Why Is the Sea Salty When the Rain Is Fresh?

Here's a puzzle that bothered me for an embarrassingly long time. Rain is fresh. Rivers are fresh (the ones I've drunk from, anyway, and I wouldn't recommend testing that casually). Yet the sea, which is fed by all those rivers, is salty enough to make you pull a face.

My first guess was a bit daft. I imagined the sea floor as a giant salt cellar, and the water had simply been poured over it at the start of time. It turns out the real answer is more interesting, and it begins with a single raindrop.

Rain isn't quite pure

Let's start with the drop itself. Rain is about as close to fresh water as nature gets, but it isn't pure. Before it even hits the ground it has picked up a few passengers.

The main one is carbon dioxide from the air. A little dissolves into every falling drop and forms a weak acid called carbonic acid. It's the same stuff that gives fizzy water its slight tang. Because of this, even clean, unpolluted rain is mildly acidic, with a pH of around 5.6 rather than the neutral 7.

Near the coast, rain also carries a pinch of sea salt, because waves and wind throw fine spray into the air. If you live somewhere with a salty westerly, you've probably noticed it on the windows. So some salt goes round the cycle without ever touching a rock.

But that's a small part of the story. The big part happens when the drop lands.

From hillside to river: how rock ends up in the water

Rock looks solid and permanent, but it isn't entirely. Weak carbonic acid, working patiently over thousands upon thousands of years, slowly attacks it. Geologists call this chemical weathering.

Take limestone, which is made mostly of calcium carbonate. Slightly acidic water reacts with it and carries it away as dissolved calcium and bicarbonate. That's why limestone country has caves and sinkholes: the rock has literally been dissolved and carried off in water.

Other rocks react more slowly, but they react. Feldspars, a very common group of minerals, are attacked by acidic water and break down into clay (which stays behind as soil) and dissolved ions (which travel on). Those ions include sodium, potassium, calcium, magnesium and some silica. (This is also where my earthworm obsession comes in. Soil isn't just rotted leaves. A good share of it is rock that has been slowly broken down this way, with the worms and everything else mixing it up. I didn't know that when I started digging around in the garden, and it rather changed how I look at a flowerbed.)

So by the time our raindrop has soaked into a hillside and trickled into a stream, it has become a very weak solution of rock. Weak enough that it still tastes like fresh water. A typical river carries only a tiny fraction of the dissolved material that seawater does, hundreds of times less on average. You'd never taste it.

But the key is that rivers never stop delivering. Every day, for an unimaginably long time, they've carried their small dissolved loads downhill and out to sea.

Where the river runs out

A river ends when it meets the ocean. And here's the part I find quietly marvellous: the water doesn't stay there.

The sun warms the sea surface, and water evaporates. It rises as vapour, forms clouds, and falls again as rain, some of it back on the sea, some of it on land where the whole journey starts again. This loop is the water cycle, and you probably drew it at school with arrows and a smiling sun.

What the school diagram usually leaves out is what doesn't go round. The dissolved ions don't evaporate with the water. They stay in the sea.

Why the salt gets left behind

This deserves a proper "why", because it's the heart of the whole puzzle.

When salt dissolves in water, it splits into charged particles called ions. Table salt, sodium chloride, becomes sodium ions (positive) and chloride ions (negative). Each one gets surrounded by water molecules, which cling to it because water molecules have slightly positive and slightly negative ends. It's a snug, stable arrangement.

Evaporation, on the other hand, is individual water molecules at the surface gaining enough energy to break free and drift off as gas. A free water molecule is easy to launch. A charged ion, held tightly by its entourage of water molecules and by its attraction to other ions, is very hard to launch. (For a sense of scale, solid sodium chloride doesn't even melt until about 800 °C, far hotter than any sunny day on the ocean.)

So the water leaves and the ions stay put. Think of a crowd at a party in a small flat. People can leave one by one, but the furniture isn't going anywhere.

A bucket thought experiment

Let me put some numbers on this, because I find it easier to believe a thing when I've done the sums. (These numbers are invented for the exercise, so don't read them as measurements.)

Picture a bucket holding 10 litres of fresh water. Every day, a tiny tap adds 1 litre of river water carrying 0.1 grams of dissolved salts per litre. Every day, the sun evaporates 1 litre from the surface, leaving all the salts behind. The water level never changes, but the salts creep up.

• After 1 day: 0.1 g of salts in 10 litres.
• After 100 days: 10 g in 10 litres, or 1 g per litre.
• After 1,000 days: 100 g in 10 litres, or 10 g per litre.

The water going in is almost fresh. The water going out contains no salt at all. And yet the bucket slowly turns salty, because the salt has a way in and no way out.

This is more or less what happens to lakes with no outlet. The Dead Sea is the famous example: rivers run in, nothing runs out, and the sun does the rest. It ends up far saltier than the ocean. (Lakes that do have an outlet, like many of ours that drain into rivers, keep flushing their dissolved material onward, which is why they stay fresh.)

So why isn't the sea getting saltier forever?

Here's where my bucket story needs an honest correction. If the ocean really worked like that bucket, it would keep getting saltier without limit. It doesn't seem to. The salinity of the open ocean is thought to have been broadly similar for a very long time, and scientists believe that's because the sea has ways of removing dissolved material as well as receiving it.

Edmund Halley, of comet fame, suggested back in the early 1700s that you might be able to work out the age of the Earth by measuring how fast the sea gets saltier. It was a clever idea, and it didn't work, partly because salt is also taken out of the sea. Good ideas can still turn out to be wrong, which I find oddly comforting.

Here are some of the ways material leaves the sea:

• Shells and skeletons. Corals, shellfish and tiny plankton pull calcium and bicarbonate out of seawater to build shells of calcium carbonate. When they die, their remains sink and become sediment, and over long ages that can turn into limestone. So the calcium that rivers deliver in quantity gets used up fast.
• Evaporation basins. In shallow, hot, enclosed seas, water can evaporate until salts crystallise out and form thick deposits. Rock salt in the ground is the remains of ancient seas that did exactly this.
• Reactions at the sea floor. Seawater circulates through hot rock beneath the ocean and exchanges some of its dissolved material along the way.

This also solves a second mini-mystery. Rivers carry mostly calcium and bicarbonate, yet seawater is dominated by sodium and chloride. Why the mismatch? Because the calcium and b