Composition of Seawater

There are many minerals in seawater. Seawater contains dissolved salts, dissolved gases and particulate matter. Almost every element is represented to some degree.

The practice of extracting salt from sea water by evaporation is very ancient. In France the Fleur de Sel is a highly prized cooking salt. It is prepared in large evaporation pans in Brittany, Nourmoutier and the Carmargue. The coveted sea salt has a taste different to that of traditional table salt because there is more in sea salt than just sodium chloride. On average each kilogram of seawater contains 35 grams of salt.

The salinity of seawater varies across the oceans. It is lower in Arctic regions, close to river mouths where there is substantial freshwater discharge, or close to melting glaciers. It is higher in tropical waters where the salts are concentrated by evaporation of the freshwater and is very high in confined waters such as the Red Sea and the Dead Sea. The salinity also varies with depth. The sea off the Alaskan coast has a salinity of about 32 grams of salt per kilogram of water. The Red Sea has a salinity of about 40 grams of salt per kilogram of water. Marine life is very sensitive to salinity.

Rather strangely, when HMS Endeavour conducted a survey of the world’s oceans between 1872 to 1876 it was discovered that although the salinity varies from place to place the composition of sea salt does not. Each kilogram of sea salt contains 553 grams of chlorine, 305 grams of sodium, 77 grams of sulphates, 37 grams of magnesium, 11 grams of calcium, 11 grams of potassium and 7 grams of other salts. Remarkably. although there is some distortion where freshwater discharges into the ocean,  this composition hardly changes across the oceans. Moreover the concentration is thought to have been constant for billions of years.

Scientists explain the chemical composition of the oceans in two ways. The traditional view taught in schools dates form the work of Sir Edmond Halley. It states that the saltiness is due to minerals that have been carried into the sea from the land and concentrated there. This is only partially correct.  The sodium and chloride concentrations are thought to have a different origin. The sodium is thought to have leached out of the sea floor when the oceans formed. The chlorine concentrations are thought to have outgassed from underwater volcanoes and hydrothermal vents. The chemical composition of the oceans is constant mechanisms that are thought to exist as part of the process of plate tectonics that remove and release minerals from the interior of the earth. Their mechanisms are not yet fully mapped out.

Unlike the salts other chemicals that are present in sea water are non-conservative. Their concentration varies from place to place and from time to time. The dissolved gases that are present in the ocean are very important for marine life. Marine plants rely upon dissolved carbon dioxide; and marine animals absorb dissolved oxygen through their gills. An excess of plants will increase the oxygen concentration of the ocean while an excess of animals (zooplankton) will reduce its concentration. Decomposing organic materials which tend to drift to the ocean floor can be brought back into the upper layers in a process known as upwelling. Upwelling can re-circulate nutrients in the upper ocean and stimulate marine life.  Upwelling is able to sustain the highly productive fisheries offshore form the coasts of Peru and Chile in the South Pacific.

The concentration of carbon in the form of carbon dioxide in the ocean is a special case because of its relevance to the issues of global warming and the acidity of the oceans. Although not listed as one of the major constituents of sea salt the ocean contains 36,000 gigatonnes of carbon mostly in the form of bicarbonate ions. Some of the carbon is removed by micro-organisms that form calcium carbonate shells. Ultimately they fall to the ocean floor and begin the formation of a chalk layer. When Fleur de Sel is extracted from salt water it is usual to either pass the product through several pools of water or scrape off just the top layer of solid to avoid a product that tastes of chalk.

A deep understanding of the chemical composition of the oceans requires a detailed understanding of oceanography. It is an important subject upon which life on earth depends.