Composition of Seawater

Seawater is a solution of salts of nearly constant composition, dissolved in variable amounts of water. It is denser than freshwater. It is risky to drink seawater because of its high salt content. More water is required to eliminate the salt through excretion than the amount of water that is gained from drinking seawater. Seawater can be turned into potable water by desalination processes or by diluting it with freshwater. The origin of sea salt is traced to Sir Edmond Halley, who in 1715 proposed that salt and other minerals were carried into the sea by rivers, having been leached out of the ground by rainfall runoff. On reaching the ocean, these salts would be retained and concentrated as the process of evaporation removed the water.

There are more than 70 elements dissolved in seawater as ions, but only six make up more than 99 percent of all the dissolved salts; namely, chloride (55.04 weight percent [wt%]), sodium (30.61 wt%), sulphate (7.68 wt%), magnesium (3.69 wt%), calcium (1.16 wt%), and potassium (1.10 wt%). Trace elements in seawater include manganese, lead, gold, and iodine. Biologically important elements such as oxygen, nitrogen, and iron occur in variable concentrations depending on utilization by organisms. Most of the elements occur in parts per million or parts per billion concentrations and are important to some positive and negative biochemical reactions. Properties such as salinity, density, and pH can be used to highlight the composition of seawater.

Salinity

Salinity is the amount of total dissolved salts present in 1 L. of water and is used to express the salt content of seawater. Normal seawater has a salinity of 35 g./L. of water; that is, 3.5 percent. The salinity of seawater is made up by the dissolved salts. Seawater is more enriched in dissolved ions

than all types than freshwater. Salts dissolved in seawater come from three main sources: volcanic

eruptions, chemical reactions between seawater and hot, newly formed volcanic rocks of spreading

zones, and chemical weathering of rocks.

Because of some chemical reactions between seawater and hot, newly formed volcanic rocks, the composition of seawater has been nearly constant over time. Salinity affects marine organisms because the process of osmosis transports water toward a higher concentration through cell walls. Marine plants and many lower organisms have no mechanism to control osmosis, which makes them very sensitive to the salinity of the water in which they live. The density of surface seawater ranges from 1,020 kg per cu. m to 1,029 kg per cu. m, depending on the temperature and salinity: the saltier the water, the higher its density. Seawater pH is limited to the range from 7.5 to 8.4 and increases with phytoplankton production. The speed of sound in seawater is about about 4,921 ft. (or 1,500 m) per second and varies with water temperature and pressure.

Carbon (IV) oxide in the sea exists in equilibrium with that of exposed rock containing limestone

(CaCO3). Seawater also contains small amounts of dissolved gases such as nitrogen, oxygen, carbon (IV) oxide, hydrogen, and trace gases. Water at a given temperature and salinity is saturated with gas when the amount of gas entering the water equals the amount leaving during the same time. Surface seawater is normally saturated with atmospheric gases such as oxygen and nitrogen. The concentrations of oxygen and carbon (IV) oxide vary with depth. The surface layers are rich in oxygen, which reduces quickly with depth to reach a minimum between 656 and 2,625 ft. (200 and 800 m) in depth. The amount of gas that can dissolve in seawater is determined by the water’s temperature and salinity. Increasing the temperature or salinity reduces the amount of gas that can be dissolved. As water temperature increases, the increased mobility of gas molecules makes them escape from the water, thereby reducing the amount of gas dissolved.

The gases dissolved in seawater are in constant equilibrium with the atmosphere, but their relative concentrations depend on each gas’ solubility. As salinity increases, the amount of gas dissolved decreases, because more water molecules are immobilized by the salt ion. Inert gases like nitrogen and argon do not take part in the processes of life and are thus not affected by plant and animal life, but gases like oxygen and carbon (IV) oxide are influenced by sea life. Plants reduce the concentration of carbon (IV) oxide in the presence of sunlight, whereas animals do the opposite in either light or darkness.

The world underwater is different from that above in the availability of important gases such as oxygen and carbon (IV) oxide. Whereas in air about one in five molecules is oxygen, in seawater this is only about four in every 1 trillion water molecules. Whereas air contains about one carbon (IV) oxide molecule in 3,000 air molecules, in seawater this ratio becomes four in every 100 million water molecules. Thus, carbon (IV) oxide is much more available in seawater than oxygen. All gases are less soluble as temperature increases, and particularly nitrogen, oxygen, and carbon (IV) oxide, which become about 40 to 50 percent less soluble with an increase of 45 degrees F (25 degrees C). When water is warmed, it becomes more saturated, resulting in bubbles leaving the liquid.