The saltiness in seawater comes from dissolved minerals (sodium, chlorine, sulfur, calcium, magnesium, and potassium) dating back to early days of our planet when vulcanism spit gases and lava from Earth's interior. Spewed carbon dioxide dissolved in rain and fog to form weak carbonic acid which dissolved rocks into salt ions. Some of those ions reached the sea via rainfall and river runoff. Some leached from rock on the seafloor. The process still happens though the amount washed from the land is negligible, an estimated 0.00005 percent of total ocean salts a year, according to Galen McKinley at the University of Wisconsin Madison. And whereas a drop of lake water and its ions might persist for a couple of centuries, ocean salts hang around for about 100 to 200 million years, surviving evaporation and uptake by marine organisms (more on that in an upcoming post). "There's geologic evidence," says McKinley, "that the saltiness of the water [~35 parts per thousand] has been the way that it is for at least a billion years." All things change. Seawater, not so much.