The reason why the Ocean is salty


Marta Fava

07 Jun 2022

We all know that the water of lakes and rivers is fresh, but if you go swimming in the ocean or sea, it tastes salty. Why is that? In the end, the water flowing into the ocean comes from streams and rivers.

In this article, we will look at the sources of salt in the sea and answer the question: why is the ocean salty?

The sources of salt in the ocean

Some have estimated that, if salt could be taken out of the ocean, and spread on land, it would form a layer more than 166 metres (500 feet) thick. Where does all this salt in the ocean and sea come from? Salt in the ocean mainly comes from two sources

  • Runoff from land

This means that rain transports mineral ions from land into water. Rainwater is slightly acidic, as some carbon dioxide in the air dissolves in it. When rain falls on rocks, mineral salts are released, which separate into ions. These ions are then carried by runoff water and eventually end up in the sea. Over 90% of all ions found in saltwater are sodium and chloride, the major ingredients of the type of salt used in cooking.

  • Openings or vents in the seafloor 

Hydrothermal fluids, another source of salts in the ocean. Ocean water seeps through seabed fissures and is heated by lava from the core of the Earth. A variety of chemical reactions are triggered by the heat. Water tends to lose oxygen, magnesium, and sulphates while picking up metals like iron, zinc, and copper from the surrounding rocks.The metals are carried away by the hot water when it is discharged from vents in the seafloor. Some ocean salts are formed by undersea volcanic eruptions that release minerals directly into the sea.

What’s the effect of salt in the water?

Seawater is denser than freshwater because of the salt it contains. Due to the increased density, people, animals, and other items float more in seawater rather than freshwater. Salts and minerals are also used in great part by marine life: organisms, for example, remove iron, zinc, and copper from water.

The list of salinity of the Ocean basins of the world

Seawater contains roughly 3.5 percent salt on average. The saltiness of seawater (called salinity by scientists) varies greatly across the ocean basins. Scientists refer to salinity as parts per thousand (ppt), representing the total amount of salt dissolved in the water (grams of salt dissolved in 1 kilogram of water). It is generally lower at the equator and poles. However, salinity rises in the spaces between. The Mediterranean Sea, for example, is saltier than the rest of the Atlantic Ocean basin. Some lakes, such as California’s Mono Lake and Asia’s Caspian Sea, are even saltier. Evaporation can cause isolated bodies of water to become extremely salty, or hypersaline. A good example is the Dead Sea. The Dead Sea’s high salt content dramatically increases the density of its waters, allowing humans to float much more than in the ocean. Salts are left behind as water evaporates from these landlocked sources of water. Salt levels continue to rise over time. Many of these saline lakes are located in dry places with little rainfall and hot temperatures during the day.

  • Pacific Ocean basin

The salinity of the Pacific Ocean basin’s surface waters is heavily influenced by wind, precipitation, and evaporation patterns. The waters in the calm-wind belt near the Equator typically have lower salinities than those in the trade-wind belts. This is due to the fact that near the Equator, there is a lot of rain and little evaporation; salinity can be as low as 34 ppt.

Salinity in the open southeastern sections, on the other hand, can reach 37 ppt, while the lowest salinities (less than 32 ppt) are found in the extreme north of the Pacific.

  • Atlantic Ocean basin

The North Atlantic’s surface waters have salt levels that exceed 37 ppt, which are among the highest in the world. Salinity levels in the South Atlantic are lower, at roughly 34.5 ppt. Precipitation and evaporation determine salinity distribution, which is also controlled by water currents. This discrepancy, for example, can be explained by the Mediterranean Sea’s strong evaporation and the discharge of high-salinity water from it, which helps keep the North Atlantic’s salinity higher. 

The Sargasso Sea, which covers roughly 2 million square miles and is located about 2,000 miles west of the Canary Islands, is the saltiest region of the North Atlantic. The floating brown seaweed “sargassum,” from which the sea derives its name, separates the Sargasso Sea from the open ocean. The high water temperature (up to 83°F) causes a high rate of evaporation, and the sea’s salinity is also in part due to its remoteness from land: it receives no fresh-water inflow. In fact, salinity in a specific part of the ocean also depends on the runoff from rivers. The Mediterranean Sea and Sargasso Sea, as mentioned above, have high salinity because evaporation is great and freshwater runoff is low. On the other hand, in the Black Sea and in the Baltic, where large rivers are empty, salinities are lower.

  • Indian Ocean basin

The salinity of the Indian Ocean basin’s surface waters ranges from 32 to 37 ppt, with substantial regional variations. The Arabian Sea has a dense, high-salinity top layer due to high evaporation rates (37 ppt). Due to the drainage of fresh water from rivers, the salinity of the Bay of Bengal’s surface layer is significantly reduced (less than 32 ppt). The Southern Hemisphere subtropical zone has high surface salinity, while low-salinity zones range from Indonesia to Madagascar along 10° S. At 60° S, the salinity of the surface water is between 33 and 34 ppt.

  • Arctic and Southern Ocean basins

Low salinities occur in polar seas where the saltwater is diluted by melting ice and continued precipitation. Partly landlocked seas or coastal inlets that receive substantial runoff from precipitation falling on the land also may have low salinities. Although the Arctic Ocean basin is generally fresher than other oceans, with salinity levels ranging from 30 to 34 psu, salinity levels vary by region, and areas with heavy river influx may have even lower salinity. On the other hand, high sea surface salinity (SSS) north of the Subtropical Front, large salinity gradients across the main polar fronts, and low surface salinity in the Antarctic Zone south of the Polar Front characterise the Southern Ocean basin.