Sea water is salty due to the presence of dissolved salts and minerals. The salt in the ocean comes from two sources: runoff from the land and openings in the seafloor. Rocks on land are the major source of salts dissolved in seawater. Rainwater that falls on land is slightly acidic due to the presence of carbon dioxide in the air, and it weathers rocks, releasing mineral salts that separate into ions. These ions are carried with runoff water and ultimately reach the ocean. Some ocean salts come from underwater volcanic eruptions, which directly release minerals into the ocean. Salt domes also contribute to the oceans saltiness. These domes, vast deposits of salt that form over geological timescales, are found underground and undersea around the world.
The two most prevalent ions in seawater are chloride and sodium, which together make up around 85 percent of all dissolved ions in the ocean. Magnesium and sulfate make up another 10 percent of the total. Other ions are found in very small concentrations. The concentration of salt in seawater (salinity) varies with temperature, evaporation, and precipitation. Salinity is generally low at the equator and at the poles.
In contrast, rivers flowing into the ocean are not salty because rain replenishes freshwater in rivers and streams, so they don’t taste salty. However, the water in the ocean collects all of the salt and minerals from all of the rivers that flow into it. It is estimated that rivers and streams flowing from the United States alone discharge 225 million tons of dissolved solids and 513 million tons of suspended sediment annually to the ocean. Throughout the world, rivers carry an estimated four billion tons of dissolved salts to the ocean annually. About the same tonnage of salt from ocean water probably is deposited as sediment on the ocean bottom, and thus yearly gains may offset yearly losses. In other words, the ocean today probably has a balanced salt input and output.
