from the microstructures of samples deformed to progressively higher strains at −10, −20 and −30 °C

Sheng Fan1, Travis Hager2, David J. Prior1, Andrew J. Cross2,3, David L. Goldsby2, Chao Qi4, Marianne Negrini1, and John Wheeler5 Sheng Fan et al.
  • 1Department of Geology, University of Otago, Dunedin, New Zealand
  • 2Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA, USA
  • 3Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, USA
  • 4Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
  • 5Department of Earth and Ocean Sciences, University of Liverpool, Liverpool, UK

Abstract. Understanding ice deformation mechanisms is crucial for understanding the dynamic evolution of terrestrial and planetary ice flow. To understand better the deformation mechanisms, we document the microstructural evolution of ice with increasing strain. We include data from deformation at relatively low temperature (−20 and −30 °C) where the microstructural evolution has never before been documented. Polycrystalline pure water ice was deformed under a constant displacement rate (equal to the strain rate of ~1.0×10−5 s−1) at temperatures of −10, −20 and −30 °C to progressively higher true axial strains (~ 3, 5, 8, 12 and 20 %). Mechanical data show peak and steady-state stresses are larger at colder temperatures as expected from the temperature dependency of creep. Cryo-electron backscattered diffraction (EBSD) analyses show distinct sub-grain boundaries in all deformed samples, suggesting activation of recovery and subgrain rotation. Deformed ice samples are characterised by big grains interlocking with small grains. For each temperature series, we separated big grains from small grains using a threshold grain size, which equals to the square mean root diameter at ~ 12 % strain. Big grains are more lobate at −10 °C than at colder temperatures, suggesting grain boundary migration (GBM) is more prominent at warmer temperatures. The small grains are smaller than subgrains at −10 °C and they become similar in size at −20 and −30 °C, suggesting bulge nucleation facilitates the recrystallization process at warmer temperature and subgrain rotation recrystallization is the nucleation mechanism at colder temperatures. At temperatures warmer than −15 °C, c-axes develop a crystallographic preferred orientation (CPO) characterized by a cone (i.e., small circle) around the compression axis. We suggest the c-axis cone forms as a result of selective growth of grains at easy slip orientations (i.e., ~ 45° to shortening direction) by strain-energy driven GBM. This particular finding is consistent with previous works. The opening-angle of the c-axis cone decreases with strain, suggesting strain-induced GBM is balanced by grain rotation. Furthermore, the opening-angle of the c-axis cone decreases with temperature. At −30 °C, the c-axis CPO transits from a narrow cone to a cluster, parallel to compression, with increasing strain. This closure of the c-axis cone is interpreted as the result of a more active grain rotation together with a less effective GBM. As the temperature decreases, the overall CPO intensity decreases, facilitated by the CPO weakening in small grains. We suggest the grain size sensitivity of grain boundary sliding (GBS) favours a faster strain rate in small grains and leads to the CPO weakening at cold temperatures. CPO development cannot provide a uniform explanation for the mechanical weakening (enhancement) after peak stress. Grain size reduction, which can be observed in all deformed samples, is most likely to cause weakening (enhancement) and should be considered to have a significant control on the rheology of natural ice flow.

Sheng Fan et al.

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Short summary
Understanding how temperature and strain control the ice deformation mechanisms is essential for predicting the ice flow rate in respond to global warming. Based on uniaxial compression experiments on synthetic ice samples, we determined that higher strains promote grain rotation and lower temperatures promote sliding between grains. The alignment of ice c-axes cannot uniformly explain the mechanical weakening during deformation. Grain size reduction likely plays a significant role in weakening.
Understanding how temperature and strain control the ice deformation mechanisms is essential for...
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