We live on a watery planet, with our oceans covering more than 70 per cent of the Earth's surface and holding 97 per cent of the Earth’s water. But why are the oceans salty?
Water enters the ocean through rivers, which bring rainwater across the land, and from rain falling over the ocean. Salts come from the weathering of rock on land; so they are carried to the oceans by the rivers.
But rain – and rivers – contain fresh water; so how come the oceans are salty?
The answer lies in the idea of 'residence time' – or how long a given substance stays in a reservoir.
Let’s imagine the ocean as a bathtub. The amount of water in this bathtub (the reservoir size) will depend on how much water comes in from the tap, and how much goes out through the drain. As long as the tap lets the water in just as fast as water goes out, the water level in the tub will not change (we call this steady state).
Now, let's go back to salts. Salts do indeed enter the ocean via the rivers, but per litre of water, the amount of salt is so small in rivers, the water remains fresh.
However, when that water enters the oceans, salts leave the ocean (or go down the drain, if you’re still thinking of the ocean as a bathtub) much more slowly than water leaves the ocean.
For instance: evaporation removes water from the ocean – but it leaves behind the salt. This means that a molecule of salt stays in the ocean a lot longer than a molecule of water, allowing the amount of the salt in the ocean at steady state to be much higher per litre of water than in the rivers.
This means that the water in the ocean is noticeably salty. Even though salt has a much longer residence time than water, as long as salt is in steady state – that is, the amount of salt coming in is the same as the amount of salt going out – the oceans will stay just as salty as they are today.