Geology Reference
In-Depth Information
crack crossed the experimental line somewhere near sta-
tion 6, but the snow cover obscured the visibility of the
crack and the relatively large opening in the ice. One of
the investigators from the weather station fell through the
crack during the routine observations on 29 May and had
to be rescued from the frigid water. This large crack could
be noticed, almost a month later, when the snow started
to melt rapidly during the days of 25-29 June and a melt
pond started to develop, as can be seen in a photograph
(Figure 5.12a) taken from Thunder Mountain northwest
of the weather station. The photograph clearly shows
long and narrow strips of snow hummocks (bright areas)
oriented parallel to the length of the bay. These hum-
mocks were most probably formed by snow drifts caused
by the prevailing wind directions. The drift patterns then
affected the melting processes and orientation of the
drainage patterns and eventual surface roughness follow-
ing the disappearance of surface flooding. Figure 5.12a
also shows a circular darker (actually turquois) area, near
the top right corner, caused by localized melting around a
breathing hole of ice inhabiting seals. Incidentally, it was
noticed later that the breathing holes also acted as drains
for the meltwater to disappear quickly from the top sur-
face of Mould Bay ice [
Digby
, 1984].
This large crack, as can be seen in the picture
(Figure 12a) was not local. It extended from the eastern
to the western shore. The actual shape of this crack across
the entire bay and the point of crossing the experimental
line can be seen clearly in the SLAR image of 3 July 1982
shown in Figure 5.12b. The oil drums placed along the
experimental line acted as targets for high backscattering
of microwave radiation and can be seen in the image as
bright spots. This image also shows that there were a
number of large cracks along the length of the channel.
These cracks appear bright principally due to higher
backscattering of microwaves from the airborne radar (to
be discussed in later chapters) from uneven and drained
topography developed by the drainage system around the
fracture lines.
The surface flooding and the percolation of snow melt
through the ice matrix affected the shape of the vertical
salinity profile in the ice. The aging processes affected
the amount of entrapped brine and certainly the micro-
structural features in a very complex manner. The iso-
thermal state, close to the melting point, makes it
“impossible” to sample ice without disturbing the finer
aspects of the microstructure and the brine content. The
pronounced change in the shape of the vertical salinity
profile within a very short time was particularly notice-
able. The prominent C‐type shape usually seen in young
ice and early stages of FY ice, illustrated earlier in
Figure 5.7, changed to a Z‐ shape and then to a reversed
c
‐shape, like Ɔ, within about one month, as illustrated in
Figure 5.13. for ice at station 3. The measured salinity
profiles at other sites across the bay for 13 July, 1982 are
shown in Figure 5.14, which also shows the site-specific
reduction in ice thickness.
The average ice salinity on 13 July had decreased to val-
ues of less than 2 ppt throughout the ice cover as com-
pared to the values between 4 and 5 ppt on 13 June.
However, the in situ salinity distributions on 13 July could
actually be significantly different from the measured val-
ues. As pointed out earlier while discussing the results in
Figure 5.7, absolutely no emphasis should be placed on
the commonly reported Ɔ‐shape of the salinity profiles
when ice sheets are at isothermal conditions and ice cores
(a)
(b)
Crack
Figure 5.12
(a) Large crack (indicated by arrow), crossing the experimental line, shown in the photograph taken
by N. K. Sinha (unpublished) on 28 June 1982 from the top of nearby Thunder Mountain. (b) AES, X‐band SLAR
image recorded on 3 July, 1982 showing several shore‐to‐shore cracks across the bay. SLAR image clearly shows
the shape of the crack that crossed the experimental line indicated by the row of bright spots caused by high
backscattering of microwave signal from the oil drums placed on the ice.
