Geology Reference
In-Depth Information
(a)
(b)
C
20 m
m
20 m
1 mm
Figure 5.24
Double‐microtomed, 100 mm × 200 mm, horizon-
tal thin section at a depth of 368 mm from the SY ice surface,
respectively, under (a) parallel and (b) cross‐polarized light; the
arrow indicates the long axis of Mould Bay (N. K. Sinha,
unpublished).
Figure 5.25
Optical micrograph of the surface of a microtomed
horizontal section showing thermally etched subgrain bounda-
ries and cross sections of irregularly shaped air bubbles inside
a grain (strictly speaking a family of subgrains) of SY ice; the
arrow indicates approximate
c
‐axis orientation of the family of
subgrains [
Sinha
, 1985b].
detailed fabric of the material. However, the presence of
brine pockets along the boundaries of the substructures in
FY ice help, somewhat, in delineating the structural details.
The microstructural details seem to disappear as brine
drains out completely, but the small mismatches in the
lattice orientations of the subgrains and air pockets remain
locked in the ice. Thermal etching technique (see sec-
tion 6.4.3) can easily be applied in the field and indeed add
a new dimension to the possibility of extending forensic
investigations to the study of ice substructure. An example
is shown in Figure 5.25. It shows etched subgrain bounda-
ries and the structural details of entrapped air pockets and
their locations with respect to these boundaries.
It is obvious that virtually complete desalination of sea
ice under isothermal conditions during the final stages of
melting occurs predominantly by gravity drainage. Unlike
the early growth period when brine channels play impor-
tant roles, drainage during the summer melt must occur
primarily through grain and subgrain boundaries because
of the absence of open brine channels in mature FY ice.
Brine channels formed near the growth front are eventually
filled as the ice‐water interface progresses downward during
the growing season. The flushing of meltwater aging pro-
cesses does not affect the fabric of the ice. The completion
of the desalination processes leads to the strengthening of
the grain and subgrain boundaries and hence closure of
these boundaries for subsequent upward migration of brine
(e.g., due to buoyancy forces) when the drainage comes to a
virtual end and the new growth starts to occur with the
advent of the winter season. The SY ice grains and sub-
grains with their basal planes oriented in the vertical planes
and parallel to the growth direction, and their
c
axis favora-
ble oriented along the direction of water current act as the
source for nucleation of the new growth. Since the SY ice
cover stayed in the bay and most probably did not rotate
more than the experimental uncertainty of determining the
axis of the bay or the direction of the tidal water current,
the new growth is subjected to the usual water current as in
the earlier growth season. The bulk structural characteris-
tics of the old ice are, therefore, preserved in the new ice
except for the enhanced entrapment of brine in the matrix
of the new growth. This observation led to an important
hypothesis that S3 type of saline ice can be grown in the
laboratory tanks, without imposing any water current, by
using plates of horizontally oriented S3 ice obtained from
the field and allowing the plates to float in the laboratory
tank water at appropriate temperatures and salinity. This
concept has, actually, been successfully applied by using S3
type of FY bulk ice samples (away from the top and the
bottom surfaces) from Mould Bay, 1981-1982 season, as
seeds by Sinha (unpublished) at the NRC laboratories in
Ottawa. A special, 2 m deep insulated, sea ice tank with
pressure‐relief devices and appropriate controlled side heat-
ing system was made for this purpose. No effort was made
to introduce any water current. Simulation of reversible
tidal water current with the appropriate thermal regime and
