Geology Reference
In-Depth Information
ice averaged between 25% and 55%. They combined tex-
tural analysis with measurements on the stable isotope
δ
(
18
O) to discriminate between snow ice and frazil ice,
similar to the 1990 to 1992 studies of
Kawamura et al
.
[1995] in Lutzow‐Holm Bay.
The application of the differential isotope,
δ
(
18
O),
method is certainly unique and very powerful. However,
the technique for using such isotopic procedures requires
accessibilities with laboratories equipped with rather
sophisticated and expensive equipment, and highly trained
operators. Moreover, only a limited number of samples
can be analyzed because of high expenses involved. An
alternate powerful and cheap method that can be per-
formed in field laboratories (e.g., inside tents) is to use the
double‐microtomed thin sectioning technique in conjunc-
tion with measurements utilizing thermal etching, chemi-
cal etching, and replicating in addition to polarized light.
Replicas can be made in the field and can also be readily
examined with optical microscopes (see Chapter 4, specifi-
cally section 4.4.1). If desired, the replicas can be examined
later with a scanning electron microscope (SEM). Since the
microstructures of granular snow ice and frazil ice are sig-
nificantly different from each other, the two types of ice
can be identified readily. A minimum of two sections (i.e.,
horizontal and vertical) are required for proper identifica-
tion. These techniques are described in detail in Chapter 6.
temperature is very low. The Canadian Ice Service (CIS)
confirmed (through personal communications) that
under a steady air temperature of −25 °C, a thin skin of
ice will thicken quickly to reach 100 mm during the first
24 h. This was actually verified by experimentally simu-
lating a lead in Mould Bay, as described in Section 5.1.3.
However, as the ice grows, snow is also accumulated on
top of the ice cover, and the growth rate decreases with
increase in ice thickness. It may take a few weeks to reach
the stage of mature first‐year ice with thickness greater
than about 1.2 m [
Canadian Coast Guard,
1999]. At
warmer temperatures below but closer to the freezing
point (in Labrador Sea, Bohai Bay and Ohkhost Sea for
examples), it would take 3-4 weeks for ice to grow up to
the stage of thin first‐year ice (0.3 m thick).
Growth and salinity profile of annual sea ice, in conjunc-
tion with microstructural investigations, have been studied
extensively for several years in Eclipse Sound near Pond
Inlet, Baffin Island, Canada [
Sinha and Nakawo,
1981
,
for 1977-1979;
Nakawo and Sinha,
1981
,
for 1977-1978].
They reported rates between 5 mm/day (i.e., 0.021 cm/h)
and 15 mm/day (0.0625 cm/h). Another extensive series of
long‐term study program, from 1981 to 1985, was carried
out in Mould Bay, Prince Patrick Island, Canada. Growth
of ice was recorded at several stations across the 7.8 km
wide bay. The details of this project will be described in
section 5.1. As an example, a growth rate of 20 mm/day
was noted during the first 10 days of growth, without any
snow cover, in September, 1981. However, for the first 200
days, after the beginning of freezing, during the 1981-1982
winter season, the average growth rate varied from 7.5 mm/
day to 9.5 mm/day, depending on the location across the
width of the bay. The differences in the growth rate were
caused by the differences in the accumulated snow depth.
Melnikov
[1995] conducted in situ measurements in the
western Weddell Sea in 1992 during the U.S.‐Russian Ice
Station Weddell 1 Expedition in the Antarctic and found
much higher growth rates of 3.8 mm/h for ice up to 90 mm
thick (19-20 May), 1.3 mm/h for thicker ice up to 280 mm
thick over the next 8 days, and only 0.3 mm/h during 81
days of observations on ice between 0.42 m and 0.97 m
thick (18 March to 7 June). The growth rate decreases as
ice thickness increases until ice reaches thermodynamic
equilibrium with the atmosphere. The ice growth then
stops. This equilibrium occurs when the thickness reaches
about 3 m in the case of Arctic ice and 1-2 m in the case
of Antarctic ice [
Whitman,
2011].
2.2.4. Thermodynamic Ice Growth
Ice grows in thickness mostly thermodynamically,
although mechanically induced growth also occurs in
rough seas. Thermodynamic growth entails increasing
thickness in response to the negative energy budget
between the ocean and the ice sheet. The ocean is the
source of the heat that is transferred to the atmosphere
through the ice. Mechanical growth entails piling up of
broken ice along the edges of ice floes due to several pro-
cesses such as rafting, ridging, and rubble pile‐up. Larger
thickness, of course, is typically generated by dynamic
processes. However, as will be seen in section 5.1.4, ther-
modynamic ice growth can indeed occur up to a depth of
5.1 m of continuous columnar‐grained ice, with four dis-
tinct interfaces indicating five growth seasons. This was
observed by
Sinha
[1986] in a multiyear floe.
Eicken et al.
[1995]
,
inclined to show that thermodynamic ice growth
leads to maximum thickness of about 3.5 m for multiyear
ice. Thermodynamic growth rate is mainly controlled by
three factors: (1) the severity and duration of cold air
temperatures, (2) snow accumulation on the surface,
and (3) ice thickness. Other factors include proxy for
long‐wave and latent heat fluxes, ocean heat flux, and
short‐wave flux in spring and summer.
Extremely rapid freezing can occur in a “lead” if an ice
sheet fractures and separates when the ambient air
2.2.4.1. Modeling Ice Growth
Sea ice growth rate and modeling is important from
oceanic, remote sensing, and marine navigation view-
points. Its oceanic impact is demonstrated in the depend-
ence of the salt rejection from the skeletal layer at the
bottom of the ice sheet to the underlying saline water
