Ocean water is salty because it contains dissolved minerals and ions that have accumulated over a very long time. The most abundant are sodium and chloride, the two components of common salt, but seawater also contains magnesium, sulfate, calcium, potassium and many other dissolved substances.
Where does the salt in the ocean come from?
Rainwater absorbs a small amount of carbon dioxide from the atmosphere and soil, making it mildly acidic. As rain and rivers move across rocks, they slowly dissolve minerals and carry ions toward the sea. Rivers do not usually taste salty because the concentration is low, but the continuous delivery of dissolved material over geological time is significant.
Part of the ocean’s dissolved material also comes from the seafloor. Hydrothermal vents circulate seawater through hot oceanic crust, where chemical reactions add and remove different elements. Underwater volcanic activity and interactions between seawater and newly formed rock also influence ocean chemistry.
Why does the salt remain when water evaporates?
Solar energy causes water molecules to evaporate from the surface, but most dissolved salts stay behind. Water later returns through rain, rivers and melting ice. This global water cycle continually moves fresh water while leaving salts in the ocean, although many ions are also removed by organisms, sediments and reactions at the seafloor.
How salty is seawater?
Average open-ocean salinity is about 35 parts per thousand, meaning roughly 35 grams of dissolved salts in one kilogram of seawater. Salinity is not identical everywhere. Strong evaporation can raise it, while heavy rainfall, river discharge and melting ice lower it. Restricted warm seas can be saltier than the open ocean, and polar or river-influenced waters are often fresher.
Why salinity matters
Salinity affects water density. Together with temperature, it helps drive the movement of water masses and the global ocean circulation that redistributes heat, oxygen and nutrients. Marine organisms are adapted to particular salinity ranges, so rapid changes can affect ecosystems. Measuring ocean salinity therefore helps scientists understand weather, climate, freshwater exchange and marine habitats.