Ocean Salinity

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Ocean Salinity

Introduction

Ocean salinity, or the saltiness of the seas, ranges from 32 to 37 parts per thousand of dissolved salts, averaging about 35 parts per thousand. Most of the saltiness of the seas is in the form of dissolved sodium chloride, which is the same as table salt. There are, however, many other dissolved salts in the seas as well. Dissolved materials in sea water are derived from weathering of minerals and rocks on land. The dissolved elements are transported to the sea by rivers. Some salts are also introduced into sea water by underwater eruptions of sea-floor volcanoes and volcanic vents. Evaporation concentrates salts in sea water, up to the point where the water becomes saturated with those salts. At the point of saturation, salt crystals precipitate from sea water and are thus removed from sea water. In this way, the saltiness of the sea is maintained over long time periods.

Historical Background and Scientific Foundations

The sea was not always salty. It is thought that the original oceans on Earth were freshwater bodies that rapidly became salty as chemical weathering of minerals and rocks on dry land contributed to salts in the sea. Over time, the seas became quite salty, and marine sediments are quite full of salts and other chemical compounds precipitated from sea water over time. In addition, the evolutionary advent of hard parts and shells of marine organisms caused those organisms to extract many chemical compounds from sea water, including calcium carbonate, a common mineral in shells and other hard parts of marine animals and plants. Over time, the chemistry of the seas has varied and this is recorded in the chemical nature of marine sediments over geological time.

There are over 70 chemical elements in sea water. The most common constituents of sea water are (in order): chlorine; sodium; sulfate; magnesium; calcium; potassium, bicarbonate; and bromide. Together these account for 99.999% of all chemical components dissolved in sea water. When evaporated, either in nature or in the laboratory, sea water yields a predictable sequence of chemical precipitates, which are (in order): calcium carbonate (calcite); hydrated calcium sulfate (gypsum); sodium chloride (halite); and then other salts including magnesium chloride and sulfate. Studies of layers of sedimentary rock formed during evaporation of sea water indicate this sequence holds in nature. There are sequences of sedimentary rock showing layers of limestone (a rock containing calcite), rock gypsum, and rock salt—in that order—in some sedimentary basins.

Impacts and Issues

Salinity of the oceans varies from place to place, but over a limited range, as noted earlier. The most saline of open ocean waters are in the mid-latitudes of the Atlantic Ocean basin. The least saline of open ocean waters are in the Northern Ocean adjacent to landmasses and in the oceans adjacent to India and southeastern Asia. In waters that are not open, for example in bays, lagoons, estuaries, and other water bodies that are connected to the sea, but partially surrounded by land, salinity can vary greatly. In such areas where there is much freshwater from rivers, ocean salinity can be substantially lowered, a condition called hyposalinity. The opposite, hypersalinity, or salinity elevated above the high end of the normal range, can occur in bays, lagoons, estuaries, or other restricted bodies of water where freshwater input is limited and/or the connection to the open sea is somehow limited. This causes evaporation to concentrate salts and other dissolved chemical compounds in the sea water.

Changes in salinity greatly affect the ocean ecosystem and many shoreline ecosystems as well. Some species are well adapted to changes in salinity, whereas others cannot survive when salinity changes. Thus, both natural and human-made salinity changes can profoundly affect ocean ecosystems or other ecosystems that are strongly connected to sea water. Natural salinity changes occur when there are long-term changes in climate and freshwater run off. Freshwater run off may be climate related or may be related to mountain building on the continents. Human-made salinity changes may be connected to human-induced climate change, but more locally are related to human-made changes in local drainage systems and the discharge of water-borne wastes. Human activities may restrict oceanic connections with bays, lagoons, estuaries, and other branches of the sea, thus causing salinity elevation. Natural processes, such as sand bar growth or sand spit growth may restrict oceanic connections and thus accomplish the same thing in terms of salinity elevation.

See Also Marine water quality; Runoff