In my Year 8 science class, we did an experiment with sea water. We removed the sand using filter paper and left the water to evaporate. When it did, there was a layer of salt crystals left behind. But if we had done the same thing with water from Lake Burley Griffin, the result would have been different. Why?
In fact, the lake water would have contained some salt. But even if it was fit to drink it would not have tasted salty; the salt concentration in so-called ‘fresh’ water is too low to be detected. When it comes to water, ‘fresh’ and ‘salty’ are judgments of taste. Water in lakes and rivers contains trace amounts of salt that has been eroded from rocks and absorbed from the ground. The salt in the world’s rivers does eventually reach the sea, but accounts for only a tiny fraction of the ocean’s salt content. It would take between 200 and 300 million years for the supply of salt from the rivers to equal the amount of salt currently in the ocean.
Another source of salt is hydrothermal vents on the ocean floor. These are places where sea water has seeped into the rock, heated up, dissolved some of the minerals in the oceanic basalt that makes up the sea floor, and begun to flow back into the ocean. It has been estimated that the entire volume of the world’s oceans could pass through these vents once every 10 million years, having a significant effect on salinity. A more dramatic salt source working on the same principle is submarine volcanism, when volcanoes in the sea floor erupt underwater.
This effect works both ways; dissolved salts can react with the rock and precipitate out of the water. In this way, the ocean has long since reached an equilibrium: salt is being absorbed out of the water at the same rate as salt is being supplied. Although balanced on the whole, with an average salt content of 35 ppm (parts per million), ocean salinity can vary from place to place. Melting ice, river outflow, evaporation, rain and snow, and ocean currents can all contribute to areas of higher or lower salinity. The world’s highest ocean salinity is in the Red Sea and the Persian Gulf, where rates of evaporation are very high. The lowest are in polar regions, due to melting ice and snowfall, and in partly landlocked areas such as the Baltic Sea, which receive large volumes of rainwater runoff from the land.
So how did the sea become salty in the first place? By evaporation. Water can evaporate from the ocean to form clouds and later rain as part of the hydrologic cycle, but salt cannot. After hundreds of millions of years of evaporation and of an inflow of salts from rivers, the ocean’s salt levels increased to what we see today. This process also explains why there are salt lakes. If a large lake – such as the Dead Sea, the Great Salt Lake, or Lake Eyre – has no outlet, its water can leave only by evaporation. Over time, the salt level will build up just as that of the ocean has.
Next time you come home from the beach with salt dried on your skin, or you add a little sea salt to your fish and chips, think of the millions of years of evaporation and precipitation that it took to get that salt there.
