Three‐dimensional observations of ice crystal characteristics in polar ice sheets

Abstract

In ice core studies, mean crystal size is generally measured on a section image of an ice specimen. However, an ice crystal with a complex shape may appear as several cells on a section image; consequently, the mean crystal size may be underestimated by overcounting the crystals on the image. We examined this phenomenon, which we called duplication effect, through three-dimensional observation of crystal shapes. In terms of results, we found that the frequency in which duplicate cells appear on a section image is linear with respect to the frequency of which randomly drawn lines on the image pass a cell more than once. On the basis of this empirical relationship, the error on the mean crystal size caused by the duplication effect can be revised from the section image itself. As to its application, we examined crystal sizes in some Antarctic ice specimens and found that the extent of change of crystal size steadily increased with depth up to 100 m and then fluctuated between 6% and 10% in the deeper pans.

Subgrain boundaries and related microstructural features in EDML (Antarctica) deep ice core

“…Thermal etching by sublimation reveals grain boundaries and subgrain boundaries as grooves on the surfaces (e.g. Nishida and Narita, 1996;Obbard and others, 2006a). The section frozen on a glass plate was then mapped at microscopic resolution (1 pixel edge length = 3.3μm) (Kipfstuhl and others, 2006).…”

Evolution of ice crystal microstructure during creep experiments

“…The bulging grain-boundary curves can be cut off from the parent grain by further grain-boundary motion, to build a new, small grain with a similar orientation to the parent (Humphreys and Hatherly, 2004, p. 251). Additionally, SIGBM can lead to apparent grain-growth reduction by three-dimensional duplication effects (studied in detail by Nishida and Narita, 1996) emerging only with interlocking textures: a single grain appears plurally in one section due to its multiple protuberances. It is inevitably counted and measured as two or more grains and therefore significantly decreases the measured mean grain size (also discussed by Gow, 1969;Alley and Woods, 1996).…”

Microstructure mapping: a new method for imaging deformation-induced microstructural features of ice on the grain scale

“…Clear surfaces are achieved by exposing the polished section to the free atmosphere. Sublimation of an ice surface works like chemical or thermal etching, a method widely employed in materials science to produce etch pits and grooves on the surfaces of metal, mineral and ceramic samples (Mullins, 1957;Kuroiwa and Hamilton, 1963;Hobbs, 1974;Nishida and Narita, 1996;Arnaud and others, 1998). During sublimation, the scratches produced by the microtome blades disappear, while certain characteristic patterns, called sublimation 'grooves', 'lines' and 'pits', start to develop at the sites where grain/subgrain boundaries (as well as other energetically unfavourable structures) meet the surface.…”

“…Arnaud and others (1998) used a classic still camera to photograph the microstructure of firn and bubbly ice in coaxial reflected light. Nishida and Narita (1996) reconstructed the threedimensional shape of ice grains using similar methods. Both used classic still cameras, and were not able to instantaneously control the image quality (contrast, resolution, etc.).…”