Salinity of Ocean Water: Sources, Factors and Distribution

Sea Water Salinity: Meaning, Average and Units

What Is Salinity of Ocean Water?

Every kilogram of seawater holds about 35 grams of dissolved salts, so salt makes up about 3.5 per cent of its weight. Salinity changes from place to place because evaporation, rain, river water, ice and currents keep adding fresh water to some parts of the sea and taking it away from others. Together with temperature, salinity decides how dense seawater is, and density decides which water sinks and how the deep ocean moves.

All natural water, rain and river water included, carries some dissolved mineral salts; seawater simply holds far more of them. The symbol ‰ counts parts out of a thousand in the way per cent counts out of a hundred, so a sample with 35 grams of salt in a kilogram holds 965 grams of pure water.

A few numbers fix the idea of "normal" sea water:

• Open ocean range: The salinity of the normal open ocean lies between 33‰ and 37‰.
• Freezing point: Salt lowers the freezing point, so seawater of typical salinity freezes at about −2 °C rather than at 0 °C.
• Density: The average density of surface seawater is about 1,025 kg per cubic metre. Seawater is heavier than fresh water because the dissolved salts add more mass than volume.
• Isohaline: A line on a map joining places of equal salinity is an isohaline, in the same way that an isotherm joins places of equal temperature.

Salinity is not only a chemical fact. With temperature and pressure it governs the density and heat capacity of seawater, and these in turn decide how ocean currents move and how the ocean exchanges heat with the atmosphere. That is why salinity, temperature and density are described as interrelated: A change in one changes the others.

Classification of Water by Salinity: Fresh, Brackish, Marine and Brine

Water bodies are grouped by how salty they are. The grouping used by marine scientists comes from the Venice system of 1959: It places brackish water between 0.5‰ and 29‰, marine water in a narrow band above it, and salt lakes far above both. The NCERT textbook draws the upper limit of brackish water lower, at 24.7‰.

The Venice bands classify water; they do not describe any one ocean. The open ocean, at 33‰ to 37‰, spans the upper part of the euhaline band and the lower part of the metahaline band, while the Mediterranean lies wholly in the metahaline band and the Red Sea reaches just above it, at 41‰ in the north.

Seas whose salinity barely changes through the year are called homoiohaline. Others swing so much with the seasons that living things there must cope with the change; these are poikilohaline, and their salinity can range from 0.5 to more than 300‰. Water so salty that salts crystallise out of it, or are about to, is called brine.

Measuring Ocean Salinity: Chlorinity, Practical Salinity and Satellites

Salinity is simple to define but hard to measure precisely, so oceanographers have used three methods in turn. Each method gives almost the same number for ordinary seawater, which is why textbooks can quote "35" for all of them.

1. Chlorinity by titration (before the 1980s): Seawater was titrated with silver nitrate to find its chlorinity, the concentration of halide ions, mainly chloride and bromide. Chlorinity multiplied by a factor gave the total salinity in parts per thousand.
2. Practical Salinity Scale 1978 (PSS‑78): Salinity was worked out from the electrical conductivity of seawater. Practical salinity has no units; adding "PSU", practical salinity unit, after the value is formally incorrect and strongly discouraged.
3. TEOS‑10 (2010): The Thermodynamic Equation of Seawater 2010 introduced absolute salinity, a mass fraction in grams per kilogram, found by combining conductivity with information on regional differences in seawater composition, or by measuring density directly.

One sample shows how close the three are: Seawater with a chlorinity of 19.37‰ has a salinity of 35.00‰ by the old method, a practical salinity of about 35.0 and an absolute salinity of about 35.2 g/kg.

Satellites now map the salinity of the sea surface from space. The European Space Agency's SMOS satellite was launched on 2 November 2009. NASA's Aquarius sensor, carried on Argentina's SAC‑D satellite, was launched on 10 June 2011 to make the first space-based observations of variations in sea surface salinity; a power supply failure on 7 June 2015 ended the mission. Before satellites, salinity came only from ships and buoys, which could not cover the whole ocean.

Composition of Sea Water and the Principle of Constant Proportions

Major Ions in Sea Water: Chloride, Sodium, Sulphate and Magnesium

Seawater contains more dissolved ions than any kind of fresh water, but only a handful matter in quantity. Two of them, chloride and sodium, make up around 85 per cent of all the dissolved ions in the ocean; together they are common salt. Magnesium and sulphate make up about another 11 per cent, and every other ion is present in very small amounts.

