85
Composition of the Major Components of Seawater
further in this chapter. Changes in the composition of pore waters caused by temperature
effects are important. Different ratios have been found for sediments when kept at in situ
temperature. This is due to changes in the solution- solid equilibrium with temperature. If
the samples are squeezed at in situ temperatures, the results are not affected.
2.7 Isotopic Variations
2.7.1 Hydrogen and Oxygen
Water is the principal constituent of sea water. In 1929, stable isotopes of oxygen were discovered, and in 1932 deuterium was discovered. Thus, it became apparent that naturally
occurring water is a mixture of several species differing in molecular weight. There are
three known isotopes of hydrogen ( 1 H, 2 H, or D [deuterium] and 3 H or T [tritium]) and six
isotopes of oxygen ( 14 O, 15 O, 16 O, 17 O, 18 O, and 19 O). Tritium is radioactive with a half- life
of 12.5 yr. The isotopes of 14 O, 15 O, and 19 O are also radioactive but are short lived and do
not occur significantly in natural waters. The precise isotopic content of natural water
depends on the origin of the sample; however, within the limits of variation, the abundance is (Table 2.11) 99.73% 1 H 2
16 O (light water), 0.2% 18 O water, 0.04% 17 O water, and 0.032%
1 H 2 H 16 O water (HDO). It should be pointed out that the equilibrium
D 2 O + H 2 O = 2HDO
(2.36)
is always maintained. The hydrogen in water is 0.032/2 = 0.015% D. The presence of these
isotopes can change some of the properties of water and thus must be considered major
components of sea water.
Mass spectrophotometry is the best method of determining D and 18 O. Prior to D determination, the water sample is converted to hydrogen gas by reaction with hot zinc or
uranium metal. In the determination of 18 O, the water sample is equilibrated with CO 2 gas,
which is then analyzed by mass spectrophotometry.
Absolute isotopic abundances cannot yet be determined with sufficient accuracy to be of
use in studies of their natural variations. In earlier work, D analyses were reported relative
Table 2.11
Composition of Water with Respect to the Different Forms of Water
Water Molecule
Portion in Total Water
Portion in Heavy Water
Comparable Concentration
1 H 2
16 O
99.73
—
—
1 H 2
18 O
0.20
73.5
Mg
1 H 2
17 O
0.04
14.7
Ca
1 H 2 H 16 O
0.032
11.8
K
1 H 2 H 18 O
6 × 10 –5
0.022
N
1 H 2 H 17 O
1 × 10 –5
0.003
Al
2 D 2
16 O
3 × 10 –6
0.001
P
2 D 2
18 O
6 × 10 –9
2 × 10 –6
Hg
2 D 2
17 O
1 × 10 –9
3 × 10 –7
Au
Composition of the Major Components of Seawater
further in this chapter. Changes in the composition of pore waters caused by temperature
effects are important. Different ratios have been found for sediments when kept at in situ
temperature. This is due to changes in the solution- solid equilibrium with temperature. If
the samples are squeezed at in situ temperatures, the results are not affected.
2.7 Isotopic Variations
2.7.1 Hydrogen and Oxygen
Water is the principal constituent of sea water. In 1929, stable isotopes of oxygen were discovered, and in 1932 deuterium was discovered. Thus, it became apparent that naturally
occurring water is a mixture of several species differing in molecular weight. There are
three known isotopes of hydrogen ( 1 H, 2 H, or D [deuterium] and 3 H or T [tritium]) and six
isotopes of oxygen ( 14 O, 15 O, 16 O, 17 O, 18 O, and 19 O). Tritium is radioactive with a half- life
of 12.5 yr. The isotopes of 14 O, 15 O, and 19 O are also radioactive but are short lived and do
not occur significantly in natural waters. The precise isotopic content of natural water
depends on the origin of the sample; however, within the limits of variation, the abundance is (Table 2.11) 99.73% 1 H 2
16 O (light water), 0.2% 18 O water, 0.04% 17 O water, and 0.032%
1 H 2 H 16 O water (HDO). It should be pointed out that the equilibrium
D 2 O + H 2 O = 2HDO
(2.36)
is always maintained. The hydrogen in water is 0.032/2 = 0.015% D. The presence of these
isotopes can change some of the properties of water and thus must be considered major
components of sea water.
Mass spectrophotometry is the best method of determining D and 18 O. Prior to D determination, the water sample is converted to hydrogen gas by reaction with hot zinc or
uranium metal. In the determination of 18 O, the water sample is equilibrated with CO 2 gas,
which is then analyzed by mass spectrophotometry.
Absolute isotopic abundances cannot yet be determined with sufficient accuracy to be of
use in studies of their natural variations. In earlier work, D analyses were reported relative
Table 2.11
Composition of Water with Respect to the Different Forms of Water
Water Molecule
Portion in Total Water
Portion in Heavy Water
Comparable Concentration
1 H 2
16 O
99.73
—
—
1 H 2
18 O
0.20
73.5
Mg
1 H 2
17 O
0.04
14.7
Ca
1 H 2 H 16 O
0.032
11.8
K
1 H 2 H 18 O
6 × 10 –5
0.022
N
1 H 2 H 17 O
1 × 10 –5
0.003
Al
2 D 2
16 O
3 × 10 –6
0.001
P
2 D 2
18 O
6 × 10 –9
2 × 10 –6
Hg
2 D 2
17 O
1 × 10 –9
3 × 10 –7
Au