By Herbert Swenson
US Geological Survey Publication
HOW SALTY IS THE OCEAN?...
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.
Illustration: Sources of salts in the ocean.
THE ORIGIN OF THE SEA...
SOURCES OF THE SALTS...
IF FRESH WATER FLOWS OUT TO THE SEA, WHY IS THE SEA STILL
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.
SEA WATER IS NOT SIMPLE...
SALINITY AND ITS VARIABILITY...
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 SALTIEST WATER...
Low salinities occur in polar seas where the salt water is diluted by melting ice and continued precipitation. Partly landlocked seas or coastal inlets that receive substantial runoff from precipitation falling on the land also may have low salinities. The Baltic Sea ranges in salinity from about 5 to 15 o/oo. The salinity of the Black Sea is less than 20 o/oo. Water of the Puget Sound in the Tacoma, Wash., area ranges in salt content from 21 to about 27 o/oo. This area is drained by a number of fresh-water streams which discharge an average of about 4.1 billion gallons of water per day into Puget Sound. Salinity of sea water along the coastal areas of the conterminous United States varies with the month of the year as well as with geographic location. For example, the salinity of the ocean water off Miami Beach, Fla., varies from about 34.8 o/oo in October to 36.4 o/oo in May and June, while diagonally across the country, off the coast of Astoria, Oregon, the salinity of sea water varies from 0.3 o/oo in April and May to 2.6 o/oo in October. The water off the coast of Miami Beach has a high salt content because it is undiluted sea water. Off the coast of Astoria, however, the sea water is less saline because it is mixed with the fresh water of the mighty Columbia.
Sometimes river water travels far from shore before it mixes with sea water. This is shown by data gathered from a study of the Columbia River, which, in an average year, carries to the ocean enough water to cover an area of 1 million acres to a depth of 197 feet. Using a radio- active tracer, scientists at Oregon State University have followed the river's water from its mouth near Astoria to a point southwest of Coos Bay, 217 miles away.
The salt content of the open oceans, free from land influences, is rarely less than 33
o/oo and seldom more than 38 o/oo. Throughout the world, the salinity of sea water
averages about 35 o/oo. This average salinity was obtained by William Dittmar in 1884 from
chemical analyses of 77 sea water samples collected from many parts of the world during
the scientific expedition of the British corvette, H.M.S. Challenger. The Challenger
expedition, organized by the British Government at the suggestion of the Royal Society,
set out to study the biology of the sea, examine the chemical and physical properties of
the water, sample deposits on the ocean floor, and measure water temperatures. The voyage
began in 1872 and ended almost 4 years later after covering 68,890 nautical miles. This
expedition remains today the longest continuous scientific investigation of the ocean
basins. Dittmar's 77 samples are still the only worldwide set of samples of sea water for
which complete data (each principal constituent) on chemical composition are available.
More recent data, reflecting improvements in analytical and sampling techniques, show
slight deviations from Dittmar's results, but these changes do not affect the overall
usefulness of his work. The average composition of the 77 samples is as shown on the
The salinity of water in the open sea is not fixed at 35 o/oo even in areas distant from land; that figure is only an average. On a worldwide basis, a maximum salinity of 36 o/oo occurs at about latitudes 20º N. and 20º S. The average salinity of sea water, 35 o/oo , occurs at the Equator. A minimum salinity of 31 o/oo corresponds approximately with latitude 60º N., whereas lowest salinities of 33 o/oo in the Southern Hemisphere occur at latitude 60º S. At the Equator, where salinity is 35 o/oo, the dilution of sea water by rain is offset by the loss of water by evaporation. But in the latitudes bordering the Equator the opposite condition prevails -- evaporation exceeds rainfall because high temperatures plus increased winds accelerate evaporation losses.
(2) Rivers carry to the sea more calcium than chloride, but the oceans nevertheless contain about 46 times more chloride than calcium.
(3) Silica is a significant constituent of river water but not of sea water.
(4) Calcium and bicarbonate account for nearly 50 percent of the dissolved solids in river water yet constitute less than 2 percent of the dissolved solids in ocean water. These variations seem contrary to what one would expect.
Part of the explanation is the role played by marine life -- animals and plants -- in ocean water's composition. Sea water is not simply a solution of salts and dissolved gases unaffected by living organisms in the sea. Mollusks (oysters, clams, and mussels, for example) extract calcium from the sea to build their shells and skeletons. Foraminifers (very small one-celled sea animals) and crustaceans (such as crabs, shrimp, lobsters, and barnacles) likewise take out large amounts of calcium salts to build their bodies. Coral reefs, common in warm tropical seas, consist mostly of limestone (calcium carbonate) formed over millions of years from the skeletons of billions of small corals and other sea animals. Plankton (tiny floating animal and plant life) also exerts control on the composition of sea water. Diatoms, members of the plankton community, require silica to form their shells and they draw heavily on the ocean's silica for this purpose.
Some marine organisms concentrate or secrete chemical elements that are present in such minute amounts in sea water as to be almost undetectable: Lobsters concentrate copper and cobalt; snails secrete lead; the sea cucumber extracts vanadium; and sponges and certain seaweeds remove iodine from the sea.
Sea life has a strong influence on the composition of sea water. However, some elements in sea water are not affected to any apparent extent by plant or animal life. For example, no known biological process removes the element sodium from the sea.
In addition to biological influences, the factors of solubility and physical-chemical reaction rates also help to explain the composition of sea water. The solubility of a constituent may limit its concentration in sea water. Excess calcium (more calcium than the water can hold) may be precipitated out of the water and deposited on the sea floor as calcium carbonate. Presumably as a result of physical-chemical reactions not well understood, the metal manganese occurs as nodules in many places on the ocean floor. Similarly, phosphorite (phosphate rock) is found in large amounts on the sea bottom off southern California and in lesser amounts in several other places.
NEAR-CONSTANT RATIOS OF MAJOR CONSTITUENTS...
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.