Why is the ocean salty? Where sea salt comes from

The ocean is salty because it is the end of the line for water on land. Rain slowly dissolves minerals out of rock, rivers carry them to the sea, and seafloor vents and volcanoes add more. Water leaves the ocean mainly by evaporating, and evaporation takes only the water, which is why the salt stays behind. Over hundreds of millions of years that has built up to about 35 grams of salt in every kilogram of seawater.

In short

• Sea salt comes mostly from rock on land, worn away by slightly acidic rain and delivered by rivers, with extra from vents and volcanic gases on the seafloor.
• Rivers taste fresh because their water is always moving on, while the ocean keeps what arrives and loses water only as pure vapor.
• The ocean is not getting steadily saltier: over geological time, salt is removed about as fast as it is added.

Where does the salt in the ocean come from?

The biggest source is rock on land. Rain picks up a little carbon dioxide from the air as it falls, which makes it slightly acidic, and over time that weak acid breaks down minerals in rock and soil. The released pieces are ions, atoms or small groups of atoms carrying an electric charge, and they dissolve in the water. Streams and rivers carry them to the sea, as NOAA's Ocean Service explains.

The second source is the seafloor. Seawater seeps into cracks in the ocean crust, is heated by magma beneath, and comes back out at hydrothermal vents changed: it has lost magnesium and sulfate and picked up metals such as iron, zinc and copper. Underwater eruptions release minerals directly. And some of the ocean's chloride, the most common ion in it, came from gases released by volcanoes early in Earth's history, not from rivers at all.

Edmond Halley, the astronomer better known for his comet, proposed the river explanation in 1715. He pointed out that lakes with no outlet to the sea, such as the Dead Sea, are very salty, which is what you would expect if rivers bring salt in and only evaporation takes water out. According to the Wikipedia summary of seawater chemistry, he was partly right: rivers matter, but so do the seafloor and volcanic gases.

How salty is seawater, in numbers?

Average salinity is about 35 parts per thousand, so about 3.5% of seawater's weight is dissolved salt. At the surface it ranges from about 32 to 37, according to NASA's Aquarius mission, which mapped it from space.

Worked example: the salt in a liter

1. A liter of seawater weighs a little more than a liter of fresh water, about 1.025 kilograms.
2. At 35 grams of salt per kilogram, that liter holds 1.025 × 35 ≈ 36 grams of dissolved salts.
3. A level tablespoon of table salt weighs roughly 17 to 18 grams, so the liter holds about two tablespoons' worth.
4. Not all of it is table salt. Chloride and sodium make up about 85% of the dissolved ions; magnesium and sulfate about 10%; calcium, potassium and others the rest.

Scale it up and the numbers become hard to picture. The ocean holds roughly 1.3 billion cubic kilometers of water, so at about 36 kilograms of salt per cubic meter, that is on the order of 50 million billion tonnes of dissolved salt.

Why aren't rivers salty if they carry the salt?

Because a river's water is just passing through. A river is mostly recent rain and snowmelt on its way downhill, so it never sits long enough to collect much. Water counts as fresh when it holds less than about 1 gram of dissolved minerals per liter, and most rivers carry far less than that, too little to taste. The ocean is different because nothing flows out of it: water leaves the ocean mostly by evaporation, and evaporation leaves the salt behind. The vapor falls as rain, runs down rivers carrying a little more dissolved rock, and the cycle repeats.

Rivers and the sea also differ in what is dissolved, not just how much. The table shows why seawater tastes of sodium chloride when river water, concentrated, would not.

	River water	Seawater
Total dissolved minerals	Usually well under 1 gram per liter	About 35 grams per kilogram
Main ions	Bicarbonate and calcium	Chloride and sodium
Share that is bicarbonate	About 48%	About 0.14%
Why	Weathering rock releases a lot of these	Shell-building life and chemistry remove calcium and bicarbonate quickly; sodium and chloride stay

The difference comes down to how long each ion stays. Corals, plankton and shellfish pull calcium and bicarbonate out to build calcium carbonate shells, so those ions don't build up. Sodium and chloride have no such quick exit and stay dissolved for many millions of years, so they dominate.

Is the ocean getting saltier?

Not overall, as far as the evidence shows. If rivers have delivered salt for billions of years, the sea might be expected to get saltier forever, and in 1899 the Irish physicist John Joly even tried to date the Earth by dividing the ocean's sodium by the yearly river supply. His answer, about 80 to 100 million years for the age of the oceans, was far too young, because he assumed nothing ever leaves. Salt does leave. It is buried in seafloor mud as the water trapped between grains, it reacts with fresh volcanic rock at the ridges, and it crystallizes out as thick salt beds where shallow seas dry up. Salinity has stayed roughly stable for a very long time because removal keeps pace with supply, the way a bathtub with the tap on and the drain open settles at one level.

What does change is the pattern from place to place. Surface water is saltier in the subtropics, where evaporation beats rainfall, and fresher near the equator, where it rains heavily, and near the poles, where ice melts and rivers pour in. Salt also makes water denser, and in the North Atlantic cold, salty water sinks and drives a slow circulation through the deep ocean, the subject of the note on the AMOC.

Why is the Dead Sea so much saltier?

The Dead Sea follows Halley's rule to the extreme. It lies in a basin with no outlet, fed mainly by the Jordan River, and it loses water only by evaporation in a hot, dry climate. In 2011 its salinity was measured at about 342 grams per kilogram, roughly 9.6 times the ocean's, according to the Wikipedia entry on the Dead Sea. So much is dissolved that people float high in it. The Great Salt Lake in Utah and other salt lakes formed the same way, and they show what the ocean would become if salt never left it.

What does sea salt tell us about Earth?

Seawater is a record of the planet's chemistry. Its sodium and much of its calcium come from the rock described in the note on what the Earth is made of; its chloride is left over from volcanic gases; and the elements dissolved in it are the same ones listed in the periodic table. The vents that add metals sit along ocean ridges and in places as deep as those around the Mariana Trench. The ocean is salty, in the end, because rock weathers, water evaporates, and the sea is where both processes meet.

Check yourself

• Without looking back: why does evaporation make the sea saltier, while the rivers that bring the salt stay fresh?
• Why is seawater mostly sodium and chloride when river water is mostly bicarbonate and calcium?
• What would have to be true for the ocean to get steadily saltier, and why isn't it?

Sources

• Why is the ocean salty? — NOAA National Ocean Service
• Salinity overview — NASA Aquarius mission
• Seawater — Wikipedia
• Dead Sea — Wikipedia
• John Joly — Wikipedia