Geology Reference
In-Depth Information
nonbasal lattice dislocations in ice [
Sinha
, 1977b] and
near‐surface and internal structures of external stress/
strain‐induced crystalline defects, such as pileup of dislo-
cations [
Sinha
, 1978a], and, especially for sea ice, the sub-
grain structures and entrapped brine and air in the form
of pockets. This method has recently been extended to
make sections with thicknesses down to about 0.1 mm
required for fine‐grained granular or snowice or snow
masses with crystal sizes of fractions of 1 mm [
Satyawali
et al.,
2003;
Klein‐Paste et al
., 2007].
Double microtoming refers to the method of finishing
the top and the bottom surfaces of the thin sections sepa-
rately without exposing the sections to any melting at any
stages during the section preparation. Consequently, the
entire process is solid state and ideal for carrying out thin
sectioning of ice (freshwater as well as seawater) at any
temperature without subjecting the material to any ther-
mal and hence thermomechanical shocks. Mechanical
shocks, such as slight increase in surface defects in the
form of lattice dislocations and dislocation loops, induced
during the removal of materials, can be minimized or
reduced to negligible levels [
Sinha
, 1977b]. DMT is ideal
for sea ice and can be used well below the eutectic point
of the predominant salts in the brine pockets.
Sea ice samples in the form of cores and/or rectangular
blocks, with markings indicating their physical orienta-
tion (such as north), should be recovered only when the
ambient temperatures are as cold as possible. The ice
samples must be shipped and stored at −30 °C or below to
avoid any possibility of brine migration. Before com-
mencing any sectioning work on sea ice, the core or the
block of ice must be thermally stabilized by storing them
inside the cold laboratory, maintained as close as possible
to the working temperature. They are to be stabilized in
order to minimize mechanical shocks during band sawing
and also to prevent changes of geometrical properties of
brine and gas inclusions. Ideally, as shown by
Sinha
[1977a], the working temperature of −30 °C was the best
for sea ice, but, then, he also collected the samples at
ambient air temperature of about −30 °C. Such a work-
ing temperature is extremely difficult for manual dexter-
ity required for thin sectioning. A compromising
temperature could be around −20 °C, close to the eutectic
temperature of NaCl.
The thin sections are prepared from thick sections. The
latter are cut from thermally stabilized samples of ice
blocks, using a band saw, approximately 5 or 10 mm in
thickness, depending on the conditions of the saw and
blade, as described earlier for hot‐plate techniques. Thin
sections are prepared from thick sections by removing
layers of ice using a sledge microtome. This is an instru-
ment that allows removal of very thin layers of a speci-
men using a steel or diamond blade. The blade shaves off
a thin layer with each pass. This instrument is commonly
used in medical and biological laboratories and was
developed for obtaining thin layers of frozen tissue or
other organic objects.
In the DMT technique, a thick section (say, 5 mm) is
mounted on a clear glass plate by freezing a few drops of
cold near‐freezing water at the edge; making sure that no
water enters the space between the glass and the section.
An eyedropper with a long nose is most appropriate for
this purpose. Moreover, the water should be cooled to
about 0 °C by making use of an ice bath consisting of
distilled water with crushed ice in it. The exposed surface
of the section is then microtomed to a mirror finish in
four stages. In the first stage, layers of 50
μ
m thick each
are removed in each “pass” of the microtome's blade as it
continues to shave the surface. The second stage starts
when the thickness of the section reaches approximately
3 mm. Here, a layer of 500
μ
m is removed from the sur-
face, taking only 10
μ
m at each pass of the blade. In the
third stage the next 200
μ
m is removed in 5
μ
m layers
while cleaning the microtome's blade with a soft tissue
paper once after a few passes. The final stage, which gives
the final finish to the section, is achieved by removing
another 50
μ
m from the surface in 1-2
μ
m layers, ensur-
ing that the surface is clean before each pass. The quality
of the finished surface has to be visually examined using
reflected light from a distant source and an optical
microscope. If no cracks are found, the section is removed
from the glass plate by carefully cutting of the bonding
ice at the edge with a sharp razor blade. It should then be
remounted on another clean glass plate with the finished
surface facing the glass. It is preferable at this stage to
build up a dam of built‐up ice completely around the
edge of the ice sample, making certain that no water
enters beyond the outer edges of the thin section. This
dam of extremely fine‐grained freshwater ice prevents
the slice from sliding during the rest of the thinning pro-
cess. It also prevents moisture going and freezing between
the glass surface and the bottom of the ice section. The
other surface is then microtomed in steps, following the
above procedures, until the thickness reaches the desired
level and the top surface has a mirror finish. The thin
section should optimally be between 0.4 and 0.8 mm as
pointed out earlier; the thinner the section the better the
information that can be revealed and resolved. However,
a thicker specimen, around 0.8 mm, is often desirable for
measurements related to brine layer spacings and related
studies in FY sea ice [
Nakawo and Sinha
, 1984;
Sinha and
Zhan
, 1996].
The success of microtoming depends upon the condi-
tion of the microtome's blade and the care with which the
surface is prepared. Slight damage to the blade can cause
the formation of rows of dislocations (removal of atoms)
in the microtoming direction. Microtoming should
produce flat and smooth surface without introducing
