Geology Reference
In-Depth Information
Figure 2.18a. These holes were created by debris blown
during the previous summers from the nearby mountain,
visible at the top-left corner in Figure 2.18a.
2.3.1. Compositional (Constitutional) Supercooling
and Brine Pocket Formation
Due to low salinities, freshwater lake and river waters
reject only very small amounts of salts during freezing
compared to seawater. As a result, the composition of
water under the ice‐water interface in freshwater ice
remains essentially unchanged during ice growth. This
causes ice growth to be almost entirely determined by
the heat flux from the warmer water to the colder ice.
Consequently, the ice‐water interface remains planer
under “normal” growth rates [
Woodruff,
1973]. However,
under very rapid growth rate, this planar interface becomes
unstable and develops into a cellular (dendritic) shape
though this is not commonly observed.
The situation is different in the case of seawater, which
contains a significant amount of dissolved inorganic salts
in addition to dissolved air and suspended impurities.
As the freezing front progresses, a considerable amount
of salts and gases are rejected to the underlying seawater.
The diffusion of the salt flux away from the ice‐water
interface propagates in an opposite direction to the heat
flow from the warmer water well below the ice interface
toward the colder interface. These two influences lead to
a phenomenon known as compositional or constitutional
supercooling. It refers to layers in the underlying seawater
where the temperature is lower than its freezing tempera-
ture. The term “constitutional” is more frequently used
than “compositional.” It is used because the freezing
temperature associated with the supercooling of seawater
is caused by compositional change of the material (i.e.,
the diffused solute in the seawater ahead of the freezing
front). The term “constitutional supercooling” was intro-
duced by
Rutter and Chalmers
[1953] to explain the
breakdown of planar into cellular interface when liquid
mixtures freeze. It was also used by
Harrison and Tiller
[1963] and
Lofgren and Weeks
[1967] to explain the
entrapment of brine between crystals and within the
same crystal along subcrystal boundaries. Compositional
supercooling appears to be an appropriate term and will
be used in this topic.
Compositional supercooling is the prime factor in
determining the vertical ice growth at the ice‐water inter-
face. It causes instability of the otherwise would‐be
planer ice‐water interface. The latter is common in lake
ice water and possible only in ice that is formed from
water of low salinity, especially when it grows at a slow
rate. The instability metamorphozes the planar interface
into a convoluted form, called dendritic or cellular
structure. This process leads to the entrapment of salt
Figure 2.18a
A small ice island in Wellington Channel near
Resolute in May 1993 exhibiting layered ice structure in the
box of the vertical crack surface, highlighted in the insert (photo
of N. K. Sinha, as a scale, taken by M. Shokr, unpublished).
100mm
Figure 2.18b
Surface of the ice Island shown in Figure 2.18a
showing holes (air bubbles) with diameter range between
10 and 100 mm.
and 50 km wide with thickness of about 40 m. There were
speculations that shock waves in the atmosphere set up by
the Russian nuclear tests on the other side of the Arctic
Ocean started to develop cracks in this shelf. Three or
four large ice islands were calved from the WHIS in 1961
[
Pounder,
1965]. They drifted away in the Arctic Ocean and
one of them, named T3, was used by the U.S. Air Force for
several years. Remotely sensed, Radarsat 2 images of
WHIS of 2010, along with images of an ice island can
be seen in section 5.2.4. Since ice islands originate from
frozen snow over land, their crystallographic structure is
characterized with layers of large air bubbles, some are
visible at the crack surface. Figure 2.18b illustrates large
holes on the surface of the same ice island shown in
