Ice Physics and Physical Processes - Sea Ice: Physics and Remote Sensing - page 50

the late afternoon from Eclipse Sound, near Pond Inlet,
Baffin Island, in March 1978, when the air temperature
was around −25°C and the sun was just above the hori-
zon. The cutting blade of the core auger is also shown to
indicate that the photograph was taken within a few sec-
onds after the recovery of the sample. The linear struc-
tural features visible in the photograph clearly show the
orientation of dendritic growth at the ice‐water interface.
Some details may be seen in the insert. The entire bottom
of the core displayed this oriented feature of the sea ice,
classified as S3 type (see the structure‐ and texture‐based
classification system described in section 4.3.3). In order
to preserve the delicate features of the dendritic growth
in the skeletal layer of natural FY sea ice and to explore
the microstructural details of the ice, a novel idea for
preservation was applied. This and the details of micro-
structural observations will be described in section 6.2.3.
These observations not only confirmed and strengthened
laboratory‐based investigations carried out in the 1960s
on saline ice, but they shed new light on the crystalline
aspects of the “grain” and “intragranular” fine structure
(subgrain or subcrystal) of directionally solidified,
columnar‐grained, natural sea ice. In short, it will be seen
later in this and subsequent sections that the traditional
concept of grain, as it is understood for freshwater or
glacier ice, is not applicable to sea ice. Grains in sea ice
represent a family of subgrains with slightly different ori-
entation of their crystallographic lattice. Physical proper-
ties of sea ice, including microcracking [ Sinha, 1985a],
creep [ Sinha et al., 1995], and strength [ Zhan et al., 1996]
response, depend strongly on the characteristics of sub-
grains, as implied originally by Sinha [1977a , 1978b].
For example, floating sea ice sheets (for bearing capacity
purposes) are found to be more flexible or elastic than
freshwater ice covers simply because of the finer structure
of the subgrains (subcrystals). This is a reality, in spite of
the fact that the cross‐sectional size of the grains in
columnar‐grained sea ice could be significantly larger
than those of freshwater ice. Strength of FY sea ice is
certainly less than freshwater ice, but it is not simply due
to the effect of brine volume, as is customarily assumed,
but also due to the significantly smaller size of the sub-
grains, compared to the size of the grains, which tend to
crack. Subgrains are subjected to shearing at their bound-
aries, leading to intersubgranular cracking and acoustic
emissions [ Sinha, 1985a, 1996]. Under similar rates of
straining, sea ice tends to emit significantly larger num-
bers of low‐amplitude acoustic emissions than freshwater
ice. An understanding of the substructure of sea ice is
extremely important for realizing the complexities of all
the physical properties, including microwave signatures,
of sea ice.
Sinha [1977a] presented a microscopic view of a hori-
zontal thin section that reveals some details on the

distribution of mostly interconnected intragranular brine
pockets in vertically oriented columnar‐grained ice in the
bulk ice above the skeletal layer (Figure 2.21a). In this
case, the ice was S3 type and sampled from Strathcona
Sound, Baffin Island, Canada, when the air temperature
was about −30 °C and thin sections were also made at
that temperature. The section was cut parallel to the sur-
face of the ice cover and hence normal to the long axis of
the columnar grains. The long axis of the grains was,
therefore, parallel to the growth direction. Figure 2.21b
shows a macroscopic view of a vertical section exhibiting
long subgrains inside one columnar grain. In this case,
thin sections were prepared at −20 °C, within a few hours
after sampling the cores, in a field laboratory near the
weather station of Mould Bay, Prince Patrick Island,
Canada (see section 5.1 on MY sea ice investigation). The
brine pockets are present primarily in arrays parallel to
the basal planes, often indicated as (0001) plane. The c
axis, indicated as < c > or <0001 > is normal to the basal
plane. Refined definitions of the basal plane and the c
axis (and also a axis) are given in the section 4.1.3 while
presenting the lattice structure of hexagonal ice and com-
monly used terms and definitions used in ice physics. As
can be seen in Figure 2.21a and 2.21b, brine pockets in
both the horizontal and vertical planes are mostly present
along lines at right angle to the c axis. This array of brine
pockets and their distribution in general seems to deline-
ate the original boundaries of dendritic structure formed
at the ice‐water interface during freezing. As the freezing

0.5 mm

Figure 2.21a Photomicrograph of a horizontal section near the
bottom surface of sea ice from Strathcona Sound, Canada, at
−30 °C, exhibiting the distribution of mostly interconnected
brine pockets along the boundaries of the subgrains associated
to the dendrites formed at the ice‐water interface during freez-
ing [ Sinha, 1977a].