Why Is the Ocean Salty?

The question contains its own paradox, and noticing it is most of the work. Rivers are fresh. Rivers are what pour into the sea. So how does a body of water fed entirely by fresh water end up salty enough to be undrinkable? The resolution is that the ocean is not a container of water so much as a one-way trap for everything dissolved in it.

Begin on land, with rain. Even in completely unpolluted air, falling rain is slightly acidic, because carbon dioxide from the atmosphere dissolves into the droplets and forms carbonic acid. It is a weak acid — nothing like the acid rain caused by industrial sulphur emissions — but it does not need to be strong, because it has geological time to work with.

When that faintly acidic water reaches the ground, it begins chemical weathering: it attacks rock, breaking down minerals and freeing their constituent ions into solution. Sodium, calcium, magnesium and potassium come out of silicate and carbonate rocks this way. Chloride has a second important source in volcanic gases and hydrothermal activity, which is why chloride and sodium — the two ions in table salt — dominate seawater while the rock-derived ions that make river water hard are comparatively minor in the sea.

Rivers then act as the conveyor. Every river on Earth carries a dissolved load as well as the visible suspended sediment. The concentration is very low, which is exactly why river water tastes fresh and is perfectly drinkable. Nothing about a river hints at the ocean's salinity.

The other half of the mechanism is what makes the difference, and it is the half people forget. Water leaves the ocean almost entirely by evaporation, and evaporation is selective: only water molecules enter the vapour phase. Dissolved ions have no way to leave with them. So the system has an entrance that admits water plus salt, and an exit that releases water alone. Salt accumulates by arithmetic. Water cycles round — evaporating, falling as rain, weathering more rock, returning through rivers — and each pass delivers another small load of ions into a basin that never gives them back through the same door.

Scale this over roughly four billion years and the result is seawater at about 35 grams of dissolved salts per kilogram, usually written as a salinity of 35 parts per thousand. Sodium and chloride together account for around 85 per cent of the dissolved material, which is why seawater tastes like salt rather than like the mixed mineral water the weathering process might suggest.

Now the correction that separates a good answer from a complete one. The account above, taken alone, implies that the ocean must be growing saltier without limit — that seawater was fresher in the Jurassic and will be brinier in a hundred million years. That is not what the evidence shows. Ocean salinity has been roughly stable over very long stretches of geological time.

The reason is that the trap leaks. Several processes remove dissolved ions at close to the rate rivers deliver them. Ions are incorporated into sea-floor sediments, both by chemical precipitation and by organisms that build shells and skeletons and then sink. Seawater circulates through hot, freshly formed crust at mid-ocean ridges, where hydrothermal reactions strip out magnesium and sulphate and exchange other elements with the rock. Salt spray carries a small amount back to land. Where a shallow sea is cut off from the open ocean and evaporates completely, it leaves behind vast beds of solid evaporite minerals — halite and gypsum — which lock the salt into the geological record. Those buried deposits are the same ones we mine for salt today.

The ocean therefore sits in what a chemist would call steady state: input roughly equals output, so the concentration holds even though individual ions are constantly arriving and departing. A useful figure for this is residence time, the average duration an ion spends dissolved in the sea before being removed. It varies enormously by element — sodium's is on the order of tens of millions of years, while iron's is measured in decades — and those differences in residence time are a large part of why seawater's composition looks so different from the composition of the rocks it was weathered out of.

One final distinction worth holding onto, because it is a favourite exam trap. Salinity varies from place to place across the ocean surface, but that variation has nothing to do with where rivers enter or how much rock is weathering nearby. It tracks the local balance of evaporation against precipitation. The subtropics, under persistent high pressure with strong evaporation and little rain, carry the saltiest surface water. The equatorial belt, with heavy rainfall, is fresher. Polar surface water is fresher again, diluted by ice melt. The total amount of salt in the ocean is a story about geology over billions of years; the pattern of salinity you would measure today is a story about weather.