Geology Reference
In-Depth Information
phases (i.e., crystals of pure ice) practically free of any sol-
utes because the hexagonal ice lattice allows only traces of
impurities. The impurities are pushed to the solid‐liquid
interfaces and the solidification front develops a cellular,
dendritic, or lamellar structure (section 2.3.2). These
solidification features are observed in both natural and
engineering material [
Bolling and Tiller,
1960] and are
known to affect their mechanical properties.
For directionally solidified (DS) growth conditions, as
described and explained in section 2.3.2, the dendritic
interface is the initial mechanism responsible for entrap-
ment of salts in the form of brine pockets between and
within the grains. To reiterate, grains and crystals cannot
be used synonymously for sea ice as can be done for fresh-
water ice. The conversion of the planer fronts to dendritic
structures with relatively enlarged surface areas slow
down the growth since the freezing temperature of brine
is lower than that of seawater. For example, brine with
230‰ salinity freezes at −21.1 °C or less. The dendritic
structure causes the grain boundaries to be convoluted
and leads to the development of intragranular substruc-
tures or subgrains with small lattice mismatches. Brine
pockets are present not only at grain boundaries but also
inside grains at subgrain boundaries of sea ice. They tend
to be located on planes parallel to the basal plane and
hence normal to the optic or
c
axis of a crystal. Brine
pockets observed in thin sections are the cross section of
what is known as 3D brine layers. The spacing between
adjacent planes or layers containing brine pockets in DS
columnar grains is called brine layer spacing (defined in
Figures 2.24 and 2.25). For a horizontal thin section, the
brine layers appear as rows of brine pockets, giving a
mosaic pattern (i.e., see Figure 2.35 in a small area and
Figure 4.28 over a large area). Since brine layers are verti-
cal in DS ice (i.e., Figures 2.21b and 2.22), their spacing
can be represented by the spacing of the brine rows found
in a horizontal section.
Brine layer spacing is one of the most important
parameters in describing the structure of sea ice because
of its impacts on the ice mechanical and, therefore, engi-
neering properties of ice and remote sensing observa-
tions. Brine layer spacing is often called the “plate width”
or “plate spacing” because it is a measure of the space
between adjacent platelet ice protrusions at the ice‐water
interface. It depends on growth rate and crystallographic
orientation, hence on the orientation of water current
under the ice. Since growth rate is related to climatology,
the vertical brine layer spacing profile in the ice provides
a record of previous weather conditions. As a rule of
thumb, brine layer spacing is inversely proportional to
the growth rate of sea ice and could also be dependent
on crystallographic orientation [
Tabata and Ono,
1962;
Weeks and Hamilton,
1962;
Paige,
1966]. Since growth
rate would generally decrease as ice grows thicker, it is
expected that brine layer spacing increases progressively
Depth=
30 mm
1 mm
2 mm
Depth=
55 mm
2 mm
Depth=
120 mm
Figure 4.49
(
left
) Horizontal thin sections of laboratory‐grown
thin ice where binary data represent ice crystals (white) and
brine (black). The correlation functions are shown at the left
column at depths of 30, 55, and 120 mm [
Perovich and Gow,
1991
, with permission from AGU
].
appear at a depth of 55 mm depth with near‐vertical ori-
entation in some parts and this is captured by the correla-
tion function, which is now asymmetric. The oriented
crystals at 120 mm depth produce a strong bell‐shaped
correlation function in the traverse direction with respect
to the crystal orientation. The promising results from this
study were not pursued perhaps because of the laborious
procedure of preparing ice thin sections and the heavy
computation time of the correlation function. It would
have been better to pursue this technique on ice grown in
the field rather than laboratories as it is impossible to cre-
ate natural solidification conditions (water current, salin-
ity profile, heat flow, etc.) in laboratories.
4.5.2. Geometric Characteristics of Brine Pockets
in First‐Year Ice
When an impure melt such as seawater is subjected
to unidirectional solidification, microsegregation occurs
at the solid‐liquid interface. The segregation processes
in case of saline water leads to the development of solid
