2 min
Why is the ocean salty — and why doesn't it get saltier every year?
The salt in the sea came from land: rain wears down rock and washes the salt into rivers, which carry it to the ocean. It does not get saltier each year because salt is added and removed at almost exactly the same rate.
Answer
The salt in the sea came from the land.
Rain is slightly acidic — it dissolves carbon dioxide from the air and becomes a weak acid — and over a very long time it wears rock down. The salt locked in that rock, mostly sodium, chlorine, magnesium, sulphur, calcium and potassium, is washed into rivers and carried to the sea.
This has been going on for about four billion years. And yet the ocean is not a giant bowl of brine, because salt is removed about as fast as it is added.
Going in: rivers deliver roughly four billion tonnes of dissolved salt to the ocean every year.
Coming out: the ocean has three ways of letting salt go.
- Living things take it. Sea creatures need calcium and silicon to build shells and skeletons. When they die, those materials settle on the sea floor.
- Hot vents underground. Seawater seeps into the crust, reacts with hot rock, and some of the salt gets locked into new minerals.
- Salt crystallises. In shallow evaporating areas such as salt flats, salt drops out of the water directly, and some is blown into the air and back onto land.
The most elegant part: the ocean's salinity sits at about 3.5% — 35 grams of salt in every kilogram of seawater. That is remarkably close to the saltiness of human blood plasma. It is not a coincidence. It is a chemical inheritance from the sea where life began.
In one line: the ocean is a soup the Earth has been seasoning for four billion years, and it stopped getting saltier because adding and removing salt happen to balance out.
A stranger fact: the Dead Sea is about 34% salt — nearly ten times saltier than the ocean. It sits inland with no outlet, so water can only leave by evaporating, and the salt keeps piling up. It is a neat proof of how important the ocean's balancing act really is.
Michael's Thought
I always assumed the sea just kept getting saltier forever. It turns out it found a balance between adding salt and letting it go.
Sources
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