Why Is the Ocean Salty? Where the Salt Comes From and Why the Sea Doesn't Get Saltier
Short answer: the salt in the sea is mostly rock. Slightly acidic rain breaks down rock on land, rivers carry the dissolved minerals to the ocean, and hot water circulating through the seafloor adds more. The sea does not keep getting saltier without limit because the same minerals are also being taken out again, by living things, by chemical reactions with seafloor rock, and by deposits that settle into sediments. Average seawater is about 35 parts per thousand salt, or roughly 3.5 percent of its weight.
This page walks through where each part of that answer comes from, what is actually dissolved in seawater, how the supply and removal balance is worked out, and where scientists say the numbers are still uncertain. Every figure comes from the sources listed at the end, and where two sources disagree we say so.
Where the salt comes from
Two government science agencies, NOAA and the U.S. Geological Survey (USGS), give the same basic account: salt reaches the ocean from the land and from below the seafloor. Rocks on land are described as the major source.
Rivers and rock weathering
Rain picks up a little carbon dioxide from the air, which makes it slightly acidic. That mild acid, together with the physical wear of flowing water, breaks down rock and releases dissolved minerals in the form of ions. Runoff carries them into streams and rivers and eventually into the sea. The USGS puts the contrast simply: the answer to why the ocean is salty "is really very simple", and it is that salt in the ocean comes from rocks on land.
River water itself is not salty. A 1963 USGS compilation of river chemistry by D. A. Livingstone estimated that the average concentration of the main dissolved constituents of the world's rivers adds up to about 120 parts per million, against roughly 35,000 parts per million in the ocean (a USGS figure). The same compilation estimated that rivers deliver about 3.9 billion tons of dissolved material to the sea each year. Livingstone was blunt about the quality of the data behind such totals, noting that the data available for these calculations were "scarce, incomplete, and not always accurate." Treat that figure as a 1963 estimate rather than a modern measurement.
Hydrothermal vents and undersea volcanoes
The second source is the seafloor. According to NOAA, ocean water seeps into cracks in the crust and is heated by magma. The heating triggers chemical reactions: the water tends to lose oxygen, magnesium and sulfate, and to pick up metals such as iron, zinc and copper from the surrounding rock. The hot fluid then comes back out through vents. NOAA adds that some ocean salts come from underwater volcanic eruptions, which release minerals directly into the water.
The USGS notes that estimates of current vent flow suggest the entire volume of the oceans could circulate through hydrothermal systems every 10 to 20 million years. It stresses that these reactions run both ways: some dissolved salts react with the rock and are removed from the seawater. That detail matters later, because it means vents are a sink for some ions as well as a source of others.
Salt deposits and groundwater
NOAA also lists salt domes, vast salt deposits that formed over geological time underground and undersea, as a contributor, and points to the northwestern Gulf of America continental shelf, where they are common. A 2025 paper in Nature Communications by Yael Kiro adds another route: groundwater that flows into the sea beneath the coast. Using budgets of elements and isotopes, the study concludes that long-term submarine groundwater discharge is a major source of calcium and alkalinity to the ocean, while removing sodium and potassium, and that its fluxes "rival" those of rivers. Kiro also states that the significance of rivers and weathering has been established for decades, whereas the other sources, including ridge hydrothermal systems and seafloor alteration, are "still under debate and less constrained."
What is in the salt
"Salt" in this context means a mix of ions, not just the sodium chloride on a dinner table. Chloride and sodium are the two most abundant. NOAA says together they make up around 85 percent of all dissolved ions, with magnesium and sulfate adding roughly another 10 percent and everything else present in very small amounts. The USGS page on the same question says chloride and sodium make up "over 90%". The two agencies do not agree on that share, so the safest reading is that chloride and sodium together are the large majority, somewhere between 85 and 90 percent according to these two pages.
For scale, the USGS says a cubic mile of seawater holds about 120 million tons of salt (counted as sodium chloride). NOAA's figure, as quoted by the USGS, is that if all the ocean's salt were removed and spread over the land, it would form a layer more than 500 feet thick. The USGS also notes a cubic mile of seawater can hold up to 25 pounds of gold, at a concentration of 0.000005 parts per million, which it describes as no reason to try alchemy given how enormous a cubic mile is.
