Why is the sea salty?
Because of the carbon dioxide in the atmosphere, rain is slightly acidic. When rain falls on land, this acidity erodes rocks, releasing mineral ions that are carried into rivers and then the sea. Some of these are sodium and chloride, which together make salt. It’s estimated that 4 billion tonnes of dissolved salt is delivered to the ocean by the water cycle each year. Vents in the seafloor and underwater volcanic eruptions are another source of salinity, releasing these ions from rocks beneath the seabed.