The salt cycle: rain wears tiny amounts of salt out of rock, rivers carry it to the sea, the sun lifts pure water back into the sky, and the salt stays behind.
π§οΈ Rain falls on rocks and hills. It picks up tiny bits of salt.
ποΈ Rivers carry the salt down to the sea.
βοΈ The sun lifts the water back up. The salt stays behind. So the sea gets salty!
Where does the salt come from?
Salt hides inside rocks. Rain is a tiny bit sour, so it slowly wears rocks away. Little bits of salt wash out and float away in rivers.
Why are rivers not salty?
A river has only a pinch of salt in it. You cannot taste it. But rivers have poured into the sea for millions and millions of years, and every drop brought a pinch.
Why does the salt stay?
The sun warms the sea. Some water floats up as vapor to make clouds. Salt is too heavy to float up, so it stays in the sea. Water leaves, salt stays, and the sea keeps every pinch!
Rocks Are Full of Salt
The sea did not start out salty. The salt came from the land, and rain brought it. Rain is a little bit sour, because it picks up a gas from the air on the way down. That sour rain slowly dissolves rock, the way a candy dissolves in your mouth, only slower. Those dissolved bits are what salt is made of. Rivers collect them and carry them to the sea.
Rivers Are Salt Trucks
River water tastes fresh because each cup holds only a speck of salt. But the rivers of the world deliver about four billion tons of dissolved salt to the sea every year, and they have been doing it for billions of years. A speck a trip, trip after trip, adds up.
Water Leaves, Salt Stays
The sun warms the sea, and some of the water turns into water vapor and floats up to make clouds. Salt cannot float. It stays behind. The clouds rain fresh water onto the land, the rivers carry a little more salt to the sea, and the sun lifts the water out again. The water goes round and round, but the salt only goes one way: in.
Is the Sea Getting Saltier?
Not any more. Some salt leaves too. Sea creatures use part of it to build shells. Waves throw tiny salt drops into the air that land on the coast. When a piece of sea dries up, it leaves thick beds of salt behind. Salt in and salt out are about even now, so the sea has been this salty for a very long time.
Try This!
Stir a spoon of salt into a cup of warm water until it disappears. Pour a little onto a dark plate and leave it in the sun. When the water is gone, look closely. White salt crystals are left behind, just like in the sea.
How Salty Is Salty?
A kilogram of seawater holds about 35 grams of dissolved salt, which is 3.5 percent, and about 85 percent of that salt is ordinary sodium chloride, the stuff in the shaker. The rest is mostly magnesium, sulfate, calcium and potassium, with traces of almost every element there is. Multiply 35 grams by the mass of the ocean and you get about 50 million billion tonnes of salt. Spread evenly over all the land on Earth, it would make a layer about 150 metres thick, the height of a 40-storey office building.
Where the Salt Comes From
Rain is slightly acidic, because carbon dioxide from the air dissolves in it and makes carbonic acid. When that rain runs over rock it slowly breaks the minerals apart, a process called chemical weathering, and the ions it frees, sodium, calcium, potassium, magnesium and more, wash into streams, then into rivers, and finally into the sea, which receives about four billion tonnes of dissolved salt from the world's rivers every year. Two more sources work under the water. At the mid-ocean ridges, seawater seeps into hot new crust, reacts with the rock, and gushes back out of hydrothermal vents loaded with dissolved metals, while volcanoes above and below the sea add gases such as hydrogen chloride, which is where most of the ocean's chloride came from, because ordinary rock holds very little chlorine.
Why the Sea Is Not Getting Saltier
If salt only ever came in, the sea would be a brine by now, and in 1715 the astronomer Edmond Halley proposed using exactly that idea as a clock: measure how fast rivers add salt, divide the total salt by that rate, and you have the age of the ocean. In 1899 the Irish physicist John Joly did the sums for sodium and got between 80 and 90 million years. The real answer is about four billion. The clock leaks. Salt also leaves the sea, and it leaves by several doors: sea creatures pull calcium out of the water to build shells and reefs, which is why river water rich in calcium ends up as sea water rich in sodium and chloride instead, waves throw salt spray onto the land, clay on the sea floor swaps ions with the water, salt is buried in the pores of sediments, and when a shallow sea is cut off and dries out it leaves salt beds hundreds of metres thick. Salt in and salt out are now roughly in balance. The ocean has probably been about this salty for hundreds of millions of years.