Principle of Constant Proportions: Why Chloride Measures Salinity

The total amount of salt changes from sea to sea, but the major ions keep a fixed ratio to one another wherever seawater is sampled. In 1884 William Dittmar analysed 77 seawater samples from around the world, brought back by the Challenger expedition of 1872 to 1876, and found the major ions in a fixed ratio. This is the principle of constant proportions, and it is why a single ion could stand for the whole: Measure the chloride, multiply by a fixed factor, and the total salinity follows. That is how chlorinity was turned into salinity until conductivity measurements took over. Dittmar found one exception, calcium, which was slightly more concentrated in the deep ocean.

River water is not simply weak seawater. Bicarbonate makes up 48 per cent of the dissolved matter in river water but only about 0.4 per cent of the salts in seawater: 0.14 grams in a litre that holds about 34.5 grams of dissolved salt. The difference comes from how long each ion stays in the ocean. Sodium and chloride have very long residence times, so they keep accumulating, while calcium and bicarbonate, which marine organisms combine into the calcium carbonate of their shells and skeletons, drop out of the water much more quickly.

Sources of Ocean Salinity: Where Sea Salt Comes From

Main Source of Salinity of the Sea: Weathering of Rocks on Land

The main source of the salinity of the sea is the weathering of rocks on land, with rivers carrying the dissolved salts to the ocean. The process runs in four steps:

1. Rain attacks the rock: Rainwater is slightly acidic, so it erodes rocks and releases ions from them.
2. Rivers carry the ions: The released ions are carried away to streams and rivers that eventually feed into the ocean.
3. Some ions are used: Many of the dissolved ions are taken up by organisms in the ocean and removed from the water.
4. The rest build up: Ions that are not removed stay in the water, so their concentrations increase over time.

The idea is old. In 1715 Edmond Halley proposed that rain washed salt and other minerals out of the ground and rivers carried them into the sea, where the salts concentrated as more arrived over time. He noted that lakes with no outlet to the ocean, such as the Dead Sea and the Caspian Sea, have high salt content, and he called the process "continental weathering". A lake with no outlet loses water only by evaporation, so whatever salt the rivers bring stays behind.

Seafloor Vents, Volcanoes and the Salt Balance of the Ocean

The second source of sea salt lies under the sea. Ocean water seeps into cracks in the seafloor and is heated by magma. The hot water tends to lose oxygen, magnesium and sulphates, picks up metals such as iron, zinc and copper from the surrounding rocks, and returns to the ocean through hydrothermal vents. Underwater volcanic eruptions release minerals directly into the ocean, and salt domes, vast deposits of salt found underground and undersea, also add to its saltiness.

Halley's river theory explains only part of sea salt, because its two main ions also came from below the sea. Sodium was leached out of the ocean floor when the ocean formed, and chloride came from outgassing of Earth's interior, released as hydrochloric acid with other gases through volcanoes and hydrothermal vents. Sodium and chloride then became the most abundant ions of sea salt, while rivers remain the main supply of the salt that reaches the sea today.

Salt keeps arriving, yet the sea is not getting steadily saltier. Ocean salinity has been stable for billions of years, most likely because the ocean removes as much salt as it receives. The main sinks are evaporite deposits left where seas dry out, salty pore water buried in seafloor sediments, and chemical reactions with the basalt of the ocean floor.

Factors Affecting Salinity of Ocean Water

Evaporation and Precipitation: The Main Control on Surface Salinity

Four factors affect the salinity of ocean water. Evaporation and precipitation act everywhere on the surface; fresh water from rivers and the freezing and thawing of ice act mainly near coasts and in polar seas; and wind and ocean currents move salty and fresh water from one area to another.

The salinity of the surface layer of the oceans depends mainly on evaporation and precipitation. Evaporation removes water but leaves the salt behind, so it raises salinity; rain adds fresh water and lowers it. The balance between the two decides whether a stretch of ocean is saltier or fresher than average.

• Where evaporation wins: Salinity is higher where evaporation exceeds precipitation, as is sometimes the case in tropical regions. Evaporation is greater than precipitation over the Mediterranean Sea, which has an average salinity of 38‰.
• Where rain wins: Salinity is lower where precipitation exceeds evaporation, as in polar and some temperate regions.
• Hot, dry extremes: Over the Red Sea, excess evaporation runs as high as 2,050 mm a year.

