Science Puzzle
Ocean Salinity Origin
Rain is essentially pure water. Rivers are nearly fresh. Yet the ocean is about 3.5% salt by mass, and has been salty for billions of years. The salt did not fall from the sky.
Where did all the ocean's salt come from, and why does it keep accumulating there?
The Answer
Rain slowly dissolves minerals from rocks as it flows over and through them. The resulting slightly salty water flows into rivers and eventually reaches the ocean. The amount of dissolved minerals in any individual river is tiny, but rivers have been flowing for billions of years.
The key is what happens at the ocean. The Sun's energy evaporates water from the ocean surface, but evaporation is a physical process that lifts only pure water molecules into the atmosphere. The dissolved salts, primarily sodium chloride and smaller amounts of magnesium, calcium and sulphate compounds, are left behind.
The water cycle then drops fresh water back onto the land, which dissolves more minerals, carries them back to the ocean, and evaporation removes the pure water again. Over billions of cycles, the salts have gradually concentrated in the ocean like water evaporating from a pot of salted water.
The ocean is in a rough steady state: some minerals are removed by seafloor sediment formation and by organisms building shells, balancing the input from rivers. But the baseline salinity has been fairly stable for several hundred million years.
The principle: Solar distillation and mineral accumulation. Evaporation removes only pure water from the ocean, leaving dissolved minerals behind. Rivers continuously deliver new mineral input from weathered rocks, concentrating salts over geological time.