Geology Reference
In-Depth Information
different depths is shown in Figure 4.56. Bubble density
number decrease with depth. The data in the figure, how-
ever, cannot be generalized because the depth of the bub-
ble‐rich layer in hummock ice varies considerably between
a few centimeters to a few tens of centimeters.
Ice covers in the small and protected waters of Allen
Bay, near Resolute, often survives the summer melts to
become second-year ice covers. Although year-round
studies of the ageing of ice in this bay and microstruc-
tural evolutions, unlike those of Mould Bay (Section 5.1)
was never performed, the proximity of SY ice allowed
the NRC researchers to perform statistical analysis of its
microstructures. However, due to the absence of tidal cur-
rents in the protected bay, the ice was found to be S2 type
instead of S3 type in M.B. In a study by
Johnston
[1998]
air bubbles in SY ice were characterized in terms of their
area, major, and minor axes. Data were obtained from
digitized images of thin section photographed between
cross polarizer with added scattered light. An example
is shown in Figure 4.57. Here the bubbles appear as dark
objects against the background of colored grain. The
circularity of the bubbles is visible. The probability distri-
butions of the area of bubbles as well as major and minor
axis in the horizontal plane are shown in Figure 4.58
(Compare the bubble structures in Figure 5.22a and influ-
ence of water current in Mould Bay SY ice). The mean
major and minor axis length is 3 and 2 mm, respectively.
These values agree with the data in Table 4.2 for M.B.
second-year ice., although the probability distributions
show a long tail that extends to 10 mm for the major axis.
The near circular cross section of the air bubbles for ice
in Allen Bay (Figure 4.57) can be associated with the
orthotropic or transverally isotropic structure of the S2‐
type columnar grains found in the small bay not affected
by any preferred direction of the water current during the
initial growth. The growth habits of the grains are known
to depend strongly on the water current. Preferred water
current produces S3 type of transversely anisotropic
columnar grains in young and FY ice. This structure pre-
vails when the ice cover ages and become SY ice. The
cross sections of air bubbles in S3 type of ice, therefore,
exhibits pronounced ellipticity in the cross‐sectional
shape of the air bubbles. This is demonstrated in thin sec-
tions at a depth of only 3 mm below the surface in
Figure 5.22 and 13 mm below the surface in Figure 5.23
of SY ice in Mould Bay to be presented in section 5.1.
0
20
40
60
80
100
120
140
0.00
0.02
0.04
0.06
0.08
0.10
0.12
Probability of bubble occurrence
Figure 4.56
Probability of occurrence of air bubbles in MY ice
at discrete depths [
Shokr and Sinha
, 1995].
1 cm
Figure 4.57
Horizontal thin section at 100 m depth from SY
ice in the Allen Bay, Canadian Arctic, obtained in April 1997,
used to quantify the geometry of air bubbles [
Johnston,
1998].
0.40
Mean area of bubbles
in the horizontal
plane = 4 mm
2
Mean minor axis length of
bubbles in the horizontal
plane = 2 mm
Mean major axis length of
bubbles in the horizontal
plane = 3 mm
0.40
0.40
0.30
0.30
0.30
0.20
0.20
0.20
0.10
0.10
0.10
0.00
0.00
0.00
0
48
Area of bubble (mm
2
)
12
16
20
24
28
32
0
235
Major axis of bubble (mm)
7810 12 13 15 17
0
134578910 12
13
Minor axis of bubble (mm)
Figure 4.58
Statistics of geometry of air bubbles in SY ice from Allen Bay (1997) measured in a horizontal plane
at 0.10 m depth [
Johnston,
1998].