The result is a clear pattern with latitude. Salinity is generally low at the equator and at the poles and high in the subtropics. At the equator heavy rain dilutes the surface water; in the subtropics evaporation exceeds rain and concentrates it; towards the poles precipitation again exceeds evaporation and melting ice adds fresh water.

River Inflow and the Freezing and Melting of Sea Ice

Surface salinity is greatly influenced in coastal regions by fresh water flowing in from rivers, and in polar regions by the freezing and thawing of ice. Both act strongly but locally, which is why coastal seas and polar seas show the widest swings.

• Baltic Sea: Water enters it from approximately two hundred rivers. The highest surface salinity, about 7‰ to 9‰, is in its south-west, and it falls to around 3‰ in the Bothnian Bay in the north.
• Black Sea: Enormous inflow of river water keeps its salinity very low; its surface water leaves through the Bosporus with a salinity of about 17‰.
• Bay of Bengal: The influx of river water gives it a lower salinity than the Arabian Sea.
• Estuaries and the Arctic: Here the salinity fluctuates from 0 to 35‰ with the seasons, as river flow and ice change through the year.

Ice works in both directions. When seawater freezes, salt is rejected from the crystal lattice of the ice and forced out into the surrounding water, a process called brine rejection. The cold, salty brine drains through narrow channels in the ice and sinks in the water below, which becomes saltier and denser, while the ice itself keeps little salt: Sea ice generally ranges from 0 at its surface to 4 at its base on the practical salinity scale. When the ice melts in summer, it releases this comparatively fresh water and lowers the salinity of the surface.

Wind, Ocean Currents and Enclosed Seas

Wind and currents do not add or remove salt; they move it. Wind influences the salinity of an area by transferring water to other areas, and ocean currents carry saltier or fresher water with them. The North Sea, in spite of its location in higher latitudes, records higher salinity because the North Atlantic Drift brings in more saline water.

The shape of a sea decides how much mixing can happen. A wide-open sea exchanges water freely with the ocean and stays close to the average. A nearly enclosed sea keeps its own balance: In a hot, dry climate it grows saltier, as the Red Sea and the Mediterranean do, and in a wet basin fed by many rivers it grows fresher, as the Baltic and the Black Sea do. No significant river drains into the Red Sea, and its narrow southern link to the Gulf of Aden, an arm of the Indian Ocean, cannot flush out the salt that evaporation leaves behind.

Distribution of Ocean Salinity: Horizontal and Vertical

Horizontal Distribution of Salinity Across the Oceans

The horizontal distribution of salinity is its spread across the surface of the oceans. For the normal open ocean it ranges between 33‰ and 37‰, while the extremes appear in nearly enclosed seas: In the Red Sea it is as high as 41‰. Ocean salinity is at its maximum in the subtropics, where evaporation exceeds rain.

• Atlantic Ocean: Average salinity is around 36‰. The open-ocean maximum of 37‰ lies in the North Atlantic between 20° N and 30° N and between 20° W and 60° W, and salinity gradually decreases towards the north.
• Pacific Ocean: Its variation comes mainly from its shape and larger areal extent. Salinity decreases from 35‰ to 31‰ in the western parts of the northern hemisphere because of Arctic meltwater, and beyond 15° to 20° south it decreases to 33‰.
• Indian Ocean: The average salinity of the Indian Ocean is 35‰. The Bay of Bengal shows a low-salinity trend because of the influx of river water, while the Arabian Sea shows higher salinity because of high evaporation and a low influx of fresh water.

Salinity of Seas and Salt Lakes: Red Sea, Dead Sea, Baltic and Black Sea

Enclosed seas and lakes show the extremes, because each is dominated by one factor. Among seas joined to the ocean, the Red Sea is the most saline: Its salinity ranges from about 36‰ in the south to 41‰ in the north around the Gulf of Suez, with an average of 40‰. Lakes with no outlet go much further, because water leaves them only by evaporation.

The Dead Sea shows how salinity responds when fresh water is cut off. A measurement of 2011 gives its salinity as 342‰, about 9.6 times as salty as the ocean. It has been shrinking rapidly since the 1960s because water from the Jordan River was diverