Crystallographic studies of sea ice in McMurdo Sound, Antarctica

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MBL/WHOI

VAM A

R 494

Technical Report :

CRYSTALLOGRAPHIC STUDIES OF
SEA ICE IN MCMURDO SOUND,

ANTARCTICA

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CRYSTALLOGRAPHIC STUDIES OF SEA ICE IN MCMURDO SOUND, ANTARCTICA
Technical Report R-494
Y-F0O15-11-01-026

by

R. A. Paige

ABSTRACT

The sea ice in McMurdo Sound is used extensively for aircraft operations,
travel, and docking areas. The safety and efficiency of utilizing the sea ice depends
upon many factors affecting its physical properties throughout the season.

Sea ice is a crystalline solid with physical properties that are highly temperature
dependent between -1.8°C and -10°C. This dependence becomes less with decreasing
temperatures. A study of various crystal parameters and structure is essential for a
better understanding of their relationship with strength properties. Horizontal
banding in the ice sheet was studied to determine the effect of temperature fluctu-
ations on band frequency. Various crystal parameters such as subcrystal platelet
width, crystal length-to-width ratios, and crystal size were measured from photographs
of thin sections.

Suberystal platelet width increased with depth from about 0.5 mm at the surface
to about 1 mm at 2.8 meters. The length- width ratio of single crystals increased
from 2 to 1 near the surface to more than 5 to 1 at depths greater than 2 meters.

The number of crystals per unit area decreased with depth. Strained ice from a
pressure ridge showed preferred c-axis orientation and wavy extinction similar to
that observed in strained quartz. There is apparently no correlation between strength
and crystal structure in a mature isothermal ice sheet.

Distribution of this document is unlimited.

Copies available at the Clearinghouse (CFSTI) $2.00
The Laboratory invites comment on this report, particularly on the
results obtained by those who have applied the information.

CONTENTS

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INTRODUCTION

The sea ice in McMurdo Sound south of McMurdo Station, Antarctica, is used
extensively for aircraft operations, travel, freight hauling, and docking areas for
cargo handling. The safety and efficiency of utilizing the sea ice depends upon a
knowledge of the many factors affecting its physical properties throughout the season.

The most important factors influencing the physical properties of the ice are
grouped into two broad classes: extrinsic and intrinsic. The extrinsic factors include
solar radiation, tides and currents, water temperature, and snow cover. Intrinsic
factors include crystal structure, thickness, salinity, brine drainage, and ice temper-
ature.

The relationship between crystal structure and strength is well known for many
crystalline solids. Sea ice is a crystalline solid with physical properties that are
highly temperature dependent, especially between -1.8° and -30°C. The crystal
parameters and strength properties of sea ice vary widely and are related to temperature
and the growth history of the ice. A detailed study of the crystal structure and
other internal features is essential for a better understanding of their relationship
with strength properties.

This report presents the results of sea ice crystal studies on that part of
McMurdo Sound that forms an embayment south and west of Hut Point Peninsula,

Ross Island, Antarctica. The studies were conducted between October 1965 and
February 1966 on sea ice 8 to 11 months old. Temperature, salinity, stratigraphy,
platelet width, and other crystal parameters were measured at selected locations

where different environmental factors may have affected the stratigraphy and
crystallography. Ring-tensile strength tests were performed after the ice sheet

became nearly isothermal in an attempt to correlate strength with the above properties.

DESCRIPTION OF SEA ICE

Sea ice is different from freshwater ice in many respects — as for example,
in salinity, crystallography, freezing temperature, density, features caused by brine
drainage, and heterogeneity of physical properties. Sea ice is difficult to describe
because it is composed of pure ice crystals, salt crystals, bubbles, and brine cavities
of various sizes and shapes, all apparently distributed at random. Sea ice crystals
are actually bundles of small, pure ice platelets separated by layers of brine and

brine drainage cavities. Ice crystallizes in the hexagonal form, and individual
subcrystal platelets are usually disc-shaped normal to the principal, or crystallographic,
c-axis. Platelet width, and usually crystal width also, is measured parallel to the
direction of the c-axis.

Seawater with a salinity of 30 to 35 parts per thousand begins to freeze at
about -1.8°C; the ice sheet then grows at a rate that is primarily dependent upon
the air temperature. As the ice sheet grows, brine cavities form that are elliptical
or circular in plan view and elongate vertically. The temperature and salinity of
the ice determines the brine content, and together with density, the air content

(Anderson, 1958, p. 148).

STUDY AREA
Location

McMurdo Sound is located at the western extremity of the Ross Ice Shelf and
is part of the Ross Sea that is bounded on the east by Ross Island, the west by the
mountains of Victoria Land, and the south by the Ross Ice Shelf. McMurdo Station
and Scott Base are located on Ross Island near the tip of Hut Point Peninsula (Figure 1).
The camp occupied by personnel of the U. S. Naval Civil Engineering Laboratory
(NCEL) is on the Ross Ice Shelf 2 miles southeast of Scott Base. The study area was
located on the embayment south of Hut Point Peninsula.

