Geology Reference
In-Depth Information
in early March of 1994 is shown in Figure 2.33. Two views
are presented here. The horizontal thin section, observed
under parallel polarized light, clearly shows several brine
channels fed by smaller tributaries or “feed arms.” The
frequency of occurrence of these star‐shaped channels
and the distances between them seem to agree well with
previous observations mentioned earlier. The channels,
however, seem to disappear when the section was viewed
under cross‐polarized light. Careful examinations can
still recognize their presence, but that is more of an after‐
effect from seeing image under parallel polarized light.
Figure 2.34a shows a double‐microtomed horizontal
thin section of a cross section of a brine channel in
matured columnar grained first‐year ice at a depth of
1.86 m in early March 1985 in Mould Bay, Canada, when
the ice sheet was still growing, but obviously, very slowly.
This brine channel was not readily noticeable in the ice
core. It was noticed only when thin sections were made.
The section was photographed by a combination of
cross‐polarized and scattered light. The entire brine
channel was confined practically within a single large
grain. A few subgrains can be seen in different gray
shades, but most of them belong to one family of lamel-
lar structure. As usual, the orientation of the
c
‐axis
(indicated by < C>) varied slightly within this grain. The
direction of the tidal water current under the ice sheet
is indicated by the double arrow. This also indicates
that the average
c
‐axis orientation of the subgrains in the
plane of the section tends to be parallel to water current,
a characteristics of S3 type of ice.
(a)
(b)
Brine channel
20 mm
Figure 2.33
Parallel‐polarized (a) and cross‐polarized (b) photographs showing star‐shaped cross-sectional views
of brine drainage channels at -20°C in columnar grained S3 type first‐year sea in Northstar Bay, Greenland, in
early March 1994 (Macrographs of N. K. Sinha, unpublished).
(a)
(b)
10 mm
1mm
Figure 2.34
(a) Horizontal cross section of a “prior” brine channel inside a large grain of first‐year columnar ice
in March 1985 at a depth of 1.86 m in Mould Bay, Canada, photographed using combined cross‐polarized and
scattered light; <c > indicates trend in c‐axis and double arrow shows water current, (b) Optical view of new
crystals inside a matrix of oriented subgrains in a prior brine channel formed within a single grain (Micrographs
by N. K. Sinha, unpublished).
