Why the sea is salt

There is an old northern story that says there is a mill at the bottom of the sea, grinding out salt, and that it has never once stopped, and that this is why the sea is salt. It is a good story, and it is good for a reason worth noticing: the sea is so salt, so evenly and endlessly and reliably salt, wherever you taste it and however much you draw off, that it feels as though there must be a source down there — some standing thing, some engine, forever making more. The truth keeps the mill and throws away the magic. There really is something grinding salt into the sea without rest, and it has been at it for most of the age of the Earth. It is only that it isn't at the bottom. It is every river you have ever seen.

Because the salt does not come from the sea. This is the part that turns the whole thing over. A river tastes fresh, but it isn't — not quite, not any of them. Every river carries a faint dissolved load of the land it drains: sodium and calcium and magnesium and silica, weathered grain by invisible grain off the rock upstream, far too dilute to taste but always, always there. A river is the continent slowly coming apart and being carried off in solution. And all of it, every river on Earth, is running to the same place.

The sea takes the water and the salt both. But it only ever gives the water back. Sunlight lifts pure vapour off the ocean — evaporation carries nothing dissolved, the salt cannot go up with the steam — and the vapour blows inland and falls as rain on the high ground and runs back down as fresh rivers, which is why rivers are fresh: they are made of sea that left its salt behind. The water goes round and round. The salt only ever arrives. The sea is salt because it is the one water on Earth with no way out: a river's water is passing through, on its way somewhere, but the sea's water leaves only as sky, and the sky will not take the salt.

You can watch this run fast wherever a body of water has no drain. The Dead Sea, the Great Salt Lake — rivers flow in and no river flows out, so the water can leave only upward, and the salt climbs and climbs until you cannot sink in it and almost nothing can live. The difference between a freshwater lake and a dead one is not the water that comes in; it is whether there is a way for water to leave except by the sky. A fresh lake has a river running out of it. A salt lake is a lake that only drinks.

And the sea is salt in a particular way — mostly plain table salt, sodium and chloride, more of it than everything else dissolved in the sea put together — even though the rock the rivers grind down is nothing much like table salt at all. That is the second turn, and the stranger one. The rivers carry off everything soluble in the land: calcium, silica, iron, the lot. But the sea keeps almost none of it. Calcium is pulled down out of the water endlessly into shells and coral and the white ooze that becomes limestone; silica is taken up by tiny glass-bodied plankton; iron rusts and settles. Sodium and chloride are the two that nothing wants — too soluble to fall out, too unreactive to be built into anything alive — so they are simply what is left in the water once everything else has been quietly removed from it. The sea is not salt because the land is salt. It is salt because salt is the part of the land that has nowhere else to go.

I should be honest that the sea is not getting saltier forever, the way the buried mill would make it. Over the long run salt leaves too — buried in the sediment on the sea floor, blown back onto the coasts as spray, laid down as flats where a shallow sea once dried. The ocean has held about the same salt for a very long time; it is a slow balance now, filling and emptying at once, not a runaway climb. But the direction of the delivery never changes and never has. It is always downhill, always land to sea, always the rivers. The taste of the ocean is that balance, held steady across an age of the world.

So the old story had everything right except the address. Something really is grinding the continents into salt and feeding it to the sea without rest — but it is not one dark mill on the sea floor. It is the rain on every hillside, and the thread of water in every ditch and brook and great brown river, and the whole slow taking-apart of the land, all of it carried down to the single place where water stops. Every river you have ever stood beside was a little bit salt, and on its way to make the sea a little more so. The taste of the sea is the taste of the mountains, dissolved and delivered, one rain at a time.