Geology Reference
In-Depth Information
plane along the direction of the maximum heat flow. This
type of oriented growth is called directional solidification
(DS) in the field of metallurgy for high‐temperature com-
plex superalloys used in making gas turbine engine com-
ponents for jet aircrafts and rockets [
Sims et al.,
1987].
In many respects, microstructure of natural sea ice is
like those of predominantly binary alloys like alpha‐beta
titanium‐based alloys [
Sinha,
2004]. Once the DS type of
growth is established in ice covers, columnar grains with
intragranular subgrains develop. The cross‐sectional areas
of the grains tend to increase with the increase in depth.
Changes in the vertical salinity distribution of salt con-
tent also occur. Structural features, related primarily to
the growth history, such as the geometry of intragranular
subgrains and their boundaries, and the size, shape, and
distribution of brine pockets, can be examined readily by
the use of quantitative metallurgy.
Although common approaches used in metallurgical
investigations are well established, their applications to ice
require special measures. Some of the ice‐specific methods
developed at the NRC of Canada will be emphasized and
explained. The most important fact to be emphasized here
is the established fact that precise forensic type of quanti-
tative metallurgical studies in ice can indeed be performed
under field conditions. For this, however, the investiga-
tions should be performed in the field when the ambient
air temperatures are close to about −20 °C. The measure-
ments should also be carried out soon after recovering
the specimens from the sea. This eliminates or significantly
reduces problems associated with storage and shipments
to distant locations.
Chronic problems in the sampling and testing of natu-
ral sea ice in general include the undesirable desalination
of sea ice when samples are shipped for conducting lab-
oratory tests, even if the cores or blocks are recovered at
extremely cold ambient temperatures. Natural decay pro-
cesses also make it impossible to take samples without
disturbing the structure and its environment when the
ambient temperatures are high. Moreover, small samples
may not represent the bulk of the material. The bulk
properties of any crystalline material (metals, metallic
alloys, and nonmetals like ceramics, rocks, and ice) can
only be determined by tests that involve a large volume of
the material containing many grains. Thus, an effort has
been made to present microstructural images obtained
from large thin sections with dimensions up to about
300 mm in order to include a large number of grains.
Examples of polycrystalline structure of young (Y) ice,
first‐year (FY) ice, second‐year (SY) ice, and old multi-
year (MYI) ice are presented below to highlight the gross
features of each type. The examples have been obtained
exclusively from field investigations in the Canadian High
Arctic (central and western) and the subarctic areas of
Labrador Sea (eastern Canada). Many persons including
0
0.5
1. 0
Bottom
0
30
60
90
Azimuth angle (deg.)
Figure 4.26
Variation of the average azimuth angle of the
c
‐axis orientation of FY ice in Eclipse Sound, Canadian Arctic,
in January 1978. The bar shows the standard deviation of the
orientation [
Nakawo and Sinha,
1984].
deviations of the orientations of the optic axes were calcu-
lated. The mean orientation is shown by a two‐headed
arrow and the standard deviation by the “bow tie” in each
fabric diagram. This representation was introduced by
Weeks and Gow
[1978] to illustrate the strong horizontal
alignment in the
c
‐axis orientation of columnar‐grained
sea ice along the coast of the Beaufort Sea. Figure 4.26
shows an example of the depth dependence of the varia-
tion of the mean azimuth angle observed in a core
extracted from Eclipse Sound [
Nakaow and Sinha,
1984]
.
Preferred orientation of the c-axis can be seen even at a
depth of about 0.2m. The mean value is about 40° (meas-
ured clockwise from the north) in the shallow sections and
decreased to about 20° near the 0.8 m level, but it increased
to about 60° toward the bottom. The preferred orientation
near the bottom, 50°-60° from the north, agrees well with
the orientation of the shorelines (60° from the north). It is
probable that the direction of the water current under the
ice is parallel to the shoreline. These observations confirm
earlier observations of
Weeks and Gow
[1980] for ice off
the Chukchi coast, Barrow, Alaska.
4.4. age‐Based sTrucTural feaTures
of naTural sea ice
As described earlier, columnar‐grained ice develops in
ice covers as a result of unidirectional growth of nucle-
ated ice crystals as seeds. The growth occurs in the vertical