Climate

The climate of the McMurdo Sound area is characterized by low temperatures,
extreme temperature fluctuations, frequent high winds, and drifting snow. Mean
annual temperatures for the years 1956 to 1961 vary from -18.5°C (-1.3°F) to 17.1°C
(1.8°F), with a mean for the 6 years of -17.8°C (-0.1°F). The coldest temperatures
occur in July and August, and vary from -40°C (-40°F) to -50°C (-59°F). Maximum
temperatures occur in December and January, and can be as high as 5.5°C (42°F)
(Climatology of McMurdo Sound, 1961).

The freezing index is the number of degree days during a freezing season and
is one of the best methods of expressing the duration and intensity of cold. The
degree days for any one day equals the difference between the average daily air
temperature and 0°C (32°F) (Linell, 1953, p: 19). The mean air freezing index at
McMurdo Station is approximately 13,300 degree days. At Point Barrow, Alaska,
the freezing index is 8,500 degree days (Péwé and Paige, 1963, p. 366). Other
factors, such as hours of sunshine and solar radiation, are also important in defining
the climate of a region and are summarized for McMurdo Sound by Paige and Lee

(1965).

Hut Point Peninsula
Ross Island

7—~\ McMurdo
<2! Station

Pram Point

Ross Ice
Shelf

Cape Armitage

McMurdo Sound

annual sea ice

Figure 1. Location of sample sites for sea ice crystal studies. Block samples
collected only from station 1.

Sea Ice

The embayment south of Hut Point Peninsula is covered by ice for at least
10 months each year. The maximum breakout commonly occurs in late January or in
February, and the sea is usually frozen over again by late March (Heine, 1963,

p. 399). During the winter months from March to November the ice sheet grows at
a fairly steady rate. In late November the growth rate decreases, and usually by
mid-December the ice reaches its maximum thickness of 8 to 11 feet. Surface
melting is negligible, but in late December the ice begins to deteriorate internally
and starts to thin because of bottom melting.

The thickness and growth rate of the sea ice vary with time and location and -
are also affected by circulation, depth of water, exposure to wind, and snow cover.
Figure 2 shows the maximum ice thickness at four different localities during the
austral summer of 1965-66. Figure 3 shows the growth rate from the end of May 1965
to breakout early in February 1966. Data on the initial stages of ice growth in the
fall of 1965 are not available; however, data from previous years (Tate, 1963) show
that the growth rate is rapid: about 3.2 centimeters per day from the end of March
to about the middle of April. A decrease in growth rate during mid-August and the
final decrease starting at the end of September are reflected by systematic changes
in crystal parameters, as discussed later in this report.

Sample Sites and Study Methods

The four sample sites for the sea ice crystal studies were located, as shown on
Figure 1, within the embayment south of Hut Point Peninsula. Samples were collected
from three sites where the ice sheet was thought to have had different growth histories
significant enough to be reflected by different crystallographic features. The fourth
site was used for thickness measurements only.

Core samples were collected with a standard coring auger and stored ina cold
room at -12°C to -9°C. Thin sections were made from the cores and from blocks cut
from the upper part of the ice sheet at station 1. Several blocks illustrating growth
features at the bottom of the ice sheet were collected from broken and overturned
ice behind the icebreaker USS Glacier in the area opposite McMurdo Station.

Large-scale features easily seen in a sea ice sheet, such as banding and large
brine drainage channels, were studied to determine their origin and relation to the
history of the ice. Data concerning the nature of ice growth, crystal size and
orientation, and the morphology of the bottom surface of a thick, growing ice sheet
are rare because direct measurements are difficult to obtain. During Deep Freeze 66,
direct observations and estimates of crystal size and relative relief at the bottom
were made from an under-ice observation chamber installed by the U. S. Antarctic
Research Program.

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Small-scale details and measurements of various crystal parameters were
obtained from thin sections of sea ice that were photographed under plain and crossed
polarized light to bring out different features. Most of the crystal measurements were
made from enlargements of the photographs. Ring-tensile strength data collected
after the ice sheet had become nearly isothermal were plotted and compared with
temperature, salinity, platelet width, and other crystal parameters in an attempt to
correlate these properties with the strength data.

LARGE-SCALE FEATURES

The most obvious features visible in a vertical section of an ice sheet are
horizontal banding (stratification), brine drainage channels and cavities, and bubble
zones. Under natural conditions these features are usually seen only in pressure
areas, where the ice may be uplifted, broken, or tilted in such a way as to expose
a vertical section. The easiest way to study these features is to extract complete
7 .62-cm-diameter cores and photograph sections of the core against a dark background.

