Why Is the Ocean Salty?

If you’ve ever tasted ocean water, you know it’s unmistakably salty. But have you ever wondered where all that salt comes from—and why seawater is salty while rivers and lakes remain fresh? The answer lies in a long and complex story involving Earth’s geology, chemistry, and hydrology—playing out over hundreds of millions of years.

Salt by the Numbers

Seawater contains about 3.5% dissolved salts by weight, or 35 parts per thousand (ppt). That means every liter of seawater holds roughly 35 grams of salts, mostly sodium (Na⁺) and chloride (Cl⁻) ions—the same chemical components that make up common table salt (NaCl).

It All Starts With Rain and Rock

Rain may seem pure, but it’s slightly acidic. As water vapor condenses in clouds, it absorbs carbon dioxide (CO₂) from the atmosphere, forming weak carbonic acid. When this rain falls onto land, it begins chemical weathering—the slow breakdown of rocks.

Illustration explaining ocean salinity: acidic rain weathers rocks, rivers transport dissolved ions (Na⁺, Cl⁻), evaporation concentrates salts, and hydrothermal vents add minerals.

As rainwater flows over and through rock and soil, it dissolves minerals like:

Sodium (Na⁺)
Calcium (Ca²⁺)
Magnesium (Mg²⁺)
Potassium (K⁺)
Bicarbonate (HCO₃⁻)
Chloride (Cl⁻)

Rivers act as delivery systems, carrying roughly 4 billion tons of dissolved ions into the ocean every year. While many minerals precipitate or get used by organisms, sodium and chloride are especially stable and soluble—so they accumulate.

Hydrothermal Vents: Salting the Sea From Below

Not all ocean salt comes from land. Seawater also enters cracks in the seafloor at mid-ocean ridges, where it’s heated by magma. These hydrothermal vents and submarine volcanoes superheat seawater to over 400°C (750°F), triggering chemical reactions between the water and basaltic crust.

This process:

Adds ions like magnesium, iron, calcium, and potassium to seawater
Removes others, altering the ocean’s chemistry
Releases metal-rich fluids and gases back into the sea

This deep-sea cycling is an important secondary source of salinity, particularly over geological time.

Evaporation Concentrates the Brine

Another critical process is evaporation. When the Sun heats the ocean’s surface, water molecules evaporate into the atmosphere—but the salts remain. Over time, this increases the salinity, especially in arid regions where evaporation exceeds freshwater input.

Notable examples:

Red Sea and Persian Gulf: Salinity can exceed 40 ppt
Dead Sea: A hypersaline basin with salinity >300 ppt (almost 10× that of seawater)

Conversely, areas with heavy rainfall or glacial melt, like the equatorial Pacific or polar seas, have slightly lower salinity.

Why Doesn’t the Ocean Keep Getting Saltier?

With billions of tons of salt entering each year, why hasn't the ocean turned into a giant brine pool?

The answer lies in the salt sinks—processes that remove salt from seawater:

Marine organisms (e.g., corals, foraminifera) use calcium and bicarbonate to build skeletons and shells
Evaporite minerals like halite (NaCl) and gypsum (CaSO₄·2H₂O) precipitate in shallow basins and become buried
Adsorption of ions onto clay minerals and deep-sea sediments
Tectonic subduction recycles ocean crust—and its chemical cargo—back into the mantle

These mechanisms keep ocean salinity roughly stable over time, at around 35 parts per thousand (ppt).

A Long Salty History

Geological records—especially evaporite formations—reveal that Earth’s oceans have been salty for hundreds of millions of years. Massive salt beds, like those in the Zechstein Basin (Europe, Permian Period) or Michigan Basin (Silurian Period), tell of times when shallow seas evaporated, leaving behind thick salt layers.

While ocean salinity has fluctuated, studies of marine fossils and fluid inclusions suggest that salinity has stayed close to modern levels (~33–37 ppt) for most of the Phanerozoic Eon (past 541 million years).

Salinity Shapes the Ocean’s Circulation and Life

Salinity isn’t just about taste—it drives major oceanic and ecological processes:

Density and Circulation: Saltier water is denser and sinks, powering the thermohaline circulation, or "global conveyor belt," which moves heat and nutrients around the world.
Marine Life: Most ocean organisms are adapted to stable salinity. Fluctuations can stress ecosystems, especially in estuaries or during climate shifts.
Climate Interactions: Salinity influences evaporation, precipitation, sea ice formation, and heat exchange—all critical to Earth’s climate system.

Rivers Are Fresh—Why Not Oceans?

Though rivers carry dissolved salts, their water remains fresh because:

It constantly refreshes through rainfall and runoff
Salt doesn’t have time to accumulate before reaching the sea
Freshwater ecosystems absorb or cycle many ions through biological activity and sedimentation

The ocean, however, is Earth’s ultimate ion reservoir, collecting what rivers deliver over millions of years.

A Changing Ocean?

Human activity may be shifting local salinity patterns:

Melting ice caps and changing rainfall due to climate change are freshening some regions (like the Arctic) and increasing evaporation elsewhere.
Agricultural runoff and mining can increase salt delivery to rivers and estuaries.
Desalination and damming alter freshwater flow, with cascading ecological impacts.

Although the global average salinity remains stable, regional imbalances could disrupt marine currents and ecosystems over time.

The Bottom Line

The ocean’s saltiness is a story written in water, rock, heat, and time. It reflects the balance between chemical weathering, hydrothermal circulation, evaporation, and biological activity, all working together across billions of years.

Seawater’s 3.5% salt content may seem small, but it’s one of Earth’s most powerful geochemical signatures—one that sustains ocean circulation, influences climate, and supports the vast web of marine life.