Seawater Composition

The properties of seawater are determined primarily by the properties of pure water and secondarily by its nature as a solution. Because water is a liquid and has great capacity as a solvent, seawater is well mixed and salty due to the ions dissolved from the continental crust's rocks. Seawater is a source of mineral wealth for humankind.

The Formation of Earth

The hydrosphere consists of the water areas of Earth, which include ponds, lakes, rivers, groundwater, and the oceans. The oceans form the largest portion of the hydrosphere, covering 71 percent of Earth’s surface. The composition of Earth’s water derives from the circumstances surrounding the formation of the solar system and the planet Earth.

In the beginning, it is hypothesized that a mass of gases and space dust came together, and gravitational eddies eventually formed separate clouds of aggregate materials, which presumably later consolidated into planets. When the young planetary mass started to cool, it formed a thin crust that allowed heat, molten material, and gaseous material to escape from the interior through numerous cracks. The gaseous material formed the first atmosphere, which was probably made up primarily of hydrogen and helium molecules. Being fairly light in weight and highly energized by atmospheric temperatures and incoming solar radiation, these molecules were probably lost to space. Eventually, they were replaced by other gases derived from the interior by the ongoing volcanic activity as the planet’s surface continued to cool. This subsequent atmosphere was composed mainly of carbon dioxide and water vapor, with higher percentages of other gases released from entrapment in rock structures than are found today. The water in the oceans and all other water on the planet are believed to have been released from within the mineral structures of the material from which the planet formed.

Clouds formed from the condensing water vapor in the ancient atmosphere, shielding Earth’s surface and allowing less than 60 percent of the Sun’s energy to penetrate. As the surface continued to cool, the water vapor condensed into liquid, falling and accumulating in depressions. With widespread volcanic activity continuing, water vapor was continuously being released, along with smaller amounts of carbon dioxide, chlorine, nitrogen, and hydrogen gas, which then underwent chemical reactions driven by sunlight to produce methane and ammonia. As the surface continued to cool, vast amounts of condensation eventually formed the oceans. Geologic evidence shows that the oceans have existed for at least 3 billion years. This evidence comes from algal fossils presumed to have grown in a marine environment.

Earth’s crust was formed by primary crystalline rocks, the first molten material that solidified into rock as the planet cooled. These rocks were weathered and eroded into the particles that became deposited and slowly dissolved in water, carried into the ocean basins, and accumulated. Also carried into the ocean were great loads of particulate material weathered from the primary crystalline rocks and deposited as sediments. Thus, the components of the primary rocks were freed by chemical weathering and dissolved in ocean water or chemically bonded with sediments and carried into the ocean by rivers. Chemical weathering took place because of the different compounds in the atmosphere, some of which combined with water vapor to form acidic rainwater, ranging in strength from slightly acidic solutions of carbonic acid to solutions of much stronger nitric and sulfuric acids. Each of these various acidic solutions could leach out various metallic ions, such as sodium, potassium, magnesium, iron, and others, from the materials that they contact. Seawater gained its characteristic saltiness through these processes.

Comparing the discharge of gases by hot springs in the United States to the average rate throughout the 3 billion years of the oceans’ existence shows that enough water vapor is produced to fill the oceans to one hundred times their present volume. Water, therefore, must be recycled and not all newly formed; the excess amount does not represent new water released from the original crystallization of magma. Only 1 percent of this water requires such an origin to account for the present volume of the oceans.

Water on land comes daily from the sea. The seas hold about 4.4 billion cubic meters of salt water. Of this amount, about 12 million cubic meters enter the atmosphere each year through evaporation and is returned by rainfall and the flow of rivers, and about 3 million cubic meters descend each year over continents, replenishing ponds, lakes, and rivers.

Gases and Solids in Seawater

Ocean water is a mixture of gases and solids dissolved in pure water (96 percent pure water and 4 percent of dissolved elements by weight). Nearly every natural element has been found or is expected to be found in seawater, although some occur only in very small amounts. The most abundant mineral found in ocean water is sodium chloride (familiar as common table salt), which makes up 85 percent of the dissolved minerals. It is interesting to note that the composition of human blood is very similar to that of seawater; in it are all the elements of the sea dispensed in different proportions.

The seven most abundant minerals in seawater include sodium chloride, 27.2 parts per thousand; magnesium chloride, 3.8 parts per thousand; magnesium sulfate, 1.7 parts per thousand; calcium sulfate, 1.3 parts per thousand; potassium sulfate, 0.9 parts per thousand; calcium carbonate, 0.1 part per thousand; and magnesium bromide, 0.1 part per thousand. It is important to note that these “minerals” do not occur in the form suggested by their molecular formulas but as the component ions dissolved in water. Six ionic species actually make up 99.3 percent of the total mass of the dissolved material: chlorine, 55.2 percent; sodium, 30.4 percent; sulfate, 7.7 percent; magnesium, 3.7 percent; calcium, 1.2 percent; potassium, 1.1 percent; and others, 0.7 percent.

The salinity of seawater is fairly uniform across the oceans at different latitudes and depths, mainly because of winds, waves, and currents. Local variations in mineral content are attributed to freshwater streams entering oceans, glacial melt, and human activity, but the variations overall are small. The salinity averages about 35 parts per thousand. This has been verified by the Glomar Challenger expedition, which used Nansen bottles to take seawater samples at different depths around the world, salinometers to measure salinity, and many other tests. There is more variation of salinity at the surface than at depths because of freshwater (rivers, glacial melt) entering the ocean at a given location, plus the biological activity and climate at different latitudes.

