Undersea volcanoes on mid-ocean ridges, and hydrothermal vents, release metallic and non-metallic ions (which together make salts) into the deep ocean. Rainwater, which is slightly acidic, erodes rocks on land, dissolving mineral ions and carries them to the sea in rivers. Also, winds blow dust containing minerals off the land and into the sea.
The water cycle is completed when water is evaporated from the sea surface to form rain clouds, but the salts are left behind in the ocean. The result, after hundreds of millions of years’ accumulation, is an average of about 3.5 per cent salinity for seawater worldwide.
The fact that this degree of saltiness remains relatively stable is due to the fact that salts are also being taken out of solution by marine animals, in sea floor sedimentary rock formation and tectonic subduction. It has been calculated that the ocean contains 5.5 trillion tons of salt – enough to cover the entire planet to a depth of 45 metres.
Oliver Crimmen, Senior Fish Curator, The Natural History Museum London