Geology Reference
In-Depth Information
presented in section 2.6. Ice classes address the traditional
age‐based ice classification system [developed by the
World Meteorological Organization (WMO)], which is
used in the operational sea ice monitoring programs. “Ice
regime,” on the other hand, includes a wide ranging phe-
nomena defined by their origin or formation processes
such as pancake ice, congealed ice, fast ice, marginal ice
zone, polynyas, and floating ice bodies of land origin.
investigators over a very long period of time and has been
documented thoroughly in many reports and topics. It
was known for a long time that if two atoms of hydrogen
are combined with one atom of oxygen under the same
temperature and atmospheric pressure, then a molecule
of water vapor forms. According to Avogadro's hypoth-
esis, this leads to the basic molecular structure of pure
water consisting of two atoms of hydrogen and one atom
of oxygen, or simply H
2
O. The ratio of the combining
volumes of oxygen (O
2
) and hydrogen (H
2
) at 0 °C and a
pressure of 0.76 mm Hg is O
2
/H
2
= 1/2.00288. Pure water,
however, may be divided into several types depending on
isotopes of oxygen and hydrogen.
Hydrogen has three isotopes:
1
H (known as H),
2
H
(known as deuterium, D) and
3
H (known as tritium), which
is radioactive and its half‐life is 2.5 years. Oxygen, on the
other hand, has six isotopes:
14
O,
15
O,
16
O,
17
O,
18
O, and
19
O
(oxygen
16
O is simply known as O). The three isotopes of
16
O,
17
O, and
18
O are stable, whereas
14
O,
15
O, and
19
O are
radioactive but short‐lived. The combination of oxygen
and hydrogen atoms leads to six stable isotopes of water
formed from stable atoms of hydrogen and oxygen.
The isotopic content of water in oceans, lakes, and riv-
ers varies depending upon its origin. It contains about
99.73% of
1
H
2
16
O, 0.2% of
1
H
2
18
O, 0.04% of
1
H
2
17
O, and
0.03% of
1
H
2
H
16
O [
Shatenshtein et al.,
1960]. The last
one is simply termed HDO. However, if both the hydro-
gen atoms in this compound are deuterium (
2
H), i.e., the
molecule is
2
H
2
16
O or D
2
O, then the compound is known
as heavy water. Based on the above percentages, ordinary
water, which is generally referred to as H
2
O, is mainly
composed of
1
H
2
16
O.
The isotopic water composition has an impact on evap-
oration and consequently on the composition of snow-
flakes in the upper atmosphere. At a given temperature,
the evaporation of the two major isotopes of water,
1
H
2
16
O and 0.2% of
1
H
2
18
O, depends on their masses.
Naturally, the heavier water evaporates at a slower rate
than the lighter one. As the temperature rises, the propor-
tionality of these two varieties of water increases. Water
containing the heavy oxygen
18
O evaporates more under
warmer atmospheric temperature, making higher percent-
age of the relevant water isotope in the falling snow. This
allows for the retrieval of paleoclimatic information to
understand the evolution of the cryosphere. Although this
information is lost when snow falls on land, rivers, lakes or
oceans it is maintained when snow is deposited on areas
that could sustain it for long periods. Such areas include
ice caps and sheets, which are formed from the massive
accumulation of snow on flat plateaus such as those found
in several islands in Canada (i.e., Ellesmere Island, Penny
Ice Cap in Baffin Island), Greenland, and Antarctica.
A chemical analysis of deep ice cores extracted from
sections of ice caps on top of flat bedrock, where ice had
2.1. InItIal Ice FormatIon
Nucleation of ice crystals in freshwater lakes, rivers, or
oceans with saline water occurs when the water surface
cools down to the freezing temperature. As described ear-
lier, the onset of freezing is seawater characterized by the
formation of crystals known as frazil ice; a collection of
loose, randomly oriented needle‐shaped ice crystals in the
sea or in freshwater. Since the formation of ice could
originate in fresh or saline water, a few relevant properties
of both types are discussed first.
2.1.1. Relevant Seawater Properties
Freshwater lakes and rivers contain very little dissolved
impurities (less than 0.5%) and their effects on the growth
of ice can be neglected for all practical purposes. That is
not the case for sea water. The two properties of seawater
that are directly related to sea ice formation are its salin-
ity and density. The isotopic composition of water in case
of freshwater ice may not be considered as important,
but it may play some role in the case of seawater and ice
formed from seawater. It is relevant because snow cover on
sea ice may have a slightly different isotopic composition
than the seawater. Consequently, this property may be
used in distinguishing snow ice, formed from solidifica-
tion of snow saturated with meltwater, from frazil ice
formed from the seawater (section 4.3.3). The isotopic
composition of glacier ice cores at decadal resolution are
routinely used to infer climate record because the land‐
based ice bodies developed from snow depositions in the
past [
Von Grafenstein et al.,
1999]. A brief account of the
molecular composition of water, including its isotopic
composition, is introduced followed by another introduc-
tion to the two major properties of water that affect the ice
formation and growth, namely salinity and density.
2.1.1.1. Molecular Composition of Water
The atomic and the molecular structure of water in all
its phases (liquid and solid) have intrigued scientists for
many centuries. Although the chemical formula of water
is very simple, its atomic structure, depending on temper-
ature and mechanical or electrical forces, is extremely
complicated and difficult to study. The stoichiometric
composition of water has been investigated by numerous
