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Why is the Ocean Salty?
All water, even rain water, contains dissolved chemicals which scientists call "salts." But not all water tastes salty. Water is fresh or salty according to individual judgment, and in making this decision man is more convinced by his sense of taste than by a laboratory test. It is one's taste buds that accept one water and reject another.

A simple experiment illustrates this. Fill three glasses with water from the kitchen faucet. Drink from one and it tastes fresh even though some dissolved salts are naturally present. Add a pinch of table salt to the second, and the water may taste fresh or slightly salty depending on a personal taste threshold and on the amount of salt held in a "pinch." But add a teaspoon of salt to the third and your taste buds vehemently protest that this water is too salty to drink; this glass of water has about the same salt content as a glass of sea water. Obviously, the ocean, in contrast to the water we use daily, contains unacceptable amounts of dissolved chemicals; it is too salty for human consumption.


How salty the ocean is, however, defies ordinary comprehension. Some scientists estimate that the oceans contain as much as 50 quadrillion tons (50 million billion tons) of dissolved solids.

If the salt in the sea could be removed and spread 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. The saltiness of the ocean is more understandable when compared with the salt content of a fresh-water lake. For example, when 1 cubic foot of sea water evaporates it yields about 2.2 pounds of salt, but 1 cubic foot of fresh water from Lake Michigan contains only one one-hundredth (0.01) of a pound of salt, or about one sixth of an ounce. Thus, sea water is 220 times saltier than the fresh lake water. What arouses the scientist's curiosity is not so much why the ocean is salty, but why it isn't fresh like the rivers and streams that empty into it. Further, what is the origin of the sea and of its "salts"? And how does one explain ocean water's remarkably uniform chemical composition? To these and related questions, scientists seek answers with full awareness that little about the oceans is understood.


In popular language, "ocean" and "sea" are used interchangeably. Today's seas are the North and South Pacific, North and South Atlantic, Indian and Arctic Oceans and the Antarctic waters or seas. Scientists believe that the seas are as much as 500 million years old because animals that lived then occur as fossils in rocks which once were under ancient seas. There are several theories about the origin of the seas, but no single theory explains all aspects of this puzzle. Many earth scientists agree with the hypothesis that both the atmosphere and the oceans have accumulated gradually through geologic time from some process of "degassing" of the Earth's interior. According to this theory, the ocean had its origin from the prolonged escape of water vapor and other gases 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. As the water drained into the great hollows in the Earth's surface, the primeval ocean came into existence. The forces of gravity prevented the water from leaving the planet.


Sea water has been defined as a weak solution of almost everything. Ocean water is indeed a complex solution of mineral salts and of decayed biologic matter that results from the teeming life in the seas. Most of the ocean's salts were derived from gradual processes such the breaking up of the cooled igneous rocks of the Earth's crust by weathering and erosion, the wearing down of mountains, and the dissolving action of rains and streams which transported their mineral washings to the sea. Some of the ocean's salts have been dissolved from rocks and sediments below its floor. Other sources of salts include the solid and gaseous materials that escaped from the Earth's crust through volcanic vents or that originated in the atmosphere.


The Mississippi, Amazon, and Yukon Rivers empty respectively into the Gulf of Mexico, the Atlantic Ocean, and the Pacific Ocean, all of which are salty. Why aren't the oceans as fresh as the river waters that empty into them? Because the saltiness of the ocean is the result of several natural influences and processes, the salt load of the streams entering the ocean is just one of these factors.

In the beginning the primeval seas must have been only slightly salty. But ever since the first rains descended upon the young Earth hundreds of millions of years ago and ran over the land breaking up rocks and transporting their minerals to the seas, the ocean has become saltier. It is estimated that the rivers and streams flowing from the United States alone discharge 225 million tons of dissolved solids and 513 million tons of suspended sediment annually to the sea. Recent calculations show yields of dissolved solids from other land masses that range from about 6 tons per square mile for Australia to about 120 tons per square mile for Europe. Throughout the world, rivers carry an estimated 4 billion tons of dissolved salts to the ocean annually. About the same tonnage of salt from the ocean water probably is deposited as sediment on the ocean bottom, and thus, yearly gains may offset yearly losses. In other words, the oceans today probably have a balanced salt input and outgo.

