Geology Reference
In-Depth Information
Figure 6.31
Thermally etched surface, normal to the long axis of
a grain of columnar‐grained S3 sea ice, exhibiting brine/air pock-
ets, subgrain boundaries, and stepped patterns, corresponding to
the network of dislocations, inside one grain; orientation of
c
axis is shown by <
c
> (micrograph by N. K. Sinha, unpublished)
Figure 6.30
Triple points of grains (actually subgrains in neigh-
boring grains) with and without brine pockets and elongated
basal dislocation pits, parallel to
c
axis, after 72 h of thermal
etching at −10 °C. Orientation of
c
axis is shown by <
c
>.
(micrograph by N. K. Sinha, unpublished).
dislocations intersecting the prismatic surfaces has to come
from chemical etching to be presented later. It should be
emphasized here that thermally etched dislocation pits like
these have not been published in the sea ice literature.
Figure 6.31 illustrates the detailed substructures within
a grain of columnar‐grained S3 type of FY sea ice with
salinity of 4.3
o
/
oo
at a depth of 1.83 m from Strathcona
Sound, Baffin Island, Canada. In this case a thin section,
cut normal to the length of the columns, was prepared
first by the DMT technique at −10 °C. Although a tem-
perature of −23 °C or lower is preferred for making thin
sections of sea ice, the higher temperature was chosen
purposely to have the brine pockets full with liquid. The
mirror finished clean surface was then coated with a thin
and uniform layer of 3% Formvar solution and allowed
to dry quickly under the ambient low humidity (about
5%) conditions of the cold room. In less than an hour,
once the film was dry, the section was kept inside a ther-
mal etching box covered with a glass plate, and the ice
surface was monitored through the dried film under an
optical microscope using oblique transmitted light.
Within one hour, the etched patterns started to develop.
The large‐angled grain boundaries (large mismatch in
both
c
axis and
a
axis)) with high energy levels can be
thermally etched within a few hours. Boundaries with
lower energy levels, such as subgrain boundaries having
very small lattice mismatch in the
c
axis across the bound-
aries, take longer durations. This is particularly applica-
ble to sea ice for which there are more subgrain boundaries
than grain boundaries. In fact, long‐range intercrystalline
boundaries with large angles of lattice mismatch,
commonly seen in freshwater ice, do not exist in FY sea
ice. The physical dimensions of the large‐angle bounda-
ries are limited by the dimensions of the subgrains.
Moreover, the entrapments of brine (and air) pockets
along these boundaries add another level of complexities.
Brine pockets are also trapped at numerous intragranular
platelet or subgrain boundaries where small mismatch of
lattice orientations also occur. These subgrain bounda-
ries are also eventually thermally etched. This is why sur-
face features can be readily seen in freshwater ice, but it
takes longer for them to develop in sea ice. Moreover,
relatively lower energy levels of subgrain boundaries in
sea ice lead to shallower and finer etching features when
thin sections of sea ice are exposed to thermal etching. In
any case, while using optical microscopes care should be
taken to focus on the etched features at the top surfaces
of the sections because the features at the bottom sur-
faces close to the glass surfaces of the plates holding the
specimens are also etched. If not properly focused, etched
boundaries at the top as well as the bottom are visible,
either overlapping each other or side by side (boundaries
are rarely vertically down in case of horizontal thin sec-
tions), creating unnecessary confusion.
Optical microscopy of the prepared surfaces after ther-
mal etching reveals that the distribution of brine pockets is
related to the subgrain boundaries or the bridging surfaces
of the dendrites formed at the ice‐water interface during
freezing. Details of brine pockets and the precipitation
pattern of the enclosed salts could be observed by replicat-
ing, to be presented in Section 6.4.4, the microtomed sur-
faces and examining the replica with a scanning electron
