Geology Reference
In-Depth Information
section minimized the mechanical or thermal damage and
protected the delicate and fragile bubbly ice. The photo-
graph taken through transmitted cross‐polarized light
(Figure 4.38a) clearly brings out the characteristics of a
few vertically oriented columnar grains—starting immedi-
ately from the top of the ice surface. The structure of the
air bubbles, or strictly, the voids can be seen in Figure 4.38a
taken with scattered light by illuminating the thin section
with diffused light from the sides and switching off the
transmitted polarized light.
Although the bright white color of the top section
of the core may lead one to think that it was snow ice,
no granular ice can be seen here. Obviously, this layer
of the hummock was part of the bulk ice that had existed
at some depth below the surface in the original FY ice.
Due to the convex shape of the hummock, it is unlikely
that newer ice is formed on the surface in subsequent
seasons. The relatively large black areas appearing at
corresponding locations in both of the two photographs
represent large voids. The horizontal line shows a crack
formed during the microtoming and handling processes.
The most important feature shown in the scattered‐light
image is the large number of highly connected voids. In this
case, the air bubbles lose their individual identity and form a
continuum. The measured density averaged over the 140 mm
depth of the top layer was 620 kg/m
3
. This corresponds to an
air volume fraction of 38%. This value is too high for air to
be considered as an inclusion within the background of
pure ice as a host material. Therefore, under this condition
the application of a two‐phase dielectric or scattering model
with a dominant host (pure ice) and a minor inclusion (air),
as presented in section 3.6.2, may not apply.
Thin‐section photographs from another hummock ice
core are presented in Figure 4.39. It shows a vertical section
from 140 to 280 mm depth. Large columnar crystals tilted
to the vertical direction are readily visible. Their major
(a)
(b)
10 mm
10 mm
(c)
(d)
10 mm
Figure 4.39
Vertical section of a MY hummock core, obtained from Lancaster Sound in May 1992 (depth range
140-290 mm), photographed (a) between crossed polarizers and (b) using diffuse light. A horizontal section at
140 mm depth is shown in (c) and (d). The white areas in the photographs taken using diffuse light represent bubbly
areas [
Shokr and Sinha,
1994]. (For color detail, please see color plate section).
