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understanding of how impurities are redistributed and how fabric forms in the shallow layers of
ice sheets.
EXPERIMENTAL
We examined six firn specimens from depths of 9.71 m, 19.545 m and 34.465 m from core 02-
SP, and depths of 13.019 m, 23.028 m and 38.533 m, from core 02-5 obtained on the United States
International Trans-Antarctic Scientific Expedition (ITASE). Core 02-SP is at the South Pole
while core 02-5 is the next core along the route, see the route map at
http://www2.umaine.edu/USITASE/html/map.html for details.
For examination in the SEM, ~25-mm- × 25-mm- × 10-mm-thick specimens were cut
perpendicular to the core axis, shaved flat with a razor blade under a HEPA-filtered, laminar-flow
hood at -10°C and frozen onto a brass plate. Specimens were then either sealed in a small
container for later examination or mounted onto a cold stage for immediate observation. For
imaging and microanalysis, the uncoated specimens were examined at -80°C ± 10°C using a JEOL
5310 low-vacuum SEM operated at 10 kV equipped with a Princeton Gamma-Tech IMIX EDS
system utilizing a lithium-drifted silicon thin-window detector. For orientation determination,
specimens were frozen to a purpose-built pre-tilted brass sled that was then mounted on a cold
stage in a field emission gun FEI XL-30 environmental SEM. The SEM was operated at 15 kV
with a beam current of 0.15 nA, and the ice was again examined at temperatures between -80°C ±
10°C at a pressure of approximately 5 × 10 –4 Pa. Secondary electron imaging was used on both
SEMs. EBSPs were captured using the techniques described by Iliescu and others (Iliescu et al.
2004; 2005) and indexed using HKL Technologies’ CHANNEL 5 software.
RESULTS AND DISCUSSION
Figure 1 shows low-magnification secondary electron images of firn from three different depths.
The images were obtained as soon as possible after the specimens were inserted into the SEM to
reduce the degree of sublimation, which can affect the appearance of the specimens. Several
features are worth noting. First, shaving the ice produced some debris, which fell into the pores in
the ice. This was much more noticeable in the higher porosity shallow ice, presumably because the
grains were less strongly bonded together and the pore volume is larger. Examples of such debris
are labeled “D” in Figure 1(a).
Second, grain boundaries grooves were observed in some images. These indicate where the
snow crystals are joined together, see Figure 1 (a). By allowing the specimens to sublimate in the
SEM it is possible to see grain boundary grooves between all the individual ice crystals and,
hence, it should be possible to determine how the grain size changes as a function of depth in the
firn. However, one needs to be aware that longer sublimation times, due to loss of mass,
drastically affect the appearance of the microstructure and hence a delicate compromise needs to
be achieved between the need to depict the microstructure in an accurate unadulterated manner
and the desired to better visualize the details within the grain boundary region.
Third, it is straightforward to determine the % areal porosity from these images by dividing the
area occupied by the shaved flat regions by the area of the image. The pore area and length of
boundary area were measured on the SEM images using Image SXM (Barrett 2005), a derivative
of NIH Image (Rasband 1997). The boundaries of pore areas were traced with the drawing tool,
and the perimeter and enclosed area measured with the pixel-counting measurement utility, see
Figure 2. As is quite common in the SEM, the edges of the flat surface tended to charge. This
analysis was performed from multiple images at each depth (except for the 9.71 m specimen,
where only one image was available). The porosity values obtained at each depth are shown in
Table 1. As might be expected, (except for the 9.71 m specimen), the porosity at a particular site
decreases with increasing depth.
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