Past accumulations of dissolved and suspended solids in the sea do not explain completely why the ocean is salty. Salts become concentrated in the sea because the Sun's heat distills or vaporizes almost pure water from the surface of the sea and leaves the salts behind. This process is part of the continual exchange of water between the Earth and the atmosphere that is called the hydrologic cycle. Water vapor rises from the ocean surface and is carried landward by the winds. When the vapor collides with a colder mass of air, it condenses (changes from a gas to a liquid) and falls to Earth as rain. The rain runs off into streams which in turn transport water to the ocean. Evaporation from both the land and the ocean again causes water to return to the atmosphere as vapor and the cycle starts anew. The ocean, then, is not fresh like river water because of the huge accumulation of salts by evaporation and the contribution of raw salts from the land. In fact, since the first rainfall, the seas have become saltier.


Scientists have studied the ocean's water for more than a century, but they still do not have a complete understanding of its chemical composition. This is partly due to the lack of precise methods and procedures for measuring the constituents in sea water. Some of the problems confronting scientists stem from the enormous size of the oceans, which cover about 70 percent of the Earth's surface, and the complex chemical system inherent in a marine environment in which constituents of sea water have intermingled over vast periods of time. At least 72 chemical elements have been identified in sea water, most in extremely small amounts. Probably all the Earth's naturally occurring elements exist in the sea. Elements may combine in various ways and form insoluble products (or precipitates) that sink to the ocean floor. But even these precipitates are subject to chemical alteration because of the overlying sea water which continues to exert its environmental influence.


Oceanographers report salinity (total salt content) and the concentrations of individual chemical constituents in sea water -- chloride, sodium, or magnesium for example -- in parts per thousand, for which the symbol o/oo is used. That is, a salinity of 35 o/oo means 35 pounds of salt per 1,000 pounds of sea water. Similarly, a sodium concentration of 10 o/oo means 10 pounds of sodium per 1,000 pounds of water.

The salinity of ocean water varies. It is affected by such factors as melting of ice, inflow of river water, evaporation, rain, snowfall, wind, wave motion, and ocean currents that cause horizontal and vertical mixing of the saltwater.


The ocean is salty because of the gradual concentration of dissolved chemicals eroded from the Earth's crust and washed into the sea. Solid and gaseous ejections from volcanoes, suspended particles swept to the ocean from the land by onshore winds, and materials dissolved from sediments deposited on the ocean floor have also contributed. Salinity is increased by evaporation or by freezing of sea ice and it is decreased as a result of rainfall, runoff, or the melting of ice. The average salinity of sea water is 35 o/oo, but concentrations as high as 40 o/oo are observed in the Red Sea and the Persian Gulf. Salinities are much less than average in coastal waters, in the polar seas, and near the mouths of large rivers.

Sea water not only is much saltier than river water but it also differs in the proportion of the various salts. Sodium and chloride constitute 85 percent of the dissolved solids in sea water and account for the characteristic salty taste. Certain constituents in sea water, such as calcium, magnesium, bicarbonate, and silica, are partly taken out of solution by biological organisms, chemical precipitation, or physical-chemical reactions. In open water the chemical composition of sea water is nearly constant. Because of the stable ratios of the principal constituents to total salt content, the determination of one major constituent can be used to calculate sea water salinity. For minor constituents and dissolved gases the composition is variable and therefore ratios cannot be used to calculate salt Circulation and mixing, density and ocean currents, wind action, water temperature, solubility, and biochemical reactions are some of the factors that explain why the composition of water in the open sea is almost constant from place to place.

This information is from a general interest publication ("Why is the Ocean Salty?" By Herbert Swenson) prepared by the U.S. Geological Survey to provide information about the earth sciences, natural resources, and the environment.

Uploaded: 2/21/2004