Geology Reference
In-Depth Information
settled with no side movement, provides insight into the
climatic conditions of the period related to the analyzed
section of the core. This can be achieved by measuring
δ
(
18
O), which is expressed as the fractional difference
between the ratio
16
O/
18
O in the sample and the ratio in
“standard mean ocean water” (SMOW) measured in
percent. In general, the values of
δ
(
18
O) in ice cap snow are
negative. Since
18
O is heavier than
16
O, then water molecules
containing the former evaporate proportionately less than
the latter. Nevertheless, under warmer atmospheric tempera-
tures, more molecules containing
18
O evaporate. This means
that higher negative values of
δ
(
18
O) found in the precipitated
snow are likely associated with a warmer climate although
there are also a number of other effects that can alter this
parameter [
Hobbs,
1974]. Since each depth segment of an
ice cap core has originated in the past, chemical analysis of
ice cores at different depths can then be used to identify
relatively warmer or colder periods of snow deposition.
Because snow has a lower content of
18
O than
16
O
compared to seawater, granular ice, formed from slush
of snow saturated with meltwater, can be chemically
distinguished from frazil ice formed from slush contain-
ing seawater and frazil crystals. These two types of ice
may have similar appearances and may not be identified
unless forensic type of microstructural analysis are
carried out on both horizontal and vertical sections.
Jefferies et al.
[1994] used both crystalline structure and
oxygen isotopic composition analysis of sea ice cores
from the Ross Sea in the Antarctic to determine the
amount of snow that contributed to the development of
sea ice. In fact, for first‐year sea ice in Okhotsk Sea,
Toyota et al.
[2007] found measurable differences in the
values of
δ
(
18
O) for snow, frazil, and columnar ice.
brackish water is usually found in estuaries, inland seas,
or lakes. Salinity is affected by the relative amount of pre-
cipitation and evaporation as well as the mixing with
freshwater in the vicinity of river mouths. If a large river
is emptying its water into a sea, the local seawater salinity
is significantly less than the typical value of 35‰.
Variation of salinity is one criterion for water classifica-
tion. It is also revealed in different mechanisms of sea ice
formation as well as structure and physical properties
of ice. Freshwater implies salinities less than 0.5‰, while
brackish water bodies have salinities more than that
of freshwater but less than the usual salinity of seawater.
Thus brackish water is defined as water having salinities
in a wide range, between 0.5‰ and 29‰ According to
this nomenclature, saline water can be defined as water
with salinity ranging from 29‰ to 50‰. Brine is a term
that describes water with salinity higher than 50‰.
The primary salt dissolved in seawater is sodium chlo-
ride (NaCl), but other salts exist such as sodium sulfate
(Na
2
SO
4
· 10H
2
O), magnesium sulfate (MgSO
4
), and mag-
nesium chloride (MgCl
2
· 12H
2
O and MgCl
2
· 8H
2
O). The
ionic proportion of chloride, sodium, sulfate, and magne-
sium in these compounds (regardless of the water salinity)
is close to 55.03%, 30.59%, 7.68%, and 3.68%, respec-
tively. The rest of the ionic proportion, namely 3.02%, is
composed of calcium (Ca), potassium (K), bromide (Br),
cobalt (CO), and other elements in negligible amounts.
Solutes in seawater are rejected during the process of
solidification. Most are rejected to the water at the ice‐
water interface. Some are entrapped as inclusions within
the ice mass. Brine inclusions are known as brine pockets.
They are located, as will be shown later, along boundaries
and subboundaries of ice crystals. The entrapped brine
naturally remains in sea ice in thermal equilibrium with
the surrounding ice. The temperature of the ice deter-
mines the salinity of the liquid in the brine pockets. With
the decrease in temperature, different solutes in the brine
start to precipitate at different temperatures. This precipi-
tation process makes the ionic concentration of the liquid
as temperature dependent, as can be seen in Table 2.1.
2.1.1.2. Seawater Salinity
Seawater holds the same isotopic water composition as
freshwater, yet it contains a considerable percentage of
dissolved salts and gases. Water salinity is the most rele-
vant property for sea ice formation, composition, and
growth. It is usually measured as the ratio of the weight
of salts (in grams) dissolved in one 1000 g (1 k) of seawa-
ter. Hence, it is usually presented in parts per thousands
(ppt or ‰). Alternatively, oceanographers define ocean
salinity in terms of the practical salinity unit (PSU),
which is the conductivity ratio of a seawater sample to
the conductivity of a solution of potassium chloride (i.e.,
the standard solution of measuring electrical conductiv-
ity). The first measure is adopted in this topic.
Seawater salinity is 35‰ on average in most marine
areas, though slightly higher values (36‰) are observed
in some regions in the Atlantic Ocean and Indian Ocean
and slightly less values (34‰) are observed in the polar
region. Within the Canadian Archipelago, water salini-
ties are often found to be in the range of 30‰. The
Table 2.1
Major salts in sea ice and their precipitation
temperatures.
Precipitating
Temp. (
°
C)
Salt Name
Composition
Calcium carbonate
CaCO
3
· 6H
2
O
−2.20
Sodium Sulfate
Na
2
SO
4
· 10H
2
O
−8.20
Magnesium chloride
MgCl
2
· 8H
2
O
−18.0
Sodium chloride
NaCl · 2H
2
O
−22.9
Magnesium chloride
MgCl
2
· 12H
2
O
−36.8
Calcium chloride
CaCl
2
· 6H
2
O
−55.0
Adapted from
Weeks and Ackley
[1982].
