Geology Reference
In-Depth Information
earlier. However, during the autumn of 1983 and before
the onset of the freezing season, several old ice floes
with wide‐ranging sizes moved northward from Crozier
Channel into the channel of Mould Bay. These ice seg-
ments were frozen‐in when the water surface in Mould
Bay started to freeze during late September of 1983.
When the ice returned to Mould Bay during the autumn
of 1983, it included in its matrix a conglomeration of old
ice floes grouped at the south end of the bay. The AES‐
Star‐1 (operated by Intera Conquest), synthetic aperture
radar (SAR) imagery in Figure 5.32 clearly highlights these
floes. This imagery of the Mould Bay ice cover was recorded
on 10 March 1984 when the winter conditions were prevail-
ing. The floes show up as relatively bright features against a
black FY ice background. The advent of these floes was
quite exciting because until then the microstructural aspects
of research at Mould Bay were concentrated on FY and SY
ice. The primary interest of the research team was to “make
history,” again for the first time, by determining the age of
the old ice floes as well as their growth characteristics from
the analysis of their microstructures.
A large number of old floes with different sizes were
frozen into the matrix of predominantly FY ice type that
grew during the1983-1984 winter. The smaller, bright
targets on the imagery are either piled up rubble, small
old ice floes, or a combination of both. The term old floe
as used here is ice that has survived two or more melt
seasons. Multiyear ice is further distinguished by two
main characteristic: (1) its rolling, hummocky surface, the
result of differential melt, and (2) its salinity profiles rang-
ing from very low to values as high as 3.0 ppt. The SAR
imagery shows a number of closely spaced old floes fro-
zen into the ice matrix at the south end of the bay. Many
of these floes were examined in a rudimentary manner,
but two (marked as MY‐R and MY in Figure 5.32) were
studied extensively. The image is also marked with a white
line across the bay along which the old experimental line,
similar to April 1983, was established with oil barrels for
10 sampling sites. The floe MY‐R along the experimental
line was a small old floe with rounded ridges. The MY
floe was rather large and had many relatively flat areas of
apparently refrozen melt pond.
Discussions on the structures and textures of the MY
ice is thought to be enhanced significantly by concurrent
presentations, if possible, on the relevant details on FY
ice subjected exactly to the same thermal and growth his-
tory for at least one season. The characteristics of the
matrix of FY ice, shown in the SAR image of Figure 5.32,
therefore, can be used as the benchmark for FY ice for
comparing the nature of new growth at the bottom of
MY ice during the winter of 1983-1984.
The annual 1.9 m thick ice at station 9 along the experi-
mental line in an area of smooth ice, close to the western
shoreline, exhibited columnar‐grained S3 type of ice
Subgrain
Boundary
Interface
1 mm
Figure 5.31
Thermally etched surface of a vertical thin section
near SY/FY interface in Mould Bay on 5 April, 1983 exhibiting
continuity of subgrain boundaries (thin lines) from SY (top half)
to the cloudy new ice (bottom half) (micrograph of N. K. Sinha,
unpublished).
subgrains (or platelets) with their
c
axis in the horizontal
plane or basal plane in the vertical plane, favorable for
growth, acted as the seed crystals. As can be seen in the
micrograph, the subgrains and hence the columnar gains
simply continue to grow down when the winter arrived due
to their favorable crystallographic orientations for growth.
However, the rapid growth rate, obviously, allowed micro-
scopic inclusions of air and brine to be trapped not only
along the subgrain boundaries but even inside the subgarins.
A measure of the width of the subgrains in the horizontal
plane of SY ice can be judged from Figure 5.25.
What can happen when an ice floe composed of S3‐type
layers of SY ice and FY ice survives through the third
summer melts and starts to grow again? Will the S3 struc-
ture continue to grow? With anticipation, visual observa-
tions were continued at the weather station of Mould Bay
on the SY‐FY ice sheet during the summer melt of 1983.
Unfortunately, the entire composite ice cover in Mould
Bay did not survive the melt season of 1983.
5.1.4. Multiyear Ice and Interfaces: Mould Bay
1984 Experience
For sea ice historians, unfortunately to some extent, due
to melt down of the entire ice cover during the summer of
1983, the history of Mould Bay during the autumn of
1981 was almost repeated during the late summer of 1983.
But, new opportunities were provided by nature. The
composite SY‐FY ice cover, investigated in March-April
1983 as described above, moved away from the channel.
Lack of strategic supports (usually the case) did not allow
the researchers any opportunity to conduct aerial survey
during this period. This was a condition very similar to
that observed during the late summer of 1981 as described