The table below puts together the numbers we could verify. Seawater concentrations and residence times come from the model table in the Kiro paper; the river figures are Livingstone's world averages. The units differ between columns, so compare the pattern rather than the raw numbers.
| Ion | Seawater (millimoles per litre) | Average world river water (parts per million) | Estimated residence time in the ocean (millions of years) |
|---|---|---|---|
| Sodium (Na+) | 475 | 6.3 | 71 |
| Chloride (Cl−) | not given in the sources we read | 7.8 | not given |
| Sulfate (SO42−) | not given in the sources we read | 11.2 | not given |
| Magnesium (Mg2+) | 53.8 | 4.1 | 14 (12 if groundwater is counted as a source) |
| Calcium (Ca2+) | 10.5 | 15.0 | 0.9 (0.5 if groundwater is counted as a source) |
| Potassium (K+) | 10.3 | 2.3 | 5 |
| Bicarbonate (HCO3−) | not given in the sources we read | 58.4 | not given |
Source: Kiro Y., Nature Communications 16, 7082 (2025), model table of end-member concentrations and residence times; Livingstone D. A., USGS Professional Paper 440-G (1963), world mean river composition. Residence times are the paper's calculated values based on its list of known sources.
The river column shows something worth noticing. River water is dominated by bicarbonate and calcium, while seawater is dominated by sodium and chloride. The ocean is therefore not just concentrated river water. Something sorts the ions after they arrive, which brings us to the question of why the sea stops getting saltier.
Why the sea does not keep getting saltier
If rivers keep delivering billions of tons of dissolved material each year, you might expect the ocean to become steadily more concentrated. The sources describe two ideas that explain why it does not run away.
Ions are removed as well as added. The USGS says many of the ions that rivers bring are used by marine organisms and so taken out of the water, while others are not used and remain, which is why their concentrations can build up. NOAA gives the same split. Kiro's paper describes the ocean's chemistry as governed by the transport of elements from land to sea through weathering and by their removal through "consumption, adsorption, and precipitation." Its budget table lists the sinks it uses: carbonate and other mineral precipitation, reactions with seafloor basalt, interaction with ocean sediments, and subduction zones. For sodium, the removal term in that table is estimated from the average rate at which large salt deposits, called salt giants, formed through geological history. Salt that once sat in the sea is, in effect, locked away in rock.
Each ion has its own timescale. Because inputs and removals differ for each element, each ion stays in the ocean for a different length of time. In Kiro's calculation, calcium has the shortest residence time of the ones in our table, under a million years, potassium is about 5 million years, magnesium about 14 million, and sodium about 71 million. A residence time is the amount of ion in the ocean divided by the rate at which it is added, so a long residence time means a huge reservoir relative to the yearly input. Sodium has the longest residence time in the table, which is consistent with it being so abundant in seawater even though river water carries comparatively little of it; the sources do not frame it exactly that way, so take that link as our reading.
A bathtub is a fair analogy, though it is ours and not from the sources: the water level stays put when the tap and the drain run at the same rate, even though water is constantly flowing through. For salinity, the "tap" is rivers, vents, volcanoes and groundwater, and the "drain" is organisms, mineral formation, sediments and altered seafloor rock. The Kiro paper builds its element budgets by first assuming steady state, meaning inputs and outputs balance, and then discussing what happens when conditions change.
Why some seas are saltier or fresher
The 35 parts per thousand figure is an average. NOAA states that salinity varies with temperature, evaporation and precipitation, and that it is generally low at the equator and at the poles and high at mid-latitudes.
Enclosed seas show the extremes. A 2025 paper in Scientific Reports by Emil Stanev separates marginal seas by their water balance. "Estuarine" seas, such as the Black Sea and the Baltic Sea, receive more water from rivers and rain than they lose to evaporation. "Lagoon" seas, such as the Mediterranean and the Red Sea, are the opposite: evaporation wins. Both kinds are limited in how much water they exchange with the open ocean. In the paper's model of the Black Sea, which connects to the Mediterranean through the Bosphorus strait, only about 30 metres deep and 1 kilometre wide, surface salinity rose from about 8 practical salinity units when the sea reconnected with the global ocean roughly 10,000 years ago to about 17 today. That is a model result, not a measurement, but it illustrates how a narrow doorway and a river-rich basin keep a sea far fresher than the open ocean.
At the other end, the USGS describes the Dead Sea as so salty and dense that a swimmer floats "like a mattress", and notes that salt water also exists underground far from any coast. In New Mexico, for example, about 75 percent of groundwater is too saline for most uses without treatment, and some may be left over from ancient seas that once covered the region.
What scientists are still working out
- The totals are uncertain. The river figures we quote date from 1963, and their author warned that the data were scarce and that calculated results can look more reliable than they are. We did not find a modern global river-load figure in the sources we could verify, so we have not stated one.