Banding

Horizontal bands of alternating blue and white ice are a prominent feature
in uplifted or tilted blocks of sea ice, especially when the ice has been exposed for
several days and the bands are emphasized by sublimation. Banding, or stratification,
in arctic sea ice has been briefly discussed by Schwarzacher (1959) and Smith (1964).
Bennington (1963) presents an excellent discussion of banding in arctic sea ice and
states (pp. 681-682) that bands are caused by one of the following processes: (1) an
accumulation of crystals with vertical c-axis orientation, (2) high-porosity zones
consisting of brine pockets along staggered intersections of platelets, and (3) the
expansion of trapped brine pockets.

Banding in the sea ice of McMurdo Sound was seen at all levels, but occurred
most frequently and with closer spacing in the upper half of the ice sheet. The
frequency of the bands (number of bands per foot) and the spacing between them varied
from place to place and apparently reflected differences in the growth history of the
ice. The bands varied in width from 1 to several centimeters and all had diffuse
boundaries. The ice between bands was generally clear, although zones of a slightly
milky ice were common. Long, vertical, small-diameter brine drainage channels
were common throughout the ice sheet but increased in size and number towards
the bottom. The bands were easily seen in cores or blocks but were almost indistin-
guishable in thin section.

Banding in the sea ice of McMurdo Sound consists of high-porosity zones
formed by horizontal rows of minute, closely spaced, elongated bubbles and brine
drainage cavities. They are apparently caused by the downward drainage of high-
density brine formed during platelet growth under supercooled conditions at the

ice- water interface, According to Bennington (1963) the growing platelets separate
brine with a progressively increasing concentration and density. This unstable brine
layer then cascades to a lower level and leaves a greater proportion of brine pockets
(p. 683). In discussing a possible driving mechanism for brine drainage, Bennington
(1966) also states that temperature fluctuations create internal pressure changes that
result in brine expulsion from the zone of growing platelets. Each temperature-related
episode of brine expulsion would, therefore, form a band and, in turn, record the
temperature fluctuation. Dykins (1966, p. 19) describes banding that occurred while
freezing seawater in a laboratory tank and states that it may have been caused by
varying the temperature in the freezing chamber.

That the development of banding is related to temperature effects during the
growth of the ice is strikingly evident in the different band frequency seen in the
cores from stations 1, 2, and 4. At station 1 (Figure 4), where the ice grew over the
deep, open water of McMurdo Sound, there were 19 bands in the upper 60 cm (2 feet).
At station 2 (Figure 5), where the ice grew in a protected zone of deep, quiet water
between the ice shelf and a stranded iceberg, there were eight bands in the upper
60 cm. At station 4 (Figure 6), where an early snow cover 91 cm thick dampened
the effect of temperature fluctuations, there were only five bands in the upper 60 cm.

A temperature decrease promotes the growth of platelets and the attendant
expulsion of brine. As the temperature of the ice becomes colder during the early
stages of ice growth, pure ice platelets are able to grow from seawater of increasing
brine concentration. Zones of clear ice between bands probably represent periods
of steady growth when convection, tidal currents, or other circulation allowed
platelets to grow from seawater of normal salinity.

In McMurdo Sound the decrease in frequency and the increased spacing
between bands from the top of the ice sheet downward results from the dampening of
temperature fluctuations by the thickening ice sheet. From the 1.2-meter depth
(4 feet) to the bottom of the ice sheet, banding is either rare or absent. Thus, it
appears that banding accurately records the temperature fluctuations of an ice sheet
during its growth.

Brine Drainage

During the early growth stages of sea ice, brine is expelled by the growing
platelets of pure ice and becomes trapped as layers and as vertical elongated cells
at interplatelet boundaries. It is this localization of brine cells and cavities that
so clearly defines the subcrystal structure of sea ice. Figure 7 is a horizontal thin
section of sea ice that shows brine cells and layers outlining the platelets of pure
ice. While the ice sheet grows during the coldest part of the winter, most brine
features are small and closely spaced except for occasional large vertical drainage
channels. As the ice sheet warms during the summer, the size and shape of brine
drainage features change considerably.

Depth in Ice Sheet (feet)

Depth in Ice Sheet (cm)

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Figure 4. Stratigraphy of annual sea ice
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Depth in Ice Sheet (feet)

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Figure 5. Stratigraphy of annual sea ice

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at station 4, December 1965.

Depth in Ice Sheet (feet)

Depth in Ice Sheet (cm)

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Figure 6. Stratigraphy of annual sea ice
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11

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Figure 7. Thin section of sea ice from a depth of 50 cm showing crystal

structure, platelets, and brine features. Grid is 1 cm.

By late December high ambient temperatures and strong solar radiation in the
McMurdo area caused the ice sheet to become nearly isothermal and to approach the
melting point of sea ice. Brine drainage was accelerated as the cells enlarged and
coalesced (Figure 8) and formed long columnar brine drainage channels normal to
the surface of the ice. Many of the brine drainage channels were open to the sea
below and filled with seawater to a level depending upon hydrostatic conditions of
the ice sheet. Solar radiation transmits considerable energy to the ice sheet, especially
if the ice has no protective layer of snow. Because of this, a zone of unusually
weak ice occurred between the 30- and 91-cm depth, where brine cavities became
numerous and large.

