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+
1,000 g 965 g
seawater water 35 g salts

FIGURE 24.13 Salinity is defined as the mass of salts
dissolved in 1.0 kg of seawater. Thus, if a sample of seawater has
a salinity of 35‰, a 1,000 g sample will evaporate 965 g of water
and leave 35 g of sea salts behind.

chloride, but it also contains salts of the four metal ions (sodium,
magnesium, calcium, and potassium) combined with the different
negative ions of chlorine, sulfate, bicarbonate, and so on.
The amount of dissolved salts in seawater is measured
as salinity. Salinity is defined as the mass of salts dissolved in
1.0 kg, or 1,000 g, of seawater. Since the salt content is reported FIGURE 24.14 Air will dissolve in water, and cooler water will
dissolve more air than warmer water will. The bubbles you see here
in parts per thousand, the symbol ‰ is used (% means parts
are bubbles of carbon dioxide that came out of solution as the soda
per hundred). Thus, 35‰ means that 1,000 g of seawater con- became warmer.
tains 35 g of dissolved salts (and 965 g of water). This is the
same concentration as a 3.5 percent salt solution (Figure 24.13). If you have ever allowed a glass of tap water to stand for a pe-
Oceanographers use the salinity measure because the mass of a riod of time, you may have noticed tiny bubbles collecting as the
sample of seawater does not change with changes in the water water warms. These bubbles are atmospheric gases, such as nitro-
temperature. Other measures of concentration are based on the gen and oxygen, that were dissolved in the water (Figure 24.14).
volume of a sample, and the volume of a liquid does vary as it Seawater also contains dissolved gases in addition to the dissolved
expands and contracts with changes in the temperature. Thus, salts. Near the surface, seawater contains mostly nitrogen and oxy-
by using the salinity measure, any corrections due to tempera- gen, in similar proportions to the mixture that is found in the at-
ture differences are eliminated. mosphere. There is more carbon dioxide than you would expect,
The average salinity of seawater is about 35‰, but the
concentration varies from a low of about 32‰ in some locations however, as seawater contains a large amount of this gas. More
carbon dioxide can dissolve in seawater because it reacts with wa-
up to a high of about 36‰ in other locations. The salinity of sea- ter to form carbonic acid, H 2 CO 3 , the same acid that is found in
water in a given location is affected by factors that tend to increase a bubbly cola. In sea water, carbonic acid breaks down into bicar-
or decrease the concentration. The concentration is increased by bonate and carbonate ions, which tend to remain in solution. Wa-
two factors: evaporation and the formation of sea ice. Evapo ration ter temperature and the salinity have an influence on how much
increases the concentration because it is water vapor only that gas can be dissolved in sea water, and increasing either or both will
evaporates, leaving the dissolved salts behind in a greater concen- reduce the amount of gases that can be dissolved. Cold, lower-
tration. Ice that forms from freezing seawater increases the con- salinity seawater in colder regions will dissolve more gases than
centration because when ice forms, the salts are excluded from the the warm, higher-salinity seawater in tropical locations. Abun-
crystal structure. Thus, sea ice is freshwater, and the removal of dant algae and seaweeds in the upper, sunlit water tend to reduce
this water leaves the dissolved salts behind in a greater concentra- the concentration of carbon dioxide and increase the concentra-
tion. The salinity of seawater is decreased by three factors: heavy tion of dissolved oxygen through the process of photosynthesis.
precipitation, the melting of ice, and the addition of freshwater by With increasing depth, less light penetrates the water, and below
a large river. All three of these factors tend to dilute seawater with about 80 m (about 260 ft), there is insufficient light for photosyn-
freshwater, which lowers the concentration of salts. thesis. Thus, more algae and seaweeds and more dissolved oxygen
Note that increases or decreases in the salinity of seawater
are brought about by the addition or removal of freshwater. This are found above this depth. Below this depth, there are no algae or
seaweeds, more dissolved carbon dioxide, and less dissolved oxy-
changes only the amount of water present in the solution. The gen. The oxygen-poor, deep ocean water does eventually circulate
kind or proportion of the ions present (Table 24.1) in seawater back to the surface, but the complete process may take several
does not change with increased or decreased amounts of fresh- thousand years. (For a worked example on this material, see the
water. The same proportion, meaning the same chemical com- chapter 24 resources at www.mhhe.com/tillery.)
position, is found in seawater of any salinity of any sample taken
from any location anywhere in the world, from any depth of the
ocean, or taken any time of the year. Seawater has a remarkably MOVEMENT OF SEAWATER
uniform composition that varies only in concentration. This Consider the enormity of Earth’s ocean, which has a surface
2
2
means that the ocean is well mixed and thoroughly stirred area of some 361 million km (about 139 million mi ) and a vol-
3
3
around the entire Earth. ume of 1,370 million km (about 328 million mi ) of sea water.
24-13 CHAPTER 24 Earth’s Waters 609