Why Is the Ocean Salty When Rivers Are Fresh?
Ocean salt comes from rocks worn down by rain and carried by rivers, and from hot springs on the seafloor. Evaporation removes only water, so the salt builds up.
The ocean is salty because, over geological time, it has collected salts that rain wore away from rocks and rivers carried to the sea, plus minerals released by hot springs on the seafloor. Water evaporates and returns to land as fresh rain, but the salt stays behind. Today, every kilogram of seawater holds about 35 grams of salts.
Where does the salt in rivers come from?
It starts with rain, which is not pure water. As it falls, it dissolves a little carbon dioxide from the air and becomes slightly acidic. That weak acidity, along with the constant flow of water, slowly wears down rocks and frees ions, electrically charged particles that stay dissolved in the water. Streams and rivers carry those ions to the ocean.
The amounts are huge. According to the U.S. National Oceanic and Atmospheric Administration (NOAA), the world's rivers deliver an estimated 4 billion tons of dissolved salts to the sea every year. Marine organisms use some of those ions and remove them from the water; others have nowhere to go and build up.
The two most abundant ions in seawater are chloride and sodium, the same pair that makes up table salt. Together they account for about 85% of the dissolved ions. Magnesium and sulfate add roughly another 10%, and the rest is a long list of elements in tiny amounts.
If rivers carry salt, why is river water fresh?
Rivers do carry salts, just at concentrations too low to taste. The difference is where the water ends up. A river is constantly refilled by rain and springs, so its water flows through and moves on. The ocean is the end of the line. It receives the load from every river that drains into it and has nowhere to send it.
Water leaves the ocean mainly by evaporation, and the vapor takes only water with it. The salt stays put. That vapor forms clouds and falls back on land as fresh rain, the same rain whose drops paint a rainbow when the sun is low. Picture a pot on the stove that someone keeps adding a pinch of salt to: the water boils off, and the salt piles up.
What does the seafloor add?
Not all the salt comes from the continents. Along mid-ocean ridges, seawater seeps into cracks in the crust and is heated by magma. The heat speeds up chemical reactions with the rock: the water loses magnesium and sulfate and picks up metals such as iron, zinc and copper. It then flows back out through hydrothermal vents. Underwater volcanic eruptions also release minerals straight into the sea.
With so much salt arriving every year, shouldn't the ocean keep getting saltier? Earth's earliest seas were probably only slightly salty, and salt levels rose over the ages. Today, though, NOAA says about as much salt leaves the system by settling into seafloor sediments as arrives from rivers. Inputs and outputs are probably close to balanced.
How much salt is in seawater?
On average, about 35 parts per thousand: 35 grams of salts in every kilogram of water, or 3.5% by weight. The figure varies from place to place, as the approximate values below show.
| Location | Salts per kilogram of water |
|---|---|
| Open ocean (average) | about 35 g |
| Open ocean (typical range) | 33 to 37 g |
| Red Sea surface | up to 41 g |
| Parts of the Baltic Sea | 10 g or less |
| Dead Sea | about 300 g |
Why are some seas saltier than others?
Surface salinity depends on the balance between evaporation and fresh water coming in. In belts around 30° to 35° latitude in both hemispheres, dry air favors evaporation and the water gets saltier. Near the equator, where rain is heavy, it gets fresher. Warm seas surrounded by dry land, like the Red Sea, run above average, while waters that take in a lot of fresh water, such as parts of the Baltic, sit far below it.
In polar regions, ice joins in. When seawater freezes, most of the salt is left out of the crystals, so the surrounding water becomes saltier and denser, enough to sink. Sea ice itself floats for the same reason an ice cube does in your glass. When it melts in summer, the surface turns fresher again.
The most extreme case is the Dead Sea, between Israel and Jordan, fed by the Jordan River. NASA puts its salinity at about 300 parts per thousand, around nine times that of the ocean. It is the same process that salted the oceans, only faster: in a landlocked basin under a hot sun, the water escapes into the air and nearly all the salt stays.
