Seawater tastes salty because of the action of rain on exposed rocks. The compounds most likely to find their way into the sea are, naturally enough, the most water-soluble ones, and these are rich in chlorine and sodium ions – the raw ingredients for common salt.
As this process has been operating for billions of years, there’s no doubt oceans have got saltier over time. In fact, the real mystery is why they aren’t now saturated with the stuff, making them as lifeless as the Dead Sea. Somehow, the concentration has remains at just a few per cent for at least half a billion years.
Exactly how isn’t clear, but one theory, suggested by British ecologist James Lovelock, involves the vast, mat-like colonies of bacteria found in coastal lagoons around the world. The Sun’s heat triggers evaporation of the water, leaving its salt content trapped on the coast and unable to dissolve back into the sea.