Geology Reference
In-Depth Information
pure ice. This is about 10% less than the density 999.8 kg/m
3
for pure water at 0°C.
Seawater, as a solution, undergoes a markedly differ-
ent freezing process from that of freshwater, particularly
when the salinity exceeds the critical value of 24.69‰. It
can be seen in Figure 2.3 that for salinities below this
critical value the temperature of the maximum density is
higher than the freezing temperature of the seawater sol-
ute. As the surface of the seawater cools, the denser cooler
layer also sinks and a vertical convection current is cre-
ated similar to the case of freshwater. The difference,
however, is that this current does not stop before the
surface reaches the freezing point. The freezing point of
water with salinity 35‰ is −1.8°C, which is higher than
−3.5°C, the critical temperature, of maximum density for
the solution. This means that the entire depth of the sea-
water body must be cooled to the freezing temperature
before any formation of ice. But this does not happen
in nature. The convection current is usually limited to
the upper 50-200 m depth, called the mixing layer of the
ocean, as can be visualized from Figure 2.5.
Many areas of the ocean are stratified with increasing
salinity and hence become denser toward the bottom.
When the colder and denser upper area sinks, the
increased density of that layer may not be as high as that
of the lower layer, known as a pycnocline zone, which fea-
tures a rapid increase of density with depth due to con-
tinuous change in its temperature or salinity. This
stratification imposes a lower limit to the vertical convec-
tion in the mixing layer. This limit is about 50 m in the
Arctic Ocean, which has slightly less salinity than the
Atlantic Ocean. Any further cooling of the surface below
the freezing temperature of the isothermal mixing layer
will cause surface freezing.
Differences in the freezing processes in freshwater and
seawater can then be summarized as follows. In the case of
freshwater, the convection current stops when the surface
temperature reaches about 4°C (the value correspond-
ing to maximum density). Therefore, there is a time lag
between the end of the convection current and the actual
start of freezing when the surface temperature reaches
the freezing point. This does not happen in the case of
seawater as the convection current stops at the moment
when the surface temperature reaches the freezing point,
depending on water salinity. Consequently in order for ice
to form in seawater, more heat has to be removed com-
pared to that required for freshwater. In other words, sea
ice formation is relatively slower than freshwater ice. As
a rule of thumb, sea ice forms faster under any of the fol-
lowing three conditions: (1) in areas of low salinity such as
those near the mouths of rivers, (2) in areas where there is
no currents such as inlets, bays, and straits, and (3) in areas
of shallow waters near coasts or over shoals or banks
where only a small body of water must be cooled.
2.1.3. Initial Ice Crystals and Frazil Ice
Attaining the freezing point is not sufficient for water
to freeze because the formation of ice crystals requires
nuclei to start solidification. In primary cryospheric
regions when the ambient air temperature is below 0°C,
small ice nuclei are almost always present in the atmos-
phere. These ice particles are deposited continuously on
seawater surfaces to initiate nucleation of crystals at the
surface. If the water surface is very calm and the air tem-
perature decreases slowly to develop only small tempera-
ture gradient, the surface water may be supercooled and
crystals may be nucleated. Once nucleated, the pure single
crystal of ice takes a spherical shape because the surface‐
to‐volume ratio is a minimum for a sphere. The spherical
shape, however, is not sustained due to the anisotropic
growth rates in different planes of the hexagonal lattice
of ice crystals. The crystal starts growing parallel to its
basal plane with
c
axis normal to this plane (definitions
are given in section 4.1.3). It takes the form of a discoid
because the growth in the basal plane is preferred in hex-
agonal crystals like ice and will be described in detail in
section 4.2.1. This process of initial growth following the
nucleation processes takes place irrespective of the salin-
ity of the water. Under calm conditions, without any
mechanical or thermal disturbance, each discoid floats at
the surface. Each of the tiny disc float with their flat sur-
face parallel to the water surface or horizontal plane.
Since the major surfaces of these discs are parallel to the
basal plane, the
c
axis of these crystals tends to be ori-
ented in the vertical plane. Each of the floating discs,
therefore, is oriented favorably for growth in the horizon-
tal plane. If the initial ice crystals are nucleated around
Mixing layer
Colder water
Continues to sink
Pycnocline zone
Extremely stable
No vertical convection
Figure 2.5
Illustration of the lower limit for convection current
in seawater appropriate for the Arctic.