- The source list is not settled. Kiro notes that seafloor alteration, sediment interaction, subduction-zone discharge, large-scale groundwater flow and submarine groundwater discharge are "still under debate and less constrained" compared with rivers. A different answer for any one of them changes the calculated residence times, as the table shows for calcium and magnesium when groundwater is counted.
- Sodium is the hardest to pin down. Kiro points out that because seawater holds so much sodium, small changes in groundwater sodium often fall within the measurement error, and the table notes that hydrothermal sodium has almost no data.
- Not every budget balances. A 2020 review in Reviews of Geophysics on ocean alkalinity (Middelburg, Soetaert and Hagens) found an imbalance of 18 to 36 teramoles per year in its alkalinity budget, and said one possible explanation is that the present-day ocean is not at steady state for alkalinity. That review is about alkalinity, not salinity, but it is a reminder that "steady" is an assumption the budgets test rather than a proven fact.
- Regional salinity can shift. The Black Sea model shows a sea's salt content responding to sea level and freshwater input over thousands of years. We did not find, in the sources we could verify, a trend for the global ocean's average salinity, so we make no claim about whether it is changing today.
Common questions
How salty is the ocean compared with fresh water?
The USGS classes water below 1,000 parts per million as fresh and water from 10,000 to 35,000 parts per million as highly saline, and says the ocean holds about 35,000 parts per million. In other words, seawater sits at the top of the "highly saline" range.
Why can't people drink seawater?
The USGS says that in its raw state saline water cannot be used for drinking, most irrigation or many industrial uses, and that drinking water that is too salty is dangerous to health. It can be turned into fresh water by desalination, a process the agency says is being used more and more.
Is the salt the same everywhere in the ocean?
The total amount varies from place to place, low near the equator and the poles and high at mid-latitudes, according to NOAA. We did not verify a regional comparison of the ion proportions, so we do not state one.
If rivers carry salt in, why is river water fresh?
Because the concentration is tiny. Livingstone's average for world rivers is about 120 parts per million of dissolved solids, compared with about 35,000 in the ocean. Rivers deliver a modest amount of salt per litre but continuously, over geological time, into a reservoir that holds enormous amounts.
Does the ocean really contain gold?
Yes, but diluted almost beyond use. The USGS quotes up to 25 pounds of gold in a cubic mile of seawater, at 0.000005 parts per million, and a cubic mile holds more than a trillion gallons.
Is the ocean getting saltier over time?
The sources we could verify do not give a current trend for the global average. They describe a balance between supply and removal, with residence times of millions of years for the major ions, and Kiro treats steady state as the starting assumption. We could not confirm a measured change in average salinity, so we leave that out.
Sources and how to check
- NOAA National Ocean Service. "Why is the ocean salty?" Page last updated 23 September 2026. oceanservice.noaa.gov/facts/whysalty.html
- U.S. Geological Survey, Water Science School. "Why is the Ocean Salty?" (2019). usgs.gov/water-science-school/science/why-ocean-salty
- U.S. Geological Survey, Water Science School. "Saline Water and Salinity" (page dated 24 August 2026). usgs.gov/water-science-school/science/saline-water-and-salinity
- Livingstone D. A. "Chemical composition of rivers and lakes." U.S. Geological Survey Professional Paper 440-G, in Data of Geochemistry (1963). DOI: 10.3133/pp440G
- Kiro Y. "Coastal aquifers key contributors to ocean chemistry through solute fluxes." Nature Communications 16, 7082 (2025). DOI: 10.1038/s41467-025-62411-8 · Europe PMC: PMC12316954
- Stanev E. V. "Long-term hydrodynamic changes in marginal estuarine seas: the role of sea level rise and freshwater fluxes." Scientific Reports 15, 44637 (2025). DOI: 10.1038/s41598-025-33172-7 · Europe PMC: PMC12749219
- Middelburg J. J., Soetaert K., Hagens M. "Ocean Alkalinity, Buffering and Biogeochemical Processes." Reviews of Geophysics 58 (2020). DOI: 10.1029/2019rg000681 · Europe PMC: PMC7391262
- Pages and full texts read, and bibliographic details checked via Crossref (Livingstone, Stanev) and Europe PMC (Kiro, Middelburg and colleagues), on 22 October 2026. "Millions of years" residence times and seawater concentrations are the Kiro paper's tabulated values; the comparison between the river and seawater columns is ours.