Salty Places and Fresh Places
Salinity is not the same everywhere. Where the sun evaporates more water than the rain returns, as in the Red Sea, the water reaches about 40 grams per kilogram. Where rivers pour in and evaporation is weak, as in the nearly enclosed Baltic Sea, it drops to about 7. The Atlantic is about a gram per kilogram saltier than the Pacific, because trade winds carry evaporated Atlantic water across Central America and rain it out on the Pacific side. The Dead Sea has no outlet at all, so everything its rivers bring stays, and after ages of that it is nearly ten times saltier than the ocean, dense enough that a swimmer cannot sink in it. Salt lakes are the trap with the lid on.
Think About It
Rivers carry mostly calcium and bicarbonate, yet the sea is mostly sodium and chloride. Where did the calcium go, and what would the sea be made of if nothing had ever built a shell?
A Question of Residence Time
Seawater is a solution of about 35 grams of dissolved solids per kilogram, and its composition is remarkably fixed: chloride makes up about 55 percent of the mass of the salts, sodium 31, sulfate 8, magnesium 4, calcium and potassium about 1 each. Rivers look nothing like this. Their dissolved load is dominated by calcium and bicarbonate from the weathering of limestone and silicate rock, with sodium and chloride well behind. The difference is explained by residence time, the average time an ion stays dissolved before something removes it, which is the ocean's inventory of that ion divided by its yearly input. For sodium and chloride the answer is tens of millions of years or more, for calcium about a million, for silicon about twenty thousand, and for iron a couple of centuries. The elements that dominate seawater are simply the ones the ocean is worst at getting rid of.
Sources
Carbonic acid rain weathers continental rock and rivers carry roughly four billion tonnes of dissolved salts to the sea each year. Not all of that is new: a good share of the chloride in rivers is cyclic salt, sea spray that blew inland and washed back, plus halite from ancient salt beds dissolving again. The primary source of chloride is volcanic degassing of hydrogen chloride, largely early in Earth's history, since crustal rock holds little chlorine. The third source is hydrothermal: the whole volume of the ocean circulates through hot ridge crust roughly every ten million years, coming out stripped of magnesium and sulfate and enriched in calcium, potassium, iron and manganese.
Sinks
Halley's 1715 proposal to date the Earth by its salt, and Joly's 1899 estimate of 80 to 90 million years from the sodium delivered by rivers, both assumed that salt accumulates. It does not, or not for long. Calcium leaves as the carbonate shells of corals, molluscs, foraminifera and coccolithophores, which is why the ocean is sodium chloride and not calcium bicarbonate. Silicon leaves in the glassy skeletons of diatoms and radiolarians. Magnesium is consumed in ridge hydrothermal systems. Sodium, potassium and magnesium are taken up by clay minerals forming on the sea floor, and salt is buried with the pore water trapped in sediments. The largest sink is the most dramatic: when an arm of the sea is cut off and evaporates, it deposits evaporites, gypsum first and then halite. Between 5.97 and 5.33 million years ago the Mediterranean was repeatedly sealed at Gibraltar and dried down, leaving salt layers up to two kilometres thick that hold about 6 percent of all the salt in today's ocean. Whether the whole system is in steady state is still debated, but the isotope record suggests the ocean has been within a factor of two of its present salinity for most of the last two billion years, and perhaps somewhat saltier in the Archean, before the first salt beds had been laid down.
Salt Moves the Ocean
Dissolved salt raises density by about 2.5 percent, lowers the freezing point to about minus 1.9 degrees Celsius, and removes the density maximum at 4 degrees that fresh water has, so a salty sea must cool through its whole depth before ice can form, which is why ponds freeze over and the open ocean rarely does. Surface salinity ranges from about 33 to 37 grams per kilogram, highest under the dry subtropical high-pressure belts, lowest under the rainy equator and near river mouths. The Atlantic is about one gram per kilogram saltier than the Pacific because the trade winds export its vapour across the Isthmus of Panama. That extra salt is what allows cold North Atlantic water to become dense enough to sink, driving the deep circulation that carries heat toward Europe. Forming sea ice adds to it: ice rejects most of its salt as it freezes, and the cold brine left behind sinks. Since 2009 satellites have measured surface salinity from orbit by the faint microwave glow of the sea, and the pattern they see is the water cycle drawn in salt.
Salt Water Beyond Earth
The Cassini spacecraft flew through the plumes of Saturn's moon Enceladus and found sodium salts in the ice grains, evidence that the buried ocean there is in contact with rock and salty for the same reason ours is. The Hubble telescope detected sodium chloride on the surface of Jupiter's moon Europa in 2019. Weathering and evaporation are apparently not an Earth-only story.
What Is Still Unknown
The chloride budget is the least certain part of the story, because the volcanic input and the burial rate are both hard to measure; the salinity of the Archean ocean, and whether early life evolved in a sea twice as salty as ours, is argued from fluid inclusions in ancient halite; and the balance between evaporite burial and dissolution over the last hundred million years, which sets whether the sea is drifting slowly fresher or saltier today, is known only roughly.
