Geology Reference
In-Depth Information
integral part of the ice cover. Superimposed ice is usually
formed on top of the ice sheet (i.e., on top of the primary
ice) when wet snow, rain, surface melt freezes, or flooding.
It is a layer of new ice that has no connection to the under-
lying ice and it may grow from the bottom up. This ice type
is not included in the classification in Table 4.1.
Although the bulk of floating freshwater ice can be
divided into five basic types: S1, S2, S3, S4, and S5, the
S1 type has never been reported for sea ice. S1 type of
columnar‐grained ice, with their
c
axis in the vertical
plane, is expected to form under calm conditions without
any snow deposition activities during the initial ages of
freezing. Careful examinations of the snow‐free surface
layers of freshly frozen sea ice in Mould Bay, Prince
Patrick Island revealed no signs of nucleation and growth
of S1 type of ice at the surface level [
Sinha,
1984]. It was
concluded that the characteristics of S1 ice would not be
maintained beyond the top 1-3 mm. Moreover, sublimation
at the surface level in conjunctions with the wind removes
this thin layer as the ice cover continues to grow.
Consequently, for all intents and purposes, sea ice can be
assumed to have no S1 type of structure. (See more on ini-
tial growth structure of sea ice in section 5.1 on Mould Bay
series of experiments.)
Type S2 is the transversally isotropic columnar‐grained ice
with the
c
axis of the grains randomly oriented in the hori-
zontal plane or normal to the growth direction. It can form
in areas not affected by any water current. S3 is columnar‐
grained with the
c
axis of the grains also in the horizontal
plane but tends to be oriented in a particular preferred
direction. The bulk of the land‐fast ice usually consists of
S3 type of ice.
Peyton
[1966] described the oriented sea ice
(in seawater around Alaska) in detail and called it “bottom
Ice.” Preferred crystal orientations in bulk sea ice was well
known to the Russian scientists, but their speculations as to
the reasons for the formation of such ice types were never
confirmed. A detailed discussion on this topic can be found
in
Weeks
[2010]. It was shown later that the crystal orienta-
tion of this ice type is parallel to the water current under the
ice sheet during the growth period. This observation was
confirmed by
Weeks and Gow
[1978] who found that the
c
axis of ice in the Beaufort Sea was oriented parallel to the
water current under the ice. At about the same time
Sinha
[1977a] also observed the same in the Strathcona Sound in
Baffin Island. The congealed frazil ice and the bottom ice
are called “platelet ice” by the investigators working primar-
ily on offshore Antarctic ice and perhaps not familiar with
the existence of the classification system already developed
for ice in Canada.
Type S4 ice is prevalent in many areas and often in large
quantities. There are indications that this type of ice and/or
mixed with unfrozen dense slush could be as thick as sev-
eral meters. Icebreakers have been reported to be unable to
maneuver in seawater containing this type of conglomer-
ate. They exist not only below the secondary ice as described
in Table 4.1 but also above it. Moreover, the crystals are usu-
ally needle‐like or crushed needles rather than tabular, and
the structure may not only be equiaxed but also oriented.
The structure of oriented frazil ice was already presented
earlier and classified as S5‐type ice.
4.3.2. Extending Crystallographic Classification
of Freshwater Ice to Sea Ice
Freshwater ice forms in inland lakes and rivers when
the surface temperature of the water, with very little salt
content, drops to 0 °C or slightly lower temperatures
if some dissolved impurities are present in the water.
Seawater, however, starts to freeze when the surface tem-
perature drops to about −1.8 °C because of higher water
salinity. In the Arctic and the Antarctic, the near shore
water may have a salinity of about 30‰. This is close but
measurably lower than the average seawater salinity of
35‰. The lower water salinity is due to the freshwater
that feeds into these areas from different land sources,
including rivers, snowmelt, glaciers, etc. The underside of
an ice cover in contact with water remains close to the
freezing temperature or slightly colder depending on
brine drainage, while the exposed upper surface can be
close to the ambient air temperature. However, there is a
much wider range of environmental and oceanic condi-
tions, above and below the sea ice covers, compared to
what exists in lakes and rivers. Moreover, the growing
seasons for sea ice usually at higher latitudes is also sig-
nificantly longer and melting periods shorter than most
lake and river ice at lower latitudes. During the growing
season for sea ice, depending upon the latitude especially
for the Arctic Ocean and the High Arctic, there can be
total absence of solar radiation or 24 h of sun during the
summer months when the ice melts. The duration and
intensity of solar radiation in the polar region add
another aspect of the difference between growth and
decay of sea ice in those regions and lake ice in southern
regions such as the Great Lakes in North America. Of
course, ice covers in rivers and lakes (even in the High
Arctic) are not known to survive beyond the winter sea-
son, but that's not the case for sea ice.
Physically speaking, therefore, there are differences
between freshwater ice and sea ice. Compared to the
types of ice that form in lakes and rivers there is more
diversity in sea ice formation and its aging processes.
However, as shown in Table 4.1, the classifications of
freshwater ice can be extended to ice forming in the
oceans as long as the diversities and complexities due to
the entrapped salts are recognized. Differences between
freshwater ice and sea ice are summarized here with
emphasis on the crystallographic classification of fresh-
water ice in Table 4.1, which can be extended to sea ice.