Bottom Growth

The bottom of a growing sea ice sheet is unlike that of freshwater ice and is
characterized by an irregular surface with numerous disconnected ice platelets
protruding downward into the seawater. This layer has been termed the "skeleton
layer" by Assur (in Butkovich, 1956, p. 1) and results from the separation of pure ice
platelets freezing from seawater, as explained by Weeks (1958, p. 97).

The small amount of available information regarding the thickness of the
skeleton layer comes from various sea ice studies in the arctic and subarctic. The
skeleton layer has been observed in arctic sea ice to be 2.4 to 2.8 cm (Weeks and
Anderson, 1958, p. 644), 1 to 2 cm (Schwarzacher, 1959), and up to 2 cm (Bennington,
1963, p. 685).

Direct observation revealed that the bottom surface of the growing ice sheet
in McMurdo Sound had a skeleton layer 10 to 15 cm thick, with a maximum of 30 cm
in isolated locations. Individual sheet-like platelets that protruded downward 10 to
15 cm beyond the bottom of the ice sheet were common. The bottom surface was
undulating and extremely irregular, with a possible relative relief of at least 60 cm
(2 feet). It must be emphasized that the above observations were made when the
ice was more than 1.8 meters thick and about 10 months old.

The idealized concept of platelet-crystal growth at the bottom of a sea ice
sheet shows the platelets growing vertically downward with their c-axes truly
horizontal (Assur and Weeks, 1964, p. 4), (Stehle, 1965, p. 3), and (Peyton, 1963,

p. 109). Weeks and Anderson (1958, p. 643) describe the skeleton layer as consisting
of unconnected vertical ice plates. Two photographs by Bennington (1963, pp. 679,
682) of vertical thin sections from the bottom of thick ice clearly show that c-axis
horizontal crystals predominate.

The c-axis orientation of platelet-crystal growth at the bottom of thick sea
in McMurdo Sound was random, with only a slight majority of platelets growing
vertically downward. Figure 9 shows a slab cut vertically from the bottom of
2.44-meter-thick sea ice. The slab was about 25 cm wide, 25 cm long, and 1 cm
thick. The dendritic platelet growth with the c-axes inclined at a wide variety of

angles is quite evident. Figure 10 is a vertical thin section at a depth of 246 to
254 cm, where the ice was 2.7 meters thick. Crystals with the c-axis inclined far
from the horizontal predominate in this section. Figure 11 is a horizontal thin
section from within 5 cm of the bottom of ice 2.87 meters thick. Again, the variety of
c-axis angles is obvious, as shown by the large, irregular clear areas.

When the sea ice stopped growing and bottom melting began, the skeleton
layer was quickly destroyed and, as melting progressed upward, crystals with horizontal
c-axis orientations predominated. This suggests that during the early stages of ice
growth in McMurdo Sound, the skeleton layer is thin with mostly horizontal c-axis
crystal growth, as reported for arctic sea ice. The unusually thick skeleton layer
with its variety of c-axis orientations may not begin to form until late in the winter,
when the growth rate becomes extremely slow (about 10 cm for November) and ¢
ice is usually more than 2.4 meters thick. The bottom surface of the melting ic
sheet became smooth and attained a reversed sun-cupped appearance similar to the
surface of an ablating snowfield.

STRUCTURAL PARAMETERS

Structure, as used herein, defines the relationship of sea ice crystals to each
other as well as their size, shape, subcrystal features, and c-axis orientation. Ina
horizontal section, the structure consists of an intensely interlocking mosaic of
elongate, sharply angular crystals. Brine layers and cavities occur both at crystal
and subcrystal boundaries and vary in size and shape depending upon the age and
temperature of the ice. Ina vertical section, individual crystals appear as long,
lenticular, spindle-shaped grains with their long axis normal to the surface of the
ice sheet. Brine features are also commonly elongate ina vertical section, as
compared to round or elliptical in a horizontal section. C-axis orientation is
predominantly random in the horizontal plane except in the upper few centimeters and
at the bottom of a growing ice sheet.

Gross crystal parameters, such as length, length-width ratio, relative size in
terms of crystals per unit area, and subcrystal platelet width, were plotted against
depth in the McMurdo ice sheet. All measurements were made in the horizontal
plane from photographs of thin sections and are somewhat subjective. Each point
plotted is an average of 8 to 12 measurements. Most of the measurements are restricted
to a 7.62-cm-diameter core specimen and do not truly represent crystal dimensions
at the bottom of the thick, growing ice sheet. For example, actual crystal lengths
were as much as 15 cm (horizontally) and some crystals were so large that only a
small fraction of the crystal occupied 1 cm¢.

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Crystal Length Versus Depth

In Figure 12 the crystal length in a horizontal plane is plotted against depth
in the ice sheet. Measurements were made from thin section photographs at various
depth levels and from two locations. These measurements show a systematic and
definite increase of crystal length with depth as related to the growth rate of the
ice. This increase probably applies to length both horizontally and vertically,
although crystal dimensions in a vertical plane are difficult to obtain and are not
presently available.

