Ice Physics and Physical Processes - Sea Ice: Physics and Remote Sensing - page 53

ice crystals with very little interstitials or foreign atoms or
molecules other than water. They may have entrapment
in the form of isolated pockets of liquid or gas inclusions,
but the ice lattice is almost free from any impurities. Thus,
grains in sea ice are strictly speaking not single crystals
but an aggregate of subcrystals with small lattice mis-
matches. It will be seen later that these boundaries can be
revealed by a very simple technique called thermal etch-
ing or replicated by the dual process of chemical etching
and replicating.
While salts from seawater are rejected during freezing
at the interface, some are retained in the sea ice in the
form of brine pockets. These pockets are initiated in the
bulk ice, close to the skeletal layer, when bridging occurs
between neighboring dendrites. Brine and gas accumulate
in the spacing between the dendrites, making these areas
the last to freeze. Two neighboring dendrites may have to
grow around the entrapped pockets. The processes of
developing new ice lattice for the growing lateral front of
the subcrystals also develop lattice defects, particularly
the line defects called dislocations. Dislocations are gen-
erated along the basal as well as nonbasal crystallo-
graphic planes. These individual line defects can actually
be revealed by etching and replicating [ Sinha, 1977b,
1978a]. Dislocations are also generated due to thermal
agitations, chemical disturbances, and water movements
causing perturbations in the lattice at the growing front.
One of the possible (but questionable) sources of lattice
straining is the periodic oscillation between outflow of
brine and inflow of seawater in the skeletal layer during
growth proposed by Martin [1970] and Eide and Martin
[1975], to be discussed later in the next section. The lattice
defects could be isolated within the matrices or grouped
in the form rows or cells. Formation processes of disloca-
tions are linked strongly to the creation of small lattice
mismatches at the bridging between the dendrites and
hence the development of subgrain boundaries. Due to
the extremely hot material states of ice, as pointed out
earlier, simple thermal etching technique can be used very
effectively under field conditions in the Arctic, for the
examinations of substructural details. Since the glaciolo-
gists have not used this method to its full potential, an
effort has been made to describe the physical principles
involved and applications in sections 6.4.2 and 6.4.3.
An example of revealing microstructural details, includ-
ing the presence of subgrain boundaries, is illustrated in
Figure 2.26. It shows an optical micrograph of a horizon-
tal thin section of freshly sampled FY columnar‐grained
ice at a depth of about 2.05 m in Mould Bay, Prince Patrick
Island, Canada, in 1985. The sectioning was performed
at −20 °C inside the field laboratory of Mould Bay (Figure
5.2). Thermal etching technique was applied to reveal two
prominent subgrain boundaries with brine pockets sitting
on those boundaries. The end of a subgrain with etched

boundaries can be seen near the top left corner of this
micrograph. It should be emphasized here that the bound-
aries are actually grooves in the ice produced by the sub-
limation of high‐energy water molecules along the lines
of lattice mismatches. Note the rows of isolated brine
pockets inside the area between the two prominent sub-
grain boundaries. These inclusions may be at different
depths within the thickness of the thin section. Note that
the inclusions are also aligned parallel to the prominent
subgrain boundary. These isolated brine pockets were
obviously trapped between the dendritic spaces at the skel-
etal layer during the growth (as depicted in Figure 2.24),
but the crystallographic axis of those dendrites were
“almost” matching with each other and thereby appar-
ently no distinct subgrain boundaries developed. However,
extremely small mismatch in the lattice orientations do
occur during the growth and bridging processes and
may result in the creation of rows of lattice line defects
(dislocations). Individual dislocations can be revealed by
X‐ray tomography, suitable for single crystals of ice, and
the dual process of chemical etching and replication for
both single crystal as well as polycrystalline ice as illus-
trated in Figure 6.35 [ Sinha, 1977b, 1978a].
Controlled thermal etching techniques can actually be
applied to examine the finer aspects of the distribution
pattern of dislocations (not individual defects) inside a
grain of ice, as depicted in Figure 2. 27. The intestine‐like
alimentary canals or steps reveal the complex network of

0.5 mm

Figure 2.26 Optical micrograph of double‐microtomed ther-
mally etched surface of horizontal section of FY columnar ice,
at depth of 2.05 m in March 1985, Mould Bay, Canada at
−20 °C, showing subgrain boundaries and brine pockets, with
or without air bubbles, aligned along the boundaries and inside
the ice matrix with no boundaries (N. K. Sinha, unpublished).