In areas where freshwater is entering the ocean, the salinity will decrease. Biological activity changes the salinity according to which species and how many marine plants and animals reside in the area. In hot and dry climates, the evaporation rate is high, and rainfall is low, so the ratio of dissolved salts to water is higher, making the water more saline (saltier). Similarly, in the polar regions in winter, the water freezes, but the minerals do not, which changes the water-to-mineral ratio and thus increases salinity. In most climates, rainfall is greater than evaporation, thus diluting seawater and decreasing salinity.

The most abundant dissolved gases in the ocean are nitrogen, carbon dioxide, and oxygen. The amount varies with depth, and as oxygen and carbon dioxide are vital to life, most living plants and animals are found in the top 100 meters of the ocean, or the sunlight penetration layer. The amount of dissolved gases will also vary with temperature. Warm water holds less dissolved gas than cold water because cold water is heavier and sinks, carrying oxygen-rich water to the ocean depths, allowing fish and marine life to live in the deepest parts of the ocean. At the surface, gases (oxygen and carbon dioxide) are exchanged between the ocean and the atmosphere and between the plants and animals that live in the ocean's top layers, where the most abundant life is found. Marine plants take in carbon dioxide and water and then, with the aid of sunlight, produce the sugar glucose and oxygen in the process called photosynthesis. Marine animals and photosynthetic plants take in oxygen and exhale or release carbon dioxide in a process called respiration.

Most precipitation that falls returns to the ocean largely by rivers, bearing salts from soils and rock in solution. Many things affect the salinity of the ocean, including the exchange of water between the ocean and the atmosphere, which is determined by climate, and the absorption of salts by plants and animals. Considering the history of ocean salinity, one might ask if the oceans have possessed a relatively uniform salinity throughout their history or if they are becoming more saline. By far, the most important component of salinity is the chloride ion, which is produced in the same manner as water vapor. The ratio of chloride ions to water vapor has not fluctuated throughout geologic time, so scientists conclude, based on present evidence, that ocean water salinity has been relatively constant over the lifetime of the oceans.

It is now evident that the oceans became salty early in their history because of weathering by acidic rainfall and the erosion of primary crystalline rock. Also, through continual volcanic activity, water vapor and gases were amply supplied to the atmosphere to aid in this weathering and eroding process. These particles were carried to the ocean and dissolved in seawater, making the water salty. This process has been going on since the formation of the oceans, so it is safe to assume that the ocean has been salty since it was formed approximately 3 billion years ago.

Comparing past and present oceans' salinity by studying vents and hot springs, the salinity appears to maintain a certain balance. Salinity ranges between 33 and 38 parts per thousand, by weight. This variation is caused by atmospheric effects at different latitudes, by freshwater entering into the oceans from rivers and glacial melt, and by biological activity in the ocean itself as plants and animals remove minerals needed for their growth and development (photosynthesis, respiration, shell building, and so on) from the seawater and put into it other forms (waste products, skeletal structures, shells, casts, and so on). In local areas, such as bays, coves, and estuaries, the salinity of seawater is further affected by human activities such as industry and agriculture, and by the pollution from these activities.

Study of Seawater Composition

The investigation of seawater composition is accomplished primarily by collecting seawater samples at different depths around the world and then measuring the salinity of the samples. A variety of instruments are used to collect samples. Nansen bottles are special metal cylinders fastened at a measuring point on a strong wire, which is then lowered into the sea to the desired depth. A messenger weight is dropped down the wire; when it strikes the bottle, it releases a catch. The bottle then turns upside down, and its valves close, trapping the water at that depth inside. However, the Nansen bottle commonly used does not seal completely, making the Fjorlie sampler or Niskin bottle a better apparatus. The Fjorlie sampler or Niskin bottle is attached to a line at both ends with spring-closing hinged ends. A messenger closes the bottle with a good seal.

Corrosion of metal-lined samplers may cause changes in the water composition in an hour or so. Copper, zinc, lead, and iron in metal linings often contaminate seawater samples. Plastics have solved this problem for both the collection and storage of seawater. It is also necessary to filter out any organic matter that could alter the seawater composition. For maximum accuracy of testing, samples should be tested as soon as possible and not stored.

A salinometer is used to electronically measure salinity. It is easy to use and gives immediate readings. Because the ions dissolved in seawater affect its properties as a conductor of electricity, the more ionic mineral matter in the sample, the better it conducts. The results are then compared to a table of standard measurements to obtain the sample’s salinity. Scientists have developed new technologies for measuring seawater composition in the twenty-first century. Ultra-high-resolution density sensors can measure seawater's temperature, salinity, and pressure with increased accuracy. Optical salinity sensors use seawater's refractive index to report its composition. Electric conductivity sensors are also employed to measure seawater salinity. 

Principal Terms

element: one of several substances composed entirely of atoms that cannot be broken into smaller particles by chemical means

free oxygen: the element oxygen by itself, not combined chemically with a different element

hydrosphere: the areas of Earth that are covered by water, including the oceans, seas, lakes, and rivers

mineral: an inorganic substance occurring naturally and having definite physical properties and a characteristic chemical composition that can be expressed by a chemical formula

nodule: a lump of mineral rock typically found on the ocean floor

primary crystalline rock: the original or first solidified molten rock of Earth

salinity: a measure of the quantity of dissolved solids in ocean water, typically given in parts per thousand by weight

weathering: the breaking down of rocks by chemical, physical, and biological means

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