Why Your Kid Asked
Because a mouthful of seawater is a surprise, and the obvious follow-up, "then why is the lake not salty", has an answer that sounds like a trick until you see it: the rivers are salty too, just not enough to taste, and the sea is where all of it ends up.
The Short Version
Rain is mildly acidic and slowly dissolves rock. Rivers carry the dissolved minerals to the sea, about four billion tonnes a year, and evaporation takes only the water back out, so the salt stays, and after billions of years of that one-way delivery the sea holds about 35 grams of salt per kilogram, with a composition that is the same in every ocean. It is not still getting saltier. Salt also leaves: shells and reefs remove calcium, clays and sediments take up other ions, spray blows some onto the land, and dried-up seas leave salt beds behind, so the sodium and chloride that dominate seawater are simply the ions the ocean is slowest to remove. Most of the chloride was breathed out by volcanoes rather than washed off rock.
The Numbers
Seawater is 3.5 percent salt, about 85 percent of it sodium chloride, six teaspoons per litre, 35 kilograms per cubic metre, and roughly 50 million billion tonnes in all, enough for a 150-metre layer over every continent. River water averages about a tenth of a gram per litre, some 300 times less. The Red Sea runs to about 40 grams per kilogram, the central Baltic about 7, the Dead Sea about 340. Seawater freezes at about minus 1.9 degrees Celsius and is about 2.5 percent denser than fresh water. Rivers deliver about four billion tonnes of salt a year, and the salt "clock" built on that figure gave John Joly an Earth about 90 million years old in 1899, roughly fifty times too young. The clock leaks.
Two Common Misconceptions Worth Correcting
First, "the sea is getting saltier every year". It is not: the inputs and the sinks are close to balanced, and the sea has been roughly this salty for hundreds of millions of years. Second, "rivers are fresh, so they cannot be the source". Every river carries dissolved rock. The difference between a river and the sea is not what is in the water but how long it has been collecting, and which ions the living ocean pulls back out.
When to Pay Attention
A mouthful of seawater while swimming is harmless, and a child who swallows a gulp will at worst feel sick. Drinking it on purpose is a different matter: the body needs more water to excrete the salt than the seawater supplies, so it dehydrates, and small children are the most vulnerable to a rapid rise in blood sodium. Never give seawater to a thirsty child, however clean the water looks, and treat the "drink your own urine" and "drink seawater" survival tips from films as exactly what they are.
Something to Do Together
Make your own salt: dissolve two tablespoons of salt in a cup of warm water, pour a thin layer onto a dark plate and set it on a sunny windowsill. In a day or two the water is gone and a crust of cubic crystals is left, which is a dried-up sea in miniature, the same process that built the Mediterranean's salt beds when the strait at Gibraltar closed some six million years ago. Then read the old Norwegian tale "Why the Sea Is Salt", in which a magic hand mill that grinds whatever it is told to grind sinks to the bottom of the sea still grinding salt. The story is wrong about the mill, but it has the direction right: the salt goes in and never comes out.
Sources
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- Marcet, A. "On the specific gravity, and temperature of sea waters, in different parts of the ocean, and in particular seas; with some account of their saline contents." Philosophical Transactions 109 (1819): 161β208.
- Dittmar, W. "Report on researches into the composition of ocean water, collected by HMS Challenger." Report on the Scientific Results of the Voyage of HMS Challenger, Physics and Chemistry, vol. 1 (1884).
- Holland, H.D. The Chemistry of the Atmosphere and Oceans. Wiley, 1978.
- Broecker, W.S. and Peng, T.-H. Tracers in the Sea. Eldigio Press, 1982.
- Millero, F.J., Feistel, R., Wright, D.G. and McDougall, T.J. "The composition of Standard Seawater and the definition of the Reference-Composition Salinity Scale." Deep-Sea Research I 55 (2008): 50β72.
- HsΓΌ, K.J., Ryan, W.B.F. and Cita, M.B. "Late Miocene desiccation of the Mediterranean." Nature 242 (1973): 240β244.
- Knauth, L.P. "Temperature and salinity history of the Precambrian ocean: implications for the course of microbial evolution." Palaeogeography, Palaeoclimatology, Palaeoecology 219 (2005): 53β69.
- Postberg, F. et al. "Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus." Nature 459 (2009): 1098β1101.
- Trumbo, S.K., Brown, M.E. and Hand, K.P. "Sodium chloride on the surface of Europa." Science Advances 5 (2019): eaaw7123.
- NOAA National Ocean Service. "Why is the ocean salty?" oceanservice.noaa.gov; U.S. Geological Survey Water Science School. "Why is the ocean salty?" usgs.gov.