Crystal Length-to-Width Ratio

The ratio of crystal length to width in a horizontal plane is shown in Figure 13.
The true ratio is probably not accurately represented for ice near the bottom because
of the restricting effect of the 7.62-cm-diameter specimen. The length was measured
normal to the c-axis direction, and the width was measured parallel to the c-axis.

It was observed that the length-to-width ratio changed considerably with depth and,
for this reason, the data were arbitrarily divided at the 1.2-meter depth and were
plotted separately to emphasize this difference. The change in the length - width
ratio is not abrupt but is probably gradational downward through the ice sheet.

For the O- to 120-cm depth, the length - width ratio is approximately two to
one, which compares closely with data by Weeks and Hamilton (1962, p. 953) from
arctic sea ice 30 cm thick. However, as the ice sheet becomes thicker, the length -
width ratio becomes more nearly five to one, as shown in Figure 13. The scatter in
both plots was caused by crystals having a width greater than their length. It is also
obvious that there is only a slight increase of crystal width versus depth, and that
this parameter is of little significance.

Crystals Per Square Centimeter

The number of complete crystals in a measured area on the photograph of a
thin section were counted and plotted against depth in Figure 14. More data from
closely spaced depth intervals were available from station 1 than from station 3 and
show strong fluctuations in number of crystals per unit area in the upper 91 cm of
the ice sheet. The large number of crystals per square centimeter at the 40-cm level
probably indicates a period of extreme cold, when rapid ice growth formed small
crystals. As the ice sheet became thicker, growth rate decreased and crystal size
increased accordingly. Such an increase in crystal size with depth is discussed by
Weeks and Hamilton (1962, p. 951), who compare it with similar phenomena occurring
in metal castings. Dykins (1966, p. 25) shows a strong increase in crystal size with
depth in sea ice grown under laboratory conditions. In the reports cited above, it
is shown that there is less than 1 crystal per square centimeter in ice no more than

40 cm thick. Observations of McMurdo Sound sea ice show that, although there are
many crystals of enormous size near the bottom of a thick ice sheet, there are also
many crystals that occupy much less area than 1 cm2, When crystals of all sizes
were considered, the average was found to be about 1 crystal per square centimeter
for the bottom meter of the ice sheet.

Platelet Width Versus Depth

An average platelet width of 0.45 mm, with a range varying from 0.2 to
0.8 mm, was determined for arctic sea ice by Weeks (1958, p. 97). Schwarzacher
(1959) made about 500 measurements of arctic sea ice to get an average platelet
width of 0.902 mm (p. 2359), or about twice that determined by Weeks. Weeks and
Hamilton (1962) describe the subcrystal structure in arctic sea ice 30 cm thick and
state that one of the apparent variations in structure is the gradual increase in
platelet width with increasing depth (p. 954). They further state, however, that the
expected variation in platelet width with depth has not yet been demonstrated.
Figure 15 is a plot of platelet width versus depth in the ice sheet. Measurements
were made at various depths from thin section photographs of sea ice at stations 1,
2, and 3. Platelet widths were measured in the horizontal plane parallel to the
c-axis by counting the number of distinct platelets within a predetermined linear
distance and dividing to obtain an average width. Measurements were also made
from cores collected on three different dates at station 1, but show no apparent
differences in width that can be related to time or temperature changes. The system-
atic increase in platelet width from about 0.4 mm near the surface to more than
1.0 mm at the bottom is clearly shown in Figure 15.

Stressed Sea Ice

A sea ice specimen oriented in relation to the stress field was collected from
the crest of a small pressure ridge near station 2 to study the effect of stress on the
crystal structure. The ice in the actively growing pressure ridge was about 2.44 meters
(8 feet) thick and had been stressed for at least 8 months by horizontal pressure from
the adjacent westward=moving McMurdo Ice Shelf.

A thin section from a depth of 20 cm photographed in crossed polarized light
is shown in Figure 16. The large arrows in photograph 16a show the direction of
tensile stress. The small lines on the photograph show the strongly preferred direction
of c-axis orientation typical of many stressed materials made up of elongate, plate-
like, or tabular crystals. Photograph 16b has been rotated 20 degrees under crossed
polarized light to show the wavy extinction in the large crystal in the center of the
view. Wavy extinction is typical in thin-section photographs of strained minerals
such as quartz or olivine, and is a good criterion that the mineral has undergone
stress sufficient to cause crystal deformation. The preferred c-axis orientation and the
wavy extinction occurring in stressed sea ice indicate that the stress conditions in a
sea ice sheet can be deduced by detailed crystal studies.

20

Crystal Length (cm)

Depth (meters)

Figure 12. Crystal length versus depth at stations 1 and 3.

21

Length (mm)

L = 1.01W + 5.73

0 ca 10
Width (mm) Width (mm)
(a) 0-120-cm depth. (b) 120-262-cm depth.

Figure 13. Length-to-width ratio of sea ice crystals versus depth in thick sea ice.

