Why Is the Ocean Salty?

Where does the salt in the ocean come from?

Ocean salt has two sources: water running off the land and openings in the seafloor.^[1] Rocks on land are the main source of the salts dissolved in seawater.^[1]

Rain contains some carbon dioxide dissolved from the surrounding air, which makes it slightly acidic because of carbonic acid.^[2] The rain wears rock down physically, the acid breaks it down chemically, and the salts and minerals that come loose are carried away in dissolved form as ions.^[2] Runoff takes those ions into streams and rivers, which deliver them to the ocean.^[2]

Not everything that reaches the sea stays in the water: ocean organisms use many of the dissolved ions and remove them, while other ions are left alone and slowly become more concentrated.^[1][2] Iron, zinc and copper are among the minerals that living things remove, but sodium and chloride are not removed this way, so they have built up over time.^[4]

The earliest seas were probably only slightly salty, and the ocean became saltier over time as rain kept breaking up rocks and carrying their minerals to the sea.^[3]

How do seafloor vents and underwater volcanoes add salt?

The second source lies at the bottom of the sea, where seawater seeps into cracks in the seafloor and is heated by magma.^[1][4] The heat sets off chemical reactions in which the water tends to lose oxygen, magnesium and sulfates and to pick up metals such as iron, zinc and copper from the surrounding rocks.^[1] The hot water then flows back out through vents in the seafloor, carrying those metals into the ocean.^[1]

These openings are called hydrothermal vents, and they are found on the crest of oceanic ridges.^[2] The U.S. Geological Survey (USGS) reports estimates that the entire volume of the world's oceans could circulate through hydrothermal vent systems every 10 to 20 million years.^[2] The exchange runs in both directions, because some dissolved salts react with the rock of the ocean crust and are removed from the seawater.^[2]

Underwater volcanic eruptions also release minerals directly into the ocean, as seawater reacts with hot rock and dissolves some of its minerals.^[1][2] Salt domes, which are vast deposits of salt that form over geological timescales underground and under the sea, add to the ocean's saltiness as well.^[1]

Why are rivers and lakes not salty?

Rivers and streams do contain salt that has dissolved out of rocks on land.^[4] They do not taste salty because rain keeps refilling them with fresh water.^[3] Whatever small amount of salt the rain washes in, it delivers far more fresh water to streams, rivers and lakes, and regular rainfall keeps them from turning salty.^[4]

The ocean, by contrast, collects the salt and minerals from all of the rivers that flow into it.^[3] Around the world, rivers carry an estimated four billion tons of dissolved salts to the ocean every year.^[3]

Even so, the sea is probably no longer getting saltier, because about the same tonnage of salt is probably deposited as sediment on the ocean bottom, which gives the ocean a balanced salt input and output.^[3]

Some lakes are salty because of evaporation: the Woods Hole Oceanographic Institution (WHOI) points to Mono Lake in California and the Caspian Sea, land-locked waters where evaporation leaves the salt behind, so salt levels keep rising over time.^[4]

How salty is the ocean, and which ions make up the salt?

The average salinity of seawater is about 35 parts per thousand, which means about 3.5 percent of the weight of seawater comes from dissolved salts.^[1][2]

Chloride and sodium are the two most common ions in seawater.^[1][2] The sources differ on their exact share: the National Oceanic and Atmospheric Administration (NOAA) says the two together make up around 85 percent of all dissolved ions, while the USGS says over 90 percent.^[1][2] NOAA adds that magnesium and sulfate account for another 10 percent, and that other ions are found only in very small concentrations.^[1] Sodium and chloride are also the main ingredients in table salt.^[4]

According to the USGS, a cubic mile of seawater holds about 120 million tons of salt, counted as sodium chloride.^[2] By some estimates, if all the salt in the ocean were removed and spread evenly over the Earth's land surface, it would form a layer more than 500 feet thick (about 150 meters, by BodaWiki's conversion), about the height of a 40-story office building.^[2]

Why is some seawater saltier than other seawater?

Salinity is not the same everywhere, and the NASA Salinity website lists precipitation, evaporation, runoff, and the freezing and melting of ice as the causes.^[5] The same site puts sea surface salinity around the globe at 32 to 37 on the Practical Salinity Scale, which is roughly equal to parts per thousand.^[5] NOAA gives a range of 33 to 37 grams of dissolved salts per liter, with a typical value close to 35.^[6]

• Low end of usual range33 ppt
• Typical seawater35 ppt
• High end of usual range37 ppt
• Red Sea and Persian Gulf region40 ppt

Source: National Oceanic and Atmospheric Administration (NOAA) [6]

Near the equator, the tropics get the most consistent rain, and that fresh water lowers the salinity of the surface water.^[6] Farther from the equator there is less rain and more sunshine, so evaporation increases, and evaporating water leaves its salt behind.^[6] Toward the poles, fresh water from melting ice lowers surface salinity again.^[6]

Of the five ocean basins, the Atlantic is the saltiest.^[6] The saltiest ocean water is in the Red Sea and the Persian Gulf region, at around 40 parts per thousand, because evaporation there is very high and little fresh water flows in.^[6] River water can have the opposite effect: at the mouth of the Amazon River, fresh water pouring into the Atlantic Ocean creates a plume of low-salinity water.^[5]