Geology Reference
In-Depth Information
Table 4.2
Second‐year ice bubble characteristics measured in Mould Bay in the spring of 1983.
Percentage
Mean Dimension (mm)
Number of
Bubbles
Depth (cm)
Circular
Elliptical
Average Bubble Density
Circular
Elliptical
1048
1303
363
297
269
418
246
123
239
73.4
61.3
63.4
68.0
70.0
57.8
59.8
58.8
85.2
26.6
38.7
36.5
32.0
30.0
40.2
40.2
41.2
18.8
1.81 ± 7.9
37.2 ± 21.8
12.1 ± 7.5
9.9 ± 3.8
8.9 ± 2.8
13.9 ± 4.1
9.8 ± 3.2
4.9 ± 2.3
8.8 ± 3.4
1.15 ± 0.47
0.99 ± 0.84
0.33 ± 0.23
0.69 ± 0.29
0.64 ± 0.27
0.69 ± 0.26
0.64 ± 0.26
0.52 ± 0.20
0.62 ± 0.35
1.07 ± 0.39
1.45 ± 1.19
1.02 ± 1.05
0.94 ± 0.44
0.81 ± 0.33
1.04 ± 0.37
0.98 ± 0.39
0.75 ± 0.27
1.26 ± 0.79
0.62
3.0
3.8
5.9
8.0
10.1
12.1
14.5
15.1
Note
: Measurements were conducted on photographs of horizontal thin sections. The average bubble density is presented as
area/m
3
× 10
−4
[
Bjerkelund et al.,
1985].
Table 4.3
Geometrical parameters of air bubbles
in multiyear ice.
A few studies on the geometrical characteristics of air
bubbles in MY ice were conducted in the 1990s. Bubbles
in MY ice are much larger than those in FY ice. While the
mean value of the major axis lengths of bubbles on FY
sea ice is typically on the order of tenth of millimeters,
bubbles in MY ice are measured in millimeters.
The first results on geometric characteristics or air
bubbles in SY sea ice appeared in
Bjerkelund et al.
[1985] from a study of ice in Mould Bay, Canadian
western Arctic in 1983. This ice is described in details
in section 5.1. The authors delineated air bubbles man-
ually in photographs of horizontal thin sections and
performed the measurements also manually. They
found that the bubbles were predominantly circular
and located at the subgrain boundaries. Eighty‐two
percent of the circular bubbles had diameters of less
than 1 mm. The average diameter of the circular bub-
bles was 0.69 ± 0.2 mm, and the average long axis of the
elliptical bubbles was 1.04 ± 0.2 mm. Table 4.2 includes
characteristics of bubble geometry, dimensions, and
density at different depths.
Shokr and Sinha
[1994] identified air bubbles in MY
hummock and melt pond ice from digital images of verti-
cal thin sections. The ice was obtained from Parry Sound in
the central Arctic in May 1992. A data set of 5 hummock
and 5 melt pond cores was used. A digital image analysis
technique was developed to identify bubbles and to meas-
ure the geometrical parameters of each bubble. The small-
est bubble that can be identified has a diameter of 0.1 mm.
From a total of 520 bubbles in hummock ice and 1496 bub-
bles in melt pond ice, bubble statistics were derived in terms
of three p
aram
eters: (1) of the circle equivalent diameter
D
(equals 4
A
where
A
is the area of the bubble), (2) the
distance to the nearest bubble
S
, and (3) the compactness
factor
C
, which is a measure of bubble shape defined as
Ice Type
Parameter
Mean
Std. Dev.
Hummock ice
D
2.58
1.96
C
0.62
0.19
S
4.03
1.73
Melt Pond ice
D
2.36
0.83
C
0.68
0.12
S
—
—
Note
:
C
is the ratio defined in equation (4.10) and
D
and
s
are in millimeters.
where
p
is the bubble perimeter. The ratio
A
/
p
2
has its
maximum value (
4
π
= 0.0795) for a circle. Hence
C
is a
measure of the deviation of a bubble cross‐section shape
from a perfect circle. As
C
approaches 1, bubbles become
more circular, and as it approaches 0, they become very
narrow relative to their length (i.e., take a needle shape).
The statistics are included in Table 4.3. The average diam-
eter of air bubbles in melt pond ice (2.58 mm) is slightly
larger than that in hummock ice (2.36 mm). The differ-
ence is attributed to the presence of large bubbles in the
snow‐ice layer that commonly exist at the top of the origi-
nal ice in a melt pond ice. The average diameter of bub-
bles in the snow‐ice layer in melt pond ice is found to be
around 4.8 mm. The shape factor of bubbles in both
hummock and melt pond ice is equal (around 0.65), indi-
cating that the overall bubble shape is similar in both
types. This means that the overall bubble shape is not far
from being spherical. Nevertheless, bubbles of snow‐ice
origin in melt pond ice were found to be more spherical,
with an average
C
= 0.83. The average distance to the
nearest bubble in melt pond ice is 4.03 mm. This distance
was not calculated for bubbles in hummock ice since most
bubbles were highly convoluted with a significant overlap
in their projection. The probability of bubble existence at
2
(4.11)
CA p
/0 0795
