All water, including water in rivers and lakes contains dissolved chemicals which scientists call “salts”. However, not all water tastes salty, for the content of salt vary among different water bodies. Sodium and chloride, the components of common table salt constitute a little more than 85 percent of the dissolved solids in ocean water and give to the water its characteristic salty taste, but they represent less than 16 percent of the salt content of river water.
Seawater is 220 times saltier than the fresh lake water. For example, when 1 cubic foot of seawater evaporates it yields about 2.2 pounds of salt, but 1 cubic foot of fresh water from Lake Michigan contains only about one sixth of an ounce of salt.
Thus while the oceans of the world have high salinity (a high concentration of salt content) that makes it unpalatable to humans, the water in rivers and lakes have low salinity (a negligible concentration of salt) that does not have any impact when humans drink this water.
THE SOURCES OF OCEAN “SALTS”
Eons ago, when the earth was taking shape, water vapor and other gases escaped on a continuous basis from the molten igneous rocks of the Earth to the clouds surrounding the cooling Earth. After the Earth’s surface had cooled to a temperature below the boiling point of water, rain began to fall and continued to fall for centuries. This water drained into the great hollows in the Earth’s surface, forming oceans.
Water in these oceans is a complex solution of mineral salts and of decayed biologic matter. The salts in the ocean gathered through the following ways:
1. Over a period of centuries, the process of weathering and erosion broke up the cooled igneous rocks of the Earth’s crust, and wore down mountains. Rains and streams transported the mineral washings from these rocks and mountains into the oceans.
2. The basalt rocks that underlie the ocean are rich in salts. Water in the ocean is in perpetual reaction with such rocks. The most intense of these reactions occur in hydrothermal vents or hot springs along mid-ocean ridges. The entire volume of ocean water gradually circulates into such hydrothermal vents, reacts with hot basalt, and returns greatly changed in chemical composition. In this transit, water loses Oxygen, and gains salts like Calcium, Silicon, Iron, and Manganese.
3. The eruption of volcanoes under water also results in the normal seawater reacting with hot rocks and dissolving some of the mineral constituents. Some solid and gaseous materials that escape from the Earth’s crust through volcanic vents also contribute to the ocean’s salt component.
4. The rivers and streams that flow into the ocean also contribute to the salt content in the ocean. Rivers and streams flowing from the United States alone discharge an estimated 225 million tons of dissolved solids and 513 million tons of suspended sediment annually to the sea. Throughout the world, rivers carry an estimated 4 billion tons of dissolved salts to the ocean annually. However, this is only a tiny fraction of the total salt in the ocean.
If one were to remove all the salt in the sea and spread it evenly over the Earth’s land surface, it would form a layer more than 500 feet thick, about the height of a 40-story office building.
Salinity is low in the polar seas where the salt water dilutes by melting ice and continued precipitation. Partly landlocked seas or coastal inlets that receive substantial runoff from precipitation falling on the land also have low salinity.
THE HYDROLOGICAL CYCLE
The water in the ocean evaporates as vapor into the air, as a result on the sun’s heat. Ice and snow also sublimate directly into water vapor. The rising air currents take these water vapors up into the atmosphere where cooler temperatures cause it to condense into clouds. Air currents move clouds around the globe. When such clouds collide, they grow, and eventually fall out of the sky as precipitation. Some precipitation falls as snow and some fall as rains. The rain runs off into streams that in turn transport water to the ocean.
However, the sun’s heat distills or vaporizes only pure water from the surface of the oceans. The salts remain and concentrate in the seabed.
WHY IS THE SALINITY OF THE OCEANS CONSTANT?
With the continuous increase of salinity in the oceans through all the processes listed above, the salinity of the ocean waters should increase with the passage of time. However, this is not the case, and the concentration of salts in ocean water has remained constant over the centuries. The two major causes for this are:
1. The Marine life in the oceans plays a major role in maintaining the balance of the ocean’s salts. Mollusks like oysters, clams, and mussels, phytoplankton like cocolithophores and diatoms, crustaceans such as crabs, shrimp, lobsters, and barnacles and Foraminifers, all extract calcium and silicon from the sea to build their shells and skeletons. When such animals die, these hard particles eventually fall toward the ocean floor and remain buried in the sediment, removed from seawater for many millions of years. In the tropical areas, the skeletons of small corals and other sea animals accumulate on the banks to form coral reefs that consist mostly of limestone or calcium carbonate.
2. When ocean plates collide with continental plates, continental plates, which are less dense floats over the ocean one. In the process of pushing the ocean floor under, at least half of its mineral-rich, salty sediments end up lost deep within Earth.
WHY DO RIVERS AND LAKES HAVE LESS SALINITY?
Rains or glaciers and ice caps in the mountains feed the rivers of the world. As explained above, the water that falls down as rain originate as vapor from the oceans, and these vapors do not contain dissolved salts. Whatever salts are present in rainwater and ice caps are a result on atmospheric impurities, both natural and pollutants.
When the rainwater enters the rivers and streams, passes through soil, and percolates through rocks, some of the minerals dissolve, and the salt concentration of the water reduces to a negligible amount. Thus, as long as the land contains no salt, the water does not pick up any more salt.
Lakes are normally just wide depressions in a river channel, with water flowing in from one end and out from the other end. They are generally not deep or vast enough to affect the factors that cause salinity in the oceans, and as such, the salinity content remains low.
However, three big lakes in the world, the Great Salt Lake, the Dead Sea, and Death Valley, California, do not have any outlets to take the water away, and water escapes from these lakes only by evaporation. Just as it is with oceans, when water evaporates the dissolved salts remain, and with every drop of water that evaporates, the lake has more salt content. Since these lakes are large some of the factors that cause salinity in oceans come into play here, and this is why these lakes are ten times saltier than water in the oceans.
USEFUL WEBSITES
http://www.utdallas.edu http://www.waterencyclopedia.com http://www.palomar.edu/oceanography
REFERENCES
Chester, Roy Marine Geochemistry, 2nd ed. London, U.K.: Blackwell Publishers, 2000. Dasch, E. Julius, ed. Encyclopedia of Earth Sciences. New York: Macmillan Reference USA, 1996. Libes, Susan. An Introduction to Marine Biogeochemistry. New York: John Wiley & Sons, 1991. Pilson, Michael E. Q. An Introduction to the Chemistry of the Sea. Upper Saddle River, NJ: Prentice Hall, 1998.
