Oceans cover about 70% of Earth’s surface and hold around 97% of its water, yet remarkably, they are salty—on average about 3.5% dissolved salts by weight. That makes each liter of seawater contain roughly 35 grams of salts. Tracing the origins and steady state of ocean salinity reveals a story billions of years in the making.
The principal source of ocean salt is the chemical weathering of rocks on land. Rainwater, acidified by dissolved carbon dioxide, slowly erodes minerals and dissolves various ions—primarily sodium, chloride, magnesium, sulfate, calcium, and potassium. These substances flush into streams and rivers, ultimately reaching the sea. Over time, as freshwater evaporates from the ocean, it leaves these salts behind, causing gradual concentration.
Submarine geology also adds to salinity. Seawater seeps into cracks in the ocean floor, is heated by hot magma, and reacts with minerals—losing some compounds while leaching metals and ions. This hydrothermal exchange introduces magnesium, calcium, and other salts into the surrounding seawater. Added to that are salts released directly from undersea volcanic eruptions and large underground salt deposits, called salt domes, which eventually dissolve into nearby ocean waters.
Once dissolved, ions remain in the ocean for very long periods. Sodium and chloride, the main salts, form stable compounds and do not get used up by biological activity; they accumulate. In contrast, other ions like calcium are quickly taken up by organisms (for example in shell formation) or precipitate out. This differential behavior over time gives modern seawater a consistent chemical makeup, dominated by sodium and chloride—together making up 85 to 90 percent of dissolved ions.
Meanwhile, processes exist that export salts from the ocean. Tectonic subduction carries some seawater into the mantle; evaporation from evaporite basins deposits salt on land; and biological sequestration—like shell deposition—removes calcium and carbonate ions. These sinks counterbalance the inputs, leading to a long-term equilibrium in salinity levels.
Ocean salinity varies regionally. Evaporation-heavy zones, such as subtropical gyres, are saltier, while high-latitude and equatorial zones, which receive more rainfall or freshwater runoff, are fresher. Average seawater salinity remains around 35 parts per thousand, but local values may range from below 34 in polar regions to above 37 in semi-enclosed seas like the Mediterranean. These salinity patterns influence water density, drive ocean currents, and shape global climate systems.
Over geological time, ocean salinity has shifted. In the Precambrian era, trapped basins created massive salt deposits—evaporites—that reduced overall seawater salt concentrations. Changes in continental positions and climate during supercontinent cycles also altered salinity cycles. Despite these fluctuations, oceans have maintained an average salinity within roughly 34 to 37 parts per thousand for the last 500 million years.
Today, oceans remain in dynamic equilibrium. Rivers deliver roughly four billion tons of dissolved salts annually, while hydrothermal vents and undersea processes contribute thousands of tons more each year. At the same time, subduction and evaporation-driven salt deposition remove comparable amounts. Measurement technologies—from satellites to deep-sea sampling—confirm that salinity remains relatively stable. Yet small regional changes over the past decades reflect shifts in evaporation and precipitation tied to climate change, offering insights into evolving global water cycles.