22

Depth (cm)

station 3

Number of Crystals Per Cm2

2 3 4 5 6 7 8 9 10
it
7
A station |]
/ —7

50

100

a

oO

tT
eee ae

200
|
|
|
|
250 [—
|
|
ds
300
Figure 14,

Number of whole crystals per square centimeter
versus depth at stations | and 3.

23

Platelet Width (mm)
1.0

@——® 25 Nov
Ove O 13 Jan station 1

O— —O 17 Dec
{
I station 2
Q—-—O station 3
]
o
°
E
<
a
o
[a)
2
3

Figure 15. Subcrystal platelet width versus depth at stations 1, 2, and 3.

24

ard. Gi . 3
. 2 D> 3 cag Po? (ew *
, ny he (b) « ere ee i‘

Figure 16. Thin section of stressed sea ice from the crest of a pressure ridge.
Photographed under crossed polarized light. The small lines in (a)

indicate the direction of the crystallographic c-axis. Grid is 1 cm
(b) has been rotated 20 degrees clockwise in relation to (a).

75)

Structural Parameters Versus Depth and Growth Rate

The variation in structural parameters occurring at different depth levels
throughout an ice sheet is related to the growth rate of the ice. This is especially
true in the upper levels of a young ice sheet, where temperature fluctuations have
a pronounced effect on crystal growth. The sudden increase in crystals per square
centimeter at the 40-cm depth in Figure 14 correlates well with the decrease in
platelet width at the same depth as shown in Figure 15, and probably indicates a
period of extreme cold during the early growth history in McMurdo Sound.

CRYSTAL STRUCTURE VERSUS STRENGTH

The relationship of crystal structure to strength isa we! tablished property
of many crystalline solids. The obvious variation of certain crystal and subcrystal
parameters with depth suggests that the strength of ice should also vary with depth.
The possible reasons for the vertical variation of strength have been discussed in
detail by Assur (1958) and by Anderson and Weeks (1958). Weeks and Assur (1963)
state (p. 259) that "the principal structural parameter controlling the distribution of
brine in single crystals of sea ice is the plate width....If the plate width changes
systematically with the distance below the upper surface of the ice sheet, this
variation might conceivably account for a change in vertical strength."

The platelet width determines the primary spacing of brine layers and cylinders.
The size, shape, and spacing of brine inclusions vary with temperature, but after their
initial formation these inclusions no longer have any relation to crystal structure.

If an ice sheet becomes cold, more ice freezes out of the trapped brine and the brine
cavities become smaller; as an ice sheet warms up, ice melts and dilutes the brine.
The brine cavities and layers then become larger and migrate 7ownward through the
ice sheet under the influence of gravity and a temperature gradient. The spacing and
geometry of brine drainage features are thought to be the major factors affecting

the strength of sea ice (Anderson and Weeks, 1958, p. 632).

Assur (1958) established a correlation between brine volume and ring-tensile
strength and showed that as brine volume decreases, ring-tensile strength increases
(p. 130). Further, brine volume is shown to decrease with depth in an ice sheet as
a result of increasing platelet width (Weeks and Assur, 1963, p. 267). The data of
Graystone (1960, pp. 36-39) show a slight increase of strength with depth in sea ice
1.2 to 1.5 meters (4 to 5 feet) thick. However, Weeks and Anderson (1958, p. 643)
found no difference in strength when breaking cantilever beams by both push-up
and push-down loading of sea ice varying in thickness from 6.3 to 36.8 cm.

26

Figure 17 shows ring-tensile strength,* temperature, and salinity, versus depth.
A comparison of the ring-tensile strength curves with the data shown in Figures 12,
13, 14, and 15 indicates that there is no apparent correlation between the vertical
distribution of strength and the various crystal parameters in a mature, isothermal ice
sheet. The progressive strength decrease with time occurring in the 0.3- to 0.9-meter
depth interval (Figure 17a) results predominantly from solar radiation energy causing
accelerated brine drainage and the enlargement of brine layers and cavities. This
phenomenon also has no relation to crystal structure. The strength increase at the
surface of the 12 January 1966 plot resulted from freshening of the sea ice by almost
complete brine drainage.

FINDINGS

1. There is apparently no distinct correlation between crystal structure and ring-tensile
strength in the thick, mature isothermal ice sheet in McMurdo Sound.

2. Subcrystal platelet width increased from about 5 mm at the surface to more than
10 mm at a depth of 3 meters, but had no apparent effect on the variation of ring-
tensile strength.

3. Crystal length (in a horizontal plane) increased systematically with depth from
less than 1 cm near the surface to as much as 15 or 20 cm (as observed) at the bottom
of the ice sheet.

4, The crystal length-to-width ratio changed from about 2 to 1 in the upper half of
the ice sheet to more than 5 to 1 near the bottom.

5. The spacing and frequency of horizontal bands of cloudy, white ice alternating
with zones of clear ice was attributed to temperature fluctuations during early
growth of the ice sheet.

