Why Is the Sea Salty?
Rain is fresh, rivers are fresh, and they both drain into the sea. So where does the salt come from, why does it not keep rising, and why is the Dead Sea so much saltier?
The sea is about 3.5% salt by weight. Scaled up, every litre of seawater holds roughly 35 grams of dissolved solids, mostly sodium chloride. If you spread all the salt in the oceans evenly over the land, it would form a layer about 150 metres deep.
Where it comes from
Two sources, working on very different timescales.
Rain, slowly dissolving the land. Rainwater is slightly acidic, because it picks up carbon dioxide from the air and forms weak carbonic acid. Over millions of years that acid eats at rock, freeing ions of sodium, chloride, calcium and magnesium. Rivers carry them to the sea. Each river is only faintly salty — far too faint to taste — but they have been doing this continuously for billions of years, and the sea is where it all collects.
Hydrothermal vents, from below. Seawater seeps into cracks in the ocean floor, is heated by magma, strips minerals out of the hot rock and jets back out. Underwater volcanoes add more directly.
Why rivers stay fresh
This is the part that confuses people. The answer is that rivers are a conveyor belt, not a container. Water flows through and keeps going, carrying its dissolved minerals out to sea. The sea is the end of the line. Water leaves it only by evaporating, and evaporation takes the water and leaves the salt behind.
A lake with an outflowing river stays fresh for the same reason. A lake with no outlet does not.
So why is it not getting saltier?
Because salt leaves too. The ocean has been at roughly its present saltiness for hundreds of millions of years, in a slow balance. Salt is removed when sea spray blows inland, when shallow seas evaporate and leave salt deposits, when minerals react with fresh volcanic rock on the sea floor, and when marine organisms take up calcium and other ions to build shells that eventually become sediment.
Why the Dead Sea is different
The Dead Sea is about 34% salt, nearly ten times ordinary seawater. It sits in the lowest land basin on Earth, in a hot, dry climate, and it has no outlet. The Jordan brings water in; only evaporation takes it out. Everything dissolved stays, and keeps concentrating.
The same thing happens at the Great Salt Lake in Utah and Lake Assal in Djibouti, which is saltier still.
Things worth knowing
- The Baltic is almost drinkable in places — under 1% salt, because so many rivers feed it and it exchanges little water with the Atlantic.
- The Red Sea is unusually salty for an open sea, at around 4%, because it is hot, has high evaporation and almost no rivers flowing into it.
- Drinking seawater dehydrates you. Your kidneys cannot produce urine as salty as the sea, so they must use your own body water to flush the excess salt out.