Geology Reference
In-Depth Information
interface of 1.75 m deep FY sea ice sampled in early
March 1978 from Eclipse Sound near Pond Inlet, Baffin
Island (this application is briefly covered below in this
section). The goal was to explore the structure of the
growing front of FY sea ice, especially the thickness of
the delicate skeletal layer. For this reason an early period
in March was chosen when records showed that the ice
was still growing, albeit slowly. The primary goal was to
explore the area of transition from the dendrites of the
skeletal layer to the main body of ice near the ice‐water
interface.
Since the ice dendrites are known to be very delicate
and protrude in the water, special care was required to
preserve the structure during the sampling and storage.
The required environmental conditions included low air
temperatures (< −25 °C), extremely calm conditions to
prevent any blowing snow particles that may get depos-
ited on the ice core, and minimum of sunlight to avoid any
effects from the absorption of solar radiation. The idea
was to extract a 250 mm long section of the bottom of the
ice, drain out the excess brine while holding the core sec-
tion vertically in a bath containing kerosene. Brine, being
heavier than kerosene, sinks to the bottom of the bath.
The section was then lowered very gently in a long and
narrow bottle filled with kerosene at the ambient temper-
ature, making sure the bottom of the bottle is full with a
slush of crushed snow and kerosene in order to provide a
soft cushion. The bottle was then sealed, making sure
that the specimen is fully supported from the sides to pre-
vent any rattling inside the bottle. The bottle was stored
at −30 °C for about a month before commencing any pro-
cessing. Inside a cold lab at −30 °C, the section was care-
fully removed from the bottle and held vertically to drain
out excess kerosene and eventually allowing it to evapo-
rate as much as possible; the volatility of kerosene is low,
but it worked. The core section was first cut into two
halves, and then a vertical thick section and a horizontal
thick section were made as illustrated in Figure 6.15.
Thin sectioning was performed following the DMT
technique.
The scattered-light view of the horizontal section 50 mm
above the bottom (marked in Figure 6.15) is shown
in Figure 6.16a, and the combined view using cross‐
polarized and scattered light is shown in Figure 6.16b.
Note that the entire section area appears to be a large
“single grain” filled with an intragranular substructure
of platelets (or subgrains). The subgrain boundaries are
delineated by the presence of rows of tiny and almost
interconnected inclusions (bright objects) of air and
solidified brine pockets with salt crystals. The average
c
axis, indicated by the small double‐headed arrow (par-
allel to the cutting line of the core) is oriented in the hori-
zontal plane and normal to the longer dimension of the
subgrains in this micrograph.
Subgrains
Vertical
section
Horizontal
section
50 mm
Ice-water
interface
Figure 6.15
Sectioning of the core segment from the bottom of
the oriented S3‐type columnar‐grained sea ice from Eclipse
Sound (sketch by N. K. Sinha, unpublished).
Figure 6.17 is a vertical thin section prepared from the
thick section, as illustrated in Figure 6.15, photographed
using scattered light only. It shows the vertical section
cut parallel to the
c
axis or the smallest dimension of the
subgrains and, therefore, also parallel to the plane con-
taining the average
c
axis of the large grain. The vertical
bright streaks in Figure 6.17 are the rows of tiny brine
pockets. The white streaky areas at the bottom represent
extremely fine‐grained bubbly ice when pure water was
applied purposely after core extraction to freeze in
between the dendritic spaces. That way, the delicate
structure of the dendrites protruding originally in the
seawater, and dangling in the air when the brine drained
out immediately after removal of the core bottom, was
preserved. However, rapid freezing of the water gener-
ated extremely fine grain with extremely fine bubbles of
air. Note that many of the subgrains or platelets are as
long as 50 mm in the vertical direction, and their widths
(brine‐layer spacing or platelet width) are less than 1 mm.
The small double‐headed arrow in the inset indicates
the orientation of the average
c
axis at right angles to
the lengths of the platelets as illustrated schematically in
Figure 6.15.
Since the skeletal layer is presented in Figure 16.17 only
in terms of scattered and polarized light, it's appropriate
here to show some results obtained by usual micropho-
tography. For this purpose another core was taken from