6. Wavy extinction in single crystals and a strong preferred c-axis orientation in
ice from a pressure ridge indicated that the stress conditions in sea ice can be deduced
by detailed crystal studies.

7. The bottom surface of the thick, growing ice sheet was extremely irregular, with
a possible relative relief of at least 60 cm (2 feet).

8. Brine drainage features varied in size and shape in relation to temperature and
solar radiation intensity. This was especially true in the 0.3- to 0.9-meter depth
interval, where a zone of weakness was caused by enlarged and closely spaced brine
drainage cavities.

*The ring-tensile strength test (Butkovich, 1956) was performed by drilling a 1/2-inch-

diameter hole coaxially through a 3-inch-diameter core specimen and applying a load
normal to the axis at a loading ram rate of 8 in./min.

27

Depth (meters)

Ring Tensile Strength Temperature
(kg/cm) (°C)

Salinity
(ppt)

o———OmNeiDer RES VAS SN Ge
@A———-OA 20 Dec (6 Peeeeeceeretes ca 12 Jan

Figure 17. Comparison of ring-tensile strength, temperature, and salinity in
a thick, mature isothermal ice sheet.

28

CONCLUSIONS

1. The vertical variation of ring-tensile strength in a thick isothermal ice sheet does
not correlate well with subcrystal platelet width or other crystal parameters. The
strength variations observed in McMurdo Sound sea ice were caused by temperature
differences and varying stages of internal deterioration.

2. The growth history of the annual sea ice in McMurdo Sound varies with time

and location, and is affected by climatic differences, water circulation and depths,
exposure to wind, and snow cover. A detailed study of large- and small-scale
features such as banding, brine drainage, and crystal structure provides much informa-
tion concerning growth conditions of ice at particular locations.

3. Stressed sea ice in pressure ridges has distinct optical properties, such as wavy
extinction under crossed polarized light and preferred c-axis orientation. This
indicates that stress conditions in an ice sheet may be determined by studying the
crystal structure and optical properties. However, more data are needed under a
wider variety of stress conditions to establish better interpretive criteria.

4, Measurements of various crystal parameters, both horizontally and vertically,

are restricted in 7.62-cm-diameter core specimens. Measurements are needed from
large blocks of sea ice to further study the relationship between crystal structure and
sea ice strength.

ACKNOWLEDGMENTS

Dr. A. J. Gow and Mr. Steven Roth rendered invaluable help in photographing
many of the thin sections. Dr. W. F. Weeks and Mr. Justin Dykins have contributed
much helpful discussion and many useful suggestions. Mr. C. W. Henderson furnished
excellent assistance in the field and laboratory.

REFERENCES

Anderson, D. L. "A model for determining sea ice properties," in Arctic sea ice,
Washington, D. C., National Research Council Publication 598, Dec. 1958, pp. 148-
156.

Anderson, D. L., and Weeks, W. K. "A theoretical analysis of sea-ice strength, "
American Geophysical Union, Transactions, vol. 39, no. 4, Aug. 1958, pp. 632-640.

Assur, A. "Composition of sea ice and its tensile strength," in Arctic sea ice.

Washington, D. C., National Research Council Publication 598, Dec. 1958, pp. 106-138.

29

Assur, A. and Weeks, W. F. Growth, structure, and strength of sea ice. U. S. Army
Cold Regions Research and Engineering Laboratory, Research Report 135, Hanover,

Naini © chal 64%

Bennington, K. O. "Some crystal growth freatures of sea ice," Journal of Glaciology,

vol. 4, no. 36, Oct. 1963, pp. 669-688.
1966. Crystal and Brine Relationships in Sea Ice. Unpublished.

Butkovich, T. R. Strength studies of sea ice. U. S. Army Snow, Ice, and Permafrost
Research Establishment, Research Report 20, Wilmette, IIl., Oct. 1956.

Climatology of McMurdo Sound. U.S. Navy Weather Research Facility, Report
NWRF 16-1261-052, U. S. Naval Air Station, Norfolk, Va., Dec. 1961.

Dykins, J. E. Ice engineering — Tensile and bending properties of sea ice growth
in a confined system. U.S. Naval Civil Engineering Laboratory, Technical Report

R-415, Port Hueneme, Calif., Jan. 1966.

Graystone, P. "In situ" strength tests on sea ice of low salinity, near Churchill,
Manitoba. Defense Research Board, Northern Laboratory, DRNL Report No. 1/60,
Ottawa, Canada, Oct. 1960.

Heine, A. J. "Ice breakout around the southern end of Ross Island, Antarctica, "
New Zealand Journal of Geology and Geophysics, vol. 6, no. 3, June 1963, pp. 395-
401.

Linell, K. A. "Frost design criteria for pavements," in Soil temperature and ground
freezing, Highway Research Board Bulletin 71, Washington, D. C., 1953, pp. 18-32.

