Geology Reference
In-Depth Information
redistribution of brine within the ice sheet but not to
drain from the sheet.
The hypothesis that freezing‐induced pressure may
cause cracks along the wall of the brine pocket, in the
bulk of an ice cover, is questionable. This is based on the
knowledge gained on the physics related to the rheologi-
cal properties of ice as a material at high thermal states
and the rapidity of stress relaxation processes at high
temperatures. This statement, however, is in need for
some clarifications. In this respect, an effort will be made
next to elaborate thermal states of sea ice that will also
be useful in understanding materials science of sea ice,
especially the application of etching techniques for reveal-
ing structural damages, if any, in ice.
Realistically speaking, the bottom temperature of float-
ing sea ice sheets is always close to about −1.8 °C for ice‐
rich areas of globe. In situ measurements of vertical
temperature distribution in sea ice during the entire growth
season, for a number of years, indicated that the surface
temperature at snow‐ice interfaces rarely goes below about
−20 °C even during the coldest time of the winter [
Sinha
and Nakawo,
1981
; Nakawo and Sinha,
1981]. Thus the ice
covers floating on its melt (water) exists at temperatures
close to its melting point. Being very close to its melting
point, ice in nature always exists as crystalline solids at
extremely high thermal states in comparison to that of its
liquid state (see section 4.1 for details). For saline sea ice
the primary structural parts are always made of pure ice
because the ice lattice does not practically allow any impu-
ries. For the main component of sea ice covers, an average
ice sheet temperature,
T
, of −5 °C (268 K) is equivalent to
a homologous temperature,
T
h
of 0.982
T
m
, where
T
m
(273
K) is the melting point (see definition in section 4.1.1 and
Figure 4.1). This temperature, therefore, is only 1.83%
below
T
m
. A relatively cold temperature of −20 °C, near
the top surfaces, is equivalent to only about 7.3% below
T
m
. At these high temperatures, mechanically induced
stresses (strains) are relaxed extremely rapidly even if the
pressure is applied “instantaneously”, say in less than in
0.001 s [
Sinha and Sinha,
2011]. Strains are accommodated
by the elasto‐delayed‐elastic‐viscous (EDEV) mechanisms
[
Sinha
, 1978c]. Initial extreme rapidity in stress relaxation
process (in fractions of one second) is governed by the
high‐temperature mechanism of delayed elasticity gov-
erned by the shearing of grain and subgrain boundaries.
For sea ice delayed elasticity is enhanced significantly by
the subgrains [
Sinha
, 1979]. However, in natural sea ice
thermally induced strains are never developed rapidly,
let alone instantaneously or fractions of one second, due
mainly to the thermal inertia of ice and the insulating
effect of snow covers.
Under snow covers the sea ice surface is not sensitive
to sudden and sharp decreases of atmospheric tempera-
ture, even under drastic drop from −5 to −35 °C (see
section 3.1). Thus the changes in the vertical temperature
distribution inside the ice mass occur very slowly.
Consequently, due to the stress relaxation processes,
stress concentrations cannot develop sufficiently high to
produce cracks around the brine pocket through which
brine can be expelled. Moreover, as can be seen in the
schematic of Figure 2.25b and actual micrograph of
Figure 2.26, brine pockets often exist along the subgrain
boundaries that can also accommodate changes in the
shape of the brine pockets. This is due to the fact that
water molecules along these boundaries are at higher
energy levels than those of the ice crystals because of
small lattice mismatches at the boundaries between
neighboring subgrains. Additionally, brine pockets often
contain air bubbles. The air bubbles can also help in
accommodating any expansion due to freezing of water
molecules in the pockets. Thus development of cracks at
the brine pockets as suggested by the brine expulsion
mechanism is rather remote.
The above conclusion has been substantiated experi-
mentally by the application of thermal etching to deline-
ate grain and subgrain boundaries, and deformation
induced gross changes in the microstructure such as tilt
boundaries or polygonization (often called recrystalliza-
tion). Thermal etching of ice surfaces is possible only
because ice exists at extremely high temperatures and
water molecules can sublimate or directly transform from
the solid state to the vapor, to be seen in section 6.4.1.
Submicroscopic features related to the deformation
induced crystalline defects, such as dislocations, dislo-
cation cells, pile up of dislocations, etc., can be revealed
by chemical etching and replication described in sec-
tion 6.4.2. These experimental techniques proved vital for
performing forensic type of investigations of damages in
ice lattice around brine pockets in response to mechanical
or thermal stresses. Common damage features of defor-
mation in ice are the formation of tilt boundaries, disloca-
tion pileups and polygonization [
Sinha,
1978
a
]. However,
damage features that can be associated directly with the
brine pockets, without any ambiguities, were never seen
by the author (N. K. Sinha) in undeformed young and
FY sea ice covers (regardless of their complex thermal
histories related to ambient conditions) when examined
shortly after sampling in field laboratories. Structurally
damaged features were also not seen later following trans-
portation and storage, and naturally subjected to una-
voidable thermal changes and hence thermally induced
localized strains due to the shrinking or enlargements of
brine pockets. In fact, Figure 2.26 exemplifies subgrain
boundaries of lattice mismatch between the neighboring
subgrains in natural sea ice, but no features directly
associated or linked to the isolated brine inclusions.
However, stress (strictly strain) concentrations around
brine pockets induced by externally applied mechanical
