Geology Reference
In-Depth Information
their basal planes in the vertical plane or their
c
axis in
the horizontal plane. For such growth conditions, the
impurities can be pushed down to the ice‐water inter-
face. Note the smooth transition in the grain structure.
The air bubble contents in this MP ice core are similar
to observations in hummock ice. The only difference,
however, is the small size bubbles that are spread
between large bubbles. Similar to hummock ice, the
bubbles diminish abruptly at a location that marks the
transition from snow ice to the retextured ice.
surfaces among neighboring crystals form a continuous
network of interfaces known as grain boundaries. These
boundaries can be used to identify individual ice crys-
tals in photographs of thin sections. Subsequently, geo-
metric characteristics of ice macrostructure (grain and
subgrain scale) can be obtained. Grain boundaries can be
delineated either manually or using digital image process-
ing techniques.
Barrette and Sinha
[1994
/
96] used both
approaches to study shelf ice in the Ward Hunt Ice Shelf
in the Canadian High Arctic described in section 5.2.2.
However, it is well known that sea ice microstructural
features are significantly more complicated than glacier
or freshwater ice due to the complicated structure of
grains with overwhelming presence of subgrains dis-
cussed earlier in Chapter 2 (specifically section 2.3.2).
Johnston and Sinha
[1995] presented a quantitative
dimensional analysis of grain and subgrains of FY sea ice
using an extended technique to the methodology devel-
oped in
Barrette and Sinha
[1994
/
96]. They examined the
macrostructural, grain‐ and subgrain‐scale characteristics
of natural columnar‐grained S3‐type FY sea ice sampled
from Frederick Hyde Fjord (FHF) (83°11'N, 29°50'W)
northern Greenland in May, 1994. They were probably
the first to present a basis for quantitative metallurgical
type of analysis of FY sea ice characteristics and thus the
fundamental parameters of the scattering layer that trig-
ger the radiation or the backscatter signals measured
from microwave remote sensing (this is typically the upper
150 mm of the ice sheet). A brief description of the ice in
the FHF is introduced before presenting the techniques
and results of geometric characteristics of polycrystalline
ice structure.
Vast area of the ice in Frederick Hyde Fjord that grew
during the winter of 1993-1994 was rather unique due to
its extremely smooth surface, absence of any snow cover,
and uniformity in thickness. Lack of snow coverage
revealed deep turquoise‐blue ice throughout the fjord—
never observed by the second author of this topic during
his 30 year extensive travel experience in the Arctic.
The color of the FHF ice cover, spanning between the
longitudes of 24°W and 36°W, was deceiving when sur-
veyed by aerial reconnaissance. From the air, one could
easily conclude the central area of the long fjord as sec-
ond‐year if not older ice, but the usual surface roughness
characteristics of old ice were missing. Only a 3.5 km
long and 100 m wide section of the ice sheet was critically
examined for the purpose of using as a strip for the his-
tory making, first landing and operations of fully loaded
(64,600 kg) Boeing 727 aircraft on floating sea ice [
Pole,
1995;
Sinha,
1995]. The average thickness of the ice along
the airstrip was found to be 2.30 m with a minimum of
2.27 m and maximum of 2.39 m. Actually, the entire
fiord, with high mountains on either side, was protected
4.5. informaTion conTenTs in
PolycrysTalline ice sTrucTure
This section includes information that can be retrieved
from the crystallographic structures revealed in thin
sections of sea ice. The information is provided to cover
two items: (1) geometric characteristics of ice crystals
and inclusions (brine and air) and (2) biomass accumu-
lation at the bottom of the ice. Since information is
obtained from photographs of two‐dimensional sur-
faces of thin sections, the retrieved geometric parame-
ters are descriptions of cross sections of crystals and
inclusions (e.g., major and minor axes, cross‐section
area, brine layer spacing, etc.).
Perovich and Gow
[1996]
suggested that a three‐dimensional characterization of
the volume of ice crystals and inclusions would be ideal,
but it was not possible using current technology. This
suggestion has not found a way for fulfillment yet,
probably because of the large number of thin sections
needed construct just one 3D view. Quantitative metal-
lography or stereology is a comprehensive and widely
accepted body of experimental and analytical methods
for characterizing a three‐dimensional microstructure
from two‐dimensional sections. Yet, it has not been
used for sea ice.
In general, the studies on the geometric characteristics
of ice crystals and inclusions are limited in number and
scope, with no comprehensive review published so far.
Therefore, the information presented in this section, which
is selected from a few studies, may not be generalized.
Statistical characterization of ice crystalline structure is
needed to model the reflection, radiation, and scattering
of electromagnetic waves as they interact with sea ice. This
is important for interpretation of remote sensing observa-
tions and the retrieval of ice parameters.
4.5.1. Geometric Characteristics of Crystalline
Structure
A polycrystalline body of ice is an organization of
crystals of different shapes, sizes, and lattice orienta-
tions into a space‐filling mass. The mutual contact