Paige, R. A. and Lee, C. W. Sea ice studies on McMurdo Sound during Deep Freeze
65. U.S. Naval Civil Engineering Laboratory, Technical Note N-721, Port Hueneme,
Calif., Oct. 1965.

Péwé, T. L., and Paige, R. A. Frost heaving of piles with an example from Fairbanks,
Alaska. U. S. Geological Survey, Bulletin 1111-1, Washington, D. C., 1963.

Peyton, H.R. "Some mechanical properties of sea ice," in Ice and snow, edited by

W.D. Kingery. Cambridge, Mass., M.1.T. Press, 1963, pp. 107-113.

Schwarzacher, W. "Pack-ice studies in the Arctic Ocean," Journal of Geophysical

Research, vol. 64, no. 12, Dec. 1959, pp. 2357-2367.

Smith, D. D. "Ice lithologies and structure of ice island ARLIS II," Journal of
Glaciology, vol. 5, no. 37, Feb. 1964, pp. 17-38.

Stehle, N. S. Ice engineering — Growth rate of sea ice in a closed system. U. S.
Naval Civil Engineering Laboratory, Technical Report R-396, Port Hueneme, Calif.,
June 1965.

30

Tate, T. N., 1963. Statistical Data on Ice Growth, Salinity, Thickness and
Appearance, McMurdo Sound, Antarctica. Operation Deep Freeze 63. Unpublished.

Weeks, W. F. "The structure of sea ice: a progress report," in Arctic sea ice.

Washington, D. C., National Research Council Publication 598, Dec. 1958, pp. 96-99.

Weeks, W. F. and Anderson, D. L. "An experimental study of strength of young sea
ice," American Geophysical Union, Transactions, vol. 39, no. 4, Aug. 1958, pp. 641-
647,

Weeks, W. F. and Assur, A. "Structural control of the vertical variation of the
strength of sea and salt ice," in Ice and snow, edited by W. D. Kingery. Cambridge,
Mass., M.I.T. Press, 1963, pp. 258-276.

Weeks, W. F. and Hamilton, W. L. "Petrographic characteristic of young sea ice,
Point Barrow, Alaska," American Mineralogist, vol. 47, no. 7-8, July-Aug. 1962,
pp. 945-961. Also published as: U.S. Army Cold Regions Research and Engineering
Laboratory, Research Report 101, Hanover, N. H., Oct. 1962.

31

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Unclassified
Security Classification

DOCUMENT CONTROL DATA - R&D

(Security classification of title, body of abstract and indexing annotation must be entered when the overall report is classified)

1. ORIGINATING ACTIVITY (Corporate author) 2a. REPORT SECURITY CLASSIFICATION

U. S. Naval Civil Engineering Laboratory Unclassified

Port Hueneme, California 93041

3. REPORT TITLE
Crystallographic Studies of Sea Ice in McMurdo Sound, Antarctica

4. DESCRIPTIVE NOTES (Type of report and inclusive dates)

Not final; October 1965 - June 1966

5. AUTHOR(S) (Last name, first name, initial)
Paige, R. A.

6. REPORT DATE Ja. TOTAL NO. OF PAGES 7b. NO. OF REFS
November 1966 35 23

8a. CONTRACT OR GRANT NO. 9a. ORIGINATOR'S REPORT NUMBER(S)

b. prosect no. Y-FO15-11-01-026 TR-494

9b. OTHER REPORT NO(S) (Any other numbers that may be assigned
this report,

10. AVAILABILITY/LIMITATION NOTICES
Distribution of this document is unlimited.
Copies available at the Clearinghouse (CFSTI) $2.00.

11. SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY

Naval Facilities Engineering Command

13. ABSTRACT

The sea ice in McMurdo Sound is used extensively for aircraft operations, travel, and
docking areas. The safety and efficiency of utilizing the sea ice depends upon many
factors affecting its physical properties throughout the season.

Sea ice is a crystalline solid with physical properties that are highly temperature
dependent between -1.8°C and -10°C. This dependence becomes less with decreasing
temperatures. A study of various crystal parameters and structure is essential for a better
understanding of their relationship with strength properties. Horizontal banding in the ice
sheet was studied to determine the effect of temperature fluctuations on band frequency.
Various crystal parameters such as subcrystal platelet width, crystal length-to-width ratios,
and crystal size were measured from photographs of thin sections.

Subcrystal platelet width increased with depth from about 0.5 mm at the surface to
about 1 mm at 2.8 meters. The length -width ratio of single crystals increased from 2 to 1
near the surface to more than 5 to 1 at depths greater than 2 meters. The number of crystals
per unit area decreased with depth. Strained ice from a pressure ridge showed preferred
c-axis orientation and wavy extinction similar to that observed in strained quartz. There is
apparently no correlation between strength and crystal structure in a mature isothermal ice
sheet.

DD Rae 1473 0101-807-6800 Unclassified

Security Classification

Unclassified

Security Classification

KEY WORDS

Crystallographic studies
Antarctic sea ice

Strength vs crystal structure
Sea ice stratigraphy

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Unclassified

Security Classification