Geology Reference
In-Depth Information
the solid line with steps (25 mm) marked as curve
a
in
Figure 5.18. The top surface layer of the ice cover was
white in appearance due to its high porosity (density < 800
kg/m
3
) and could be mistaken readily as “recrystallized,”
granular, or superimposed snow ice, even by sea ice
experts, but uninitiated observers, without performing
careful cold‐state (no warm glass plates used) thin sec-
tioning and polarized light analysis. Extremely careful
thin sectioning by following the DMT (described in sec-
tion 6.2.2) showed, without any ambiguity, that the grain
structure of this highly porous layer was actually columnar‐
grained S3 type as will be seen later. About 10 mm below
the surface, as can be seen at curve
a
, the density increased
rapidly with depth. This is also shown by the broken
line
b
for which the density measurements were obtained
from the mass and volume of a set of large machined
specimens with dimensions of 50 × 101 × 250 mm used for
determining the strain rate sensitivity of biaxially con-
fined compressive strength [
Sinha
, 1985b] and thereby
more representative of the bulk ice characteristics. The
density increased from 905.0 kg/m
3
, at a depth of 0.05-
0.10 m below the top surface, to 915.9 kg/m
3
for a depth
of 0.43-0.48 m. Thus, except for the top 10 mm, density
of SY ice was very close to the density of pure ice, 917.8 kg/
m
3
(at −10°C). The average density of SY ice obtained
from 16 large specimens was 910.3 ± 4.1 kg/m
3
, which
yielded an air volume of 8.2 ± 4.5‰ (ppt) at −10°C, using
the density of pure ice as 917.8 kg/m
3
presented for refer-
ence as the broken line in Figure 5.18 for comparison.
Average density of SY ice cover at station 9 was found
to be 913 ± 5 kg/m
3
and, therefore, very close to that at
station 3.
The FY ice, outside the areas of SY ice, had an average
bulk salinity of 4.4 ppt (‰) while the underlying new
growth below the SY layer had an average bulk salinity of
about 3.7 ppt. Thus the salinities of the two varieties of
saline ice were comparable to each other. The FY ice was
rather opaque, had layers of frazil ice at the surface, and
the average density was 902 ± 15 kg/m
3
. However, below
the frazil layer, the ice was columnar‐grained S3 type. The
SY ice cover, as expected from its recorded growth his-
tory, was columnar‐grained through its entire depth
including the old/new ice interface as will be seen later.
The mean
c
axis of the grains in the horizontal plane
tended to be parallel (±15°) to the longitudinal axis
(nearly north‐south orientation) of the channel and
hence the direction of the tidal current, similar to previ-
ous observations in Mould Bay [
Sinha
, 1983b, 1984]. The
aging processes that led to the complete desalination in
the old ice did not affect the texture and the anisotropic
fabric of the ice.
One of the most common tasks performed by sea ice
engineers and other investigators is to take vertical cores
and measure the vertical profile of salinity. This is driven
by the fact that both tensile and compressive strength and
related deformation properties of sea ice can be linked
very closely to ice salinity and hence the estimated poros-
ity based on assumed salinity‐porosity‐temperature rela-
tionships. No considerations are given to the physics of
microstructure‐sensitive physical properties of sea ice. It
is natural, therefore, to present vertical salinity profiles of
sea ice without, in general, performing any time‐consum-
ing “unnecessary” forensic type of microstructural analy-
sis. Following this tradition, the most notable aspect to
be noticed in SY ice cover is the change in density
(Figure 5.19a), salinity across the transition zone between
the old (SY) and the new ice as shown in Figure 5.19b.
It is difficult to understand or explain this macroscopic
observation without performing any microstructural
investigations. However, microstructural investigations,
as illustrated in Figure 5.19c, bring out the characteristics
of fundamental importance for the ice cover and the
aging processes. Note the continuation of columnar
growth through the interface in Figure 5.19d and 5.19e
and the preservation of the fabric (
c
‐axis orientation) and
increase in the substructure across the boundary. The
transition zone between SY and the new ice (FY) is
marked not only by a rapid increase in salinity from 0‰
to 3.7‰ over a short distance of 0.05 m but also by a thin
Salinity, S,
‰
0.1 m
Density,
ρ
, kg/m
3
800
850
900
950
0
2
4
0
(a)
119
0.1
Test
no. 121
120
0.2
(b)
122
123
0.3
124
0.4
125
0.5
0.6
ρ
= 917. 8
0.7
0.8
Figure 5.18
Vertical density and salinity profile of columnar‐
grained S3‐type SY ice and salinity profile of new S3 ice
at station 3 in Mould Bay in April 1983; bubble structure in a
0.1 m wide and 0.4 m deep thick section of SY ice is shown
on the right. The vertical broken line represents density
(917.8 kg/m
3
) of pure ice as a reference [
Sinha
, 1985b].
