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Why Is the Ocean Salty?

Natural Salinity of Sea Water

Why Is the Ocean Salty?

Why Is the Ocean Salty?

The world’s oceans are salty for a variety of reasons, including the concentration of sodium and chloride ions from the earth’s crust in the water, evaporation that occurs as part of the hydrologic cycle, and chemical processes carried out by various ocean creatures. The average salinity, or salt content, of the sea water is 35 parts per thousand, which is symbolized by the marking o/oo. However, there are seas that have salinity amounts of up to 40 o/oo, including parts of the Mediterranean Sea, the Dead Sea, and the Arabian Gulf.

Erosion and Chemical Concentration

Most of the salt content in the earth’s sea water comes from the land mass that has eroded and traveled in rivers to the ocean. Acids in rainwater break down land rocks and rivers carry the sediment away as runoff, which then reaches the ocean and gradually dissolves in the water. It has been estimated that rivers supply the ocean with over 4 billion tons of dissolved salts every year, with between 225 million and 500 million of those tons coming from the United States alone. Other types of sediment that reach the ocean and increase its salinity level include solid rock ejections from above-ground and undersea volcanoes, debris that is swept into the ocean by winds or storms, and ocean floor sediment that dissolves and rises to middle and surface water levels.

Processes of Evaporation

The basic ecological process of evaporation also contributes mightily to the saltwater content in our oceans. When placid ocean water is heated by the sun and turned into a gaseous water vapor, the pure H2O evaporates but its salt content is left behind. This process is why bodies of water in hot, arid regions, such as the Mediterranean Sea or the Dead Sea, have such high salinity levels. High water temperatures lead to higher rates of evaporation, which in turn lead to higher salinity readings that can approach 40 o/oo.

Chemical Processes Utilized by Sea Creatures

The animal and plant life that flourishes in the oceans also contributes to its salt content. When river water flows into the ocean, it is stocked with many different elemental molecules, including sodium, calcium, magnesium, silica and carbon. Biological organisms such as crustaceans, plankton, and foraminifers (one-celled sea animals) use these chemical elements for food, shelter, and skeleton construction. However, there are no known biological processes that remove sodium from sea water, and very few that remove chloride. Hence, the large presence of sodium chloride, or salt, in the world’s seven major oceans. In this way, fresh water rivers are able to supply the bulk of the water supply for the world’s salt water oceans.

The ocean is not a static body of water: it is constantly being acted upon by the sun, wind, rain and the creatures living within its depths. Although the ocean’s salt content is mainly a product of dissolved sediment carried to it by fresh water rivers criss-crossing the surface of continents, processes of evaporation and the construction of exoskeletons and coral reefs by biological organisms also impact the overall salt level.


“Why Is the Ocean Salty?” National Oceanic and Atmospheric Administration. N.p., n.d. Web. 16 June 2016.

“Why Is the Ocean Salty?” U.S. Geological Survey Publication. N.p., n.d. Web. 16 June 2016.

“Ocean Salinity.” Science Learning Hub RSS. N.p., n.d. Web. 16 June 2016.


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