Geology Reference
In-Depth Information
core 86‐2 was at 36.08 m. These specimens were wrapped
in polyethylene bags and then in cardboard tubes for
shipment to Fairbanks, Alaska, by air and road using a
freezer, insulated boxes and dry ice. In November 1988
selected ice specimens were shipped to the NRC labora-
tory in Ottawa by air in insulated boxes with dry ice.
Luckily, all the shipments were completed successfully
without any melting or mechanical damages.
Prior to shipment to Ottawa, the liquid electrical con-
ductivity of the ice specimens had been measured. Once in
Ottawa, the specimens were allowed to equilibrate to the
cold‐room temperature (−10°C) before 160-170 mm long
specimens were cut from the originals. These pieces were
then mounted on a lathe and machined to final test speci-
mens with diameters of 67.7-74.8 mm, lengths of 142.4-
169.5 mm and length‐diameter ratios of 2.0-2.3. Final
dimensions were measured with a vernier caliper before
the specimens were weighed on an electronic balance prior
to bulk density determinations. Thirteen hummock and 11
depression specimens were prepared and tested.
The mean crystal diameter of one end of each speci-
men had already been measured in Fairbanks, and before
specimen preparation the mean crystal diameter of the
other end was also measured. Horizontal thin sections
were prepared by the DMT technique and photographed
in cross‐polarized light. Mean grain diameter was then
determined according to d (mm) = (4 / πN A ) −1/2 where N A is
the number of grains per unit area in mm 2 .
All the specimens from shelf ice exhibited a very low
level of dissolved impurities (a salinity of only 0.01‰ is
approximately equivalent to a conductivity of 165.0 μ S/
cm; and most specimens contain many bubbles, hence the
relatively low‐density values. Here, bulk density is treated
as a proxy for bubble‐free situation. The range of bulk
density values is 876-911 kg/m 3 , and the range of mean
ice crystal diameters is 5.3-10.1 mm. The bulk density
and ice crystal diameter data for the hummock (86‐1) and
depression (86‐2) specimens are plotted, respectively, in
Figure 5.41 and Figure 5.42. Although there is some scat-
ter in the data, the trend of increasing crystal size with
increasing bulk density is apparent. Jeffries et al. (unpub-
lished manuscript) characterize the ice in cores 86‐1 and
86‐2 as superimposed ice, the product of melting and
refreezing of snow packs on the Ward Hunt Ice Shelf.
Although the ice shelf superimposed ice forms in a man-
ner similar to superimposed ice on a glacier, it must be
emphasized that the Hobson's Choice specimens are not
glacier ice since it is known that the Ward Hunt Ice shelf
does not really have a glacier component. Since glaciers
are subjected to shearing forces as the ice body glides on
mountain slopes, ice from glaciers are subjected to com-
plex thermal and mechanical histories. Microstructural
analysis can assist greatly in identifying the texture and
structural characteristics of ice and hence the source.
0
-10
Hummock
-20
Depression
-30
East ward hunt ice shelf
(ice island) 1986 ice cores
-40
860
910
920
870
880
890
900
Density, kg·m -3
Figure 5.41 Depth dependence of density for the cores from
hummock and depression of Ward Hunt Shelf Ice of Hobson's
Choice ice island (N. K. Sinha, unpublished).
0
Hummock
-10
-20
Depression
-30
East ward hunt ice shelf
(ice island) 1986 ice cores
-40
0
5
10
15
Grain size, mm
Figure 5.42 Depth dependence of grain size for the cores from
hummock and depression of Ward Hunt Ice Shelf of Hobson's
Choice ice island (N. K. Sinha, unpublished).
Glacier ice tends to exhibit shearing features in the grains
and also different families of healed cracks as illustrated
earlier for Greenland glacier ice in Figure 4.28. Compare
the images in this figure with those of the Ward Hunt Shelf
Ice shown here in Figure 5.38 taken with cross-polarized
light and in Figure 6.13 illustrating micrographs taken in
both cross‐ and parallel‐polarized light.
In a detailed study of ice structure and stratigraphy of
the two cores, it has been found that both cores contain
equi‐axed isotropic ice as well as transversely isotropic
ice. The latter is indicative of refrozen melt ponds and
more common in the depression core, where ice speci-
mens are, on average, slightly more dense and have larger
crystals than hummock specimens (Jeffries et al., unpub-
lished). It is fair to assume that all the test specimens
comprise mostly equi‐axed isotropic ice, with a minor
transversely isotropic ice component most likely in the
depression specimens. Figure 5.43 shows the assemblage
of the grain based on boundary morphology at 5.1 m
depth of the hummock ice near a site close to the main
camp site in Figure 5.38.
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