How salty is seawater?
Seawater is typically about 3.5 percent dissolved salts by weight, which is roughly 35 grams of dissolved material in each kilogram of water. Oceanographers describe this as a salinity of about 35 parts per thousand. The figure varies from place to place but usually stays within a fairly narrow range in the open ocean.
Despite the name, “sea salt” is not only table salt. The most abundant dissolved ions are chloride and sodium, which together make up most of the dissolved material and are the components of ordinary salt, sodium chloride. The rest includes sulfate, magnesium, calcium and potassium, along with small amounts of many other elements.
Where the salt comes from
The ocean’s salts have several sources, and scientists describe their relative importance in broad terms.
Weathering of rocks on land
Rain absorbs some carbon dioxide from the air and becomes slightly acidic, as measured on the pH scale. As it falls on land and runs through soil, it slowly dissolves minerals from rocks. These chemical reactions release ions such as calcium, sodium, potassium and bicarbonate into the water. Rivers carry the dissolved ions to the sea. Rivers taste fresh because their concentration is very low, but there are enormous volumes of them delivering dissolved material for a very long time.
Vents and volcanic activity
On the seafloor, seawater seeps into cracks in the crust near spreading ridges, is heated by hot rock and reacts with it. It comes back out through hydrothermal vents carrying dissolved minerals, while also losing some ions, such as magnesium, to the rock. Volcanic gases, including chlorine compounds, and the products of volcanic eruptions on land and under the sea also add material. The chloride in the ocean is thought to have come largely from such volcanic and crustal sources over geologic time, rather than directly from river water.
Why the salt stays
When ocean water evaporates, only the water leaves. The salts are left behind. Evaporated water forms clouds, falls as rain or snow, and returns through rivers in the water cycle, but the dissolved salts mostly stay in the ocean. Over time, the sea has become salty in much the same way that a pot of water becomes saltier when you boil it for long while adding a little more salt now and then.
The ocean is not simply getting saltier without limit, though. For a long period, its overall salinity appears to have been close to stable, because salts are also removed. Some ions are taken up into sediments or incorporated into the shells of marine organisms. Some react with seafloor rock. In shallow, dry basins, evaporation can leave thick layers of salt deposits, called evaporites, which become part of the rock record and are another route in the rock cycle. The balance between additions and removals keeps the average concentration steady over very long timescales, though estimates of the exact budget are uncertain.
Why salinity varies
Local salinity depends on how much water is added or removed:
- Evaporation raises salinity. Seas in hot, dry regions, such as the Red Sea, tend to be saltier than average.
- Rain, rivers and melting ice lower it. The Baltic Sea, which receives a lot of river water and has limited exchange with the open ocean, is much less salty than typical seawater. Regions near large river mouths show a similar effect.
- Freezing sea ice excludes much of its salt. The salt is pushed into the surrounding water, making it saltier and denser.
Salinity matters because, together with temperature, it controls the density of seawater. Differences in density help drive deep ocean currents, which move heat around the planet.
Lakes: salty, fresh and in between
Whether a lake is salty depends mostly on whether it has an outlet. A lake that has rivers flowing out, such as many in rainy regions, flushes dissolved material onward and stays fresh. A lake with no outlet, called a terminal or endorheic lake, loses water only by evaporation, so salts keep accumulating. Over time, such lakes can become much saltier than the ocean. The Dead Sea and Great Salt Lake in the United States are well-known examples, with the Dead Sea several times saltier than the ocean.
Other lakes are only slightly salty or mineral-rich, depending on the local rock and the climate.
Effects of salt on water
Dissolved salt makes water denser, so objects float more easily in seawater than in fresh water, an everyday example of the upward buoyant force. In very salty lakes, this effect is striking, which is why swimmers there float high in the water. Salt also lowers the freezing point, so seawater freezes at about minus 2 degrees Celsius instead of 0.
Most animals and plants that live in fresh water cannot cope with sea salt, and marine organisms have their own ways of managing it. For humans, seawater is not safe to drink in quantity, because the body needs more fresh water to excrete the extra salt than the seawater provides.