Rain, rivers, and lakes are fresh. Tears and sweat taste salty, but only faintly. Then there is the ocean, salty enough that a mouthful makes most people gag. Why is the ocean salty in the first place, when the water that feeds it is not? The answer is not a single event but a slow, billion-year accumulation driven by two very different geological processes.
Why Is the Ocean Salty and Rivers Are Not?
Rivers actually do carry dissolved salts and minerals toward the sea, they are just too dilute to taste salty. As rain falls, it becomes slightly acidic from absorbing carbon dioxide in the air, and that mildly acidic water slowly erodes rocks and soil on its way downhill. This erosion releases ions like sodium and chloride, the building blocks of ordinary table salt, which rivers then carry to the ocean. Rivers and streams deliver an estimated 4 billion tons of dissolved salts to the ocean every single year. Individually that is a tiny concentration, but the ocean has been collecting this runoff for roughly 4 billion years, and water constantly evaporates from the ocean surface while the dissolved salt stays behind, concentrating over time.
Hydrothermal Vents Add Salt From Below
Rivers are not the only source. Along mid-ocean ridges, seawater seeps down into cracks in the ocean floor, where it is superheated by magma close to the Earth’s crust. This hot water reacts chemically with surrounding rock, stripping out minerals like iron, manganese, and various salts, before the mineral-rich fluid gushes back out through seafloor openings called hydrothermal vents, sometimes visible as dramatic black smokers. This underwater volcanic activity has been steadily adding dissolved minerals directly to the ocean for as long as the seafloor has existed, working alongside river runoff rather than instead of it.
How Salty Is the Ocean, Exactly?
Scientists measure ocean saltiness as salinity, and the global average is about 35 parts per thousand, meaning roughly 3.5 percent of seawater’s weight comes from dissolved salts. Salinity is not uniform everywhere: it runs higher in warm, evaporation-heavy regions like the subtropics, and lower near the mouths of major rivers or in polar regions fed by melting ice. Some enclosed bodies of water take this much further. The Dead Sea, which has no outlet and loses water only through evaporation, has built up to nearly 32 percent salinity, far beyond the open ocean’s average.
Why the Ocean Is Not Getting Saltier Today
Given billions of years of rivers and vents adding salt, it might seem like the ocean should keep getting saltier indefinitely, but it has actually reached a rough equilibrium. Marine organisms constantly pull dissolved ions like calcium and silica out of seawater to build shells, skeletons, and other structures, removing salts from the water at roughly the same pace new salts arrive. Salt also gets locked away in seafloor sediments and evaporite deposits over geologic time. The result is an ocean whose overall salinity has stayed fairly stable for a very long stretch of Earth’s history, even as the underlying input and removal processes never stop.
Salt Shapes Ocean Life and Currents
Ocean salinity is not just a chemistry footnote, it actively drives how the planet works. Combined with temperature, salinity determines the density of seawater, and differences in density power huge, slow-moving currents that circulate heat around the globe, including the deep ocean conveyor belt that influences climate far beyond the coastline. Salinity also shapes which species can survive where, since marine organisms are generally adapted to a fairly narrow salinity range, and sudden changes, like a large freshwater influx from melting ice, can disrupt entire ecosystems built around that balance.