The saltiness of the sea comes from dissolved minerals, especially sodium, chlorine, sulfur, calcium, magnesium, and potassium, says Galen McKinley, a UW-Madison professor of atmospheric and oceanic sciences.
Today’s ocean salt has ancient origins. As the earth formed, gases spewing from its interior released salt ions that reached the ocean via rainfall or land runoff.
Now, the ocean’s salinity is basically constant. “Ions aren’t being removed or supplied in an appreciable amount,” McKinley says. “The removal and sources that do exist are so small and the reservoir is so large that those ions just stay in the water.” For example, she says, “Each year, runoff from the land adds only 0.00005 percent of total ocean salts.”