The oceans cover more than 70 percent or the
Earth’s surface and extend to an average depth of several
kilometers. As part of the hydrologic cycle, each year approximately
1.27 × 1016 cubic feet (3.6 × 1014 m3) of water evaporates
from the oceans with about 90 percent of this returning
to the oceans as rainfall. The remaining 10 percent falls as
precipitation on the continents where it forms freshwater
lakes and streams and seeps into the groundwater system
where it is temporarily stored before eventually returning to
the sea. During its passage over and in the land, the water
erodes huge quantities of rock, soil, and sediment, and dissolves
chemical elements such as salts from the continents,
carrying these and other sediments as dissolved, suspended,
and bed load to the oceans. More than 50 million tons of
continental material is transported into the oceans each year.
Most of the suspended and bed load materials are deposited
as sedimentary layers near passive margins, but the dissolved
Name Symbol Thousand Material
Chloride Cl– 18.980 55.05
Sodium Na+ 10.556 30.61
Sulfate SO4
2– 2.649 7.68
Magnesium Mg2+ 1.272 3.69
Calcium Ca2+ 0.400 1.16
Potassium K+ 0.380 1.10
Bicarbonate HCO3– 0.140 0.41
Bromide Br– 0.065 0.19
Borate H3BO3– 0.026 0.07
Strontium Sr2+ 0.008 0.03
Fluoride F– 0.001 0.00
Total 34.447 99.99
salts and ions derived from the continents play a major role
in determining seawater chemistry. The most abundant dissolved
salts are chloride and sodium, which together with sulfate,
magnesium, calcium, potassium, bicarbonate, bromide,
borate, strontium, and fluoride form more than 99.99 percent
of the total material dissolved in seawater.
In addition to the elements listed in the table, there are a
number of minor and trace elements dissolved in seawater
that are important for the life cycle of many organisms. For
instance, nitrogen, phosphorous, silicon, zinc, iron, and copper
play important roles in the growth of tests and other
parts of some marine organisms. Gases, including nitrogen,
oxygen, and carbon dioxide are also dissolved in seawater.
The amount of oxygen dissolved in the surface layers of seawater
is about 34 percent of the total dissolved gases, significantly
higher than the 21 percent of total dissolved gases
oxygen comprises in the atmosphere. This oxygen is generated
through photosynthesis by marine plants, where it is
exchanged with the atmosphere across the air–water interface,
and also sinks where it is used by deep aerobic organisms.
The amount of carbon dioxide dissolved in seawater is
about 50 times greater than its concentration in the atmosphere.
CO2 plays an important role in buffering the acidity
and alkalinity of seawater where, through a series of chemical
reactions, it keeps the pH of seawater between 7.5 and 8.5.
Marine organisms make carbonate shells out of the dissolved
CO2, and some is incorporated into marine sediments where
it is effectively isolated from the atmosphere. The total
amount of CO2 stored in the ocean is very large, and as a
greenhouse gas, if it were to be released to the atmosphere, it
would have a profound effect on global climate.
The salinity and temperature of seawater are important
in controlling mixing between surface and deep water, and in
determining ocean currents. Temperature is controlled largely
by latitude, whereas river input, evaporation from restricted
basins, and other factors determine the total dissolved salt
concentration. Density differences caused by temperature and
salinity variations induce ocean currents and thermohaline
circulation, distributing heat and nutrients around the globe.
See also HYDROLOGIC CYCLE; OCEAN CURRENTS;
OCEANOGRAPHY; THERMOHALINE CIRCULATION.
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