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(a)
(b)
0.5mm
Figure 2.27 Controlled thermal etching technique applied to freshly sampled ice in Mould Bay to reveal brine
pockets along (a) extremely low mismatch subgrain boundaries and (b) inclusion on distorted boundary or isolated
with no links to any boundary. Note the intestine‐like alimentary canals or steps revealing complex network of basal
dislocations intersecting the surface; c-axis is indicated by <c> (N. K. Sinha, unpublished).
(a)
(b)
(0.5 mm)
20 β m
Figure 2.28 Scanning electron micrograph (SEM) of a replica of a horizontal section of columnar‐grained first‐
year sea ice from 0.3 m below the surface showing subgrain boundary with (a) brine pockets and (b) inside a brine
pockets at −30 °C [ Sinha, 1977a].
dislocations intersecting the surface. The patterns indi-
cate the crowded rows of closely spaced dislocations in
the ice lattice presumably generated inside the dendrites
during the period when the tips of the dendritic arms pro-
trude in the water. As the growth front progresses and
bridging between the arms are created, and brine and air
inclusions are trapped, the complexities in the pattern are
amplified.
The presence of subgrain boundaries in sea ice were ini-
tially revealed by Sinha [1977 a ] by making use of the dual
process of chemical etching and replicating technique
followed by scanning electron micrography, or SEM
(described in section 6.4.4), as exemplified in Figure 2.28.
Replicas should be interpreted as negatives. For example,
the grooves produced by etching of the subgrain bound-
ary appear as raised edges as can be seen in the middle of
the SEM in Figure 2.28a. Tube of brine pockets filled with
precipitated crystals look like crumpled cylinders in SEM
in Figure 2.28b. Replicas can actually be made under field
conditions and stored for examinations under comfortable
conditions of shirt‐sleeve environments (for details on
microstructural analytical methods, see section 6.4).
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