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Figure 4.40 Vertical section of the top 140 mm depth of a MY hummock core obtained from Lancaster Sound
in May 1992, photographed between ( left ) crossed polarizers and ( right ) scattered light [ Shokr and Sinha, 1994].
(For color detail, please see color plate section).
axis varies from 50 to 80 mm, although smaller dimen-
sions around 10 mm are observed near the top. Crystals
have irregular boundaries, indicating that the original ice
was highly saline. The deviation of crystal orientation
with respect to the vertical indicates that this part of the
hummock has possibly originated from a tilted ice block
in an old rubble field. The major axis of bubbles varies
from 1 to 6 mm long. Once again, the boundary between
the bubbly and bubble‐free zones is well identified at
about 220 mm below the surface. A careful examination
of the air bubble orientations in the vertical and horizon-
tal sections (the latter was made at 90 mm depth) reveals
a tendency of the bubbles to stretch parallel to the domi-
nant crystal orientation. This means that when surface
melt infiltrates ice (in summer), the meltwater follows
paths connecting brine pockets (which exist in the origi-
nal saline FY ice).
Figure  4.40 shows another possible configuration of
the polycrystalline structure of hummock ice. It features
large oriented crystals with planar boundaries with no
subgrain boundaries. This is an indication that this ice
grew from freshwater. Note that the long axis of the
crystals is inclined at approximately 45°. Since this axis
marks the crystal's growth direction (i.e., the direction
of maximum heat flow), it can then be concluded that
this ice must have grown at the edge of a melt pond and,
thereby, on a side of a hummock. The air bubbles in this
core are vertically oriented, i.e., not parallel to the domi-
nant crystal orientation. This is not uncommon in MY
hummock ice, and it is in line with the configuration
shown in Figure  2.62. The vertical orientation of air
bubbles are indicators of the flow of meltwater follow-
ing the direction of gravity since no brine drainage
channels exist in this example of freshwater ice. Irregular
and interconnected bubbles, with typical dimensions
between 1 and 4 mm, are observed in the surface layer
(top part of Figure 4.49). At greater depths the common
bubble shape becomes more elliptical with typical
dimensions of the major and minor axes being about 6
and 3 mm, respectively. Bubbles are randomly spaced in
both vertical and horizontal sections, but they tend to
cluster as shown in the figure. It is worth repeating here
that the distribution of air bubbles in hummock ice (or
MY ice for that matter) is different from the pattern of
the distribution in FY ice. In the latter, bubbles are usu-
ally arranged along the grain and subgrain boundaries
with their long axis parallel to the grain growth
direction.
As seen in the above examples, air bubbles in hummock
ice are usually highly interconnected near the surface.
They lose their individual identity and form a complicated
network, particularly near the surface. Therefore, it is pos-
sible to retrieve the geometrical characteristics of the bub-
bles when examined at depths far from the surface. Two
micrographs of air bubbles at 0.14 m depth are shown in
Figure  4.41. Bubbles appear dark and mostly located at
the subgrain boundaries, which are made visible by ther-
mal etching. A few bubbles also appear inside the grains.
The colored areas inside the dark bubbles are parts of
solid grains when the thin section cuts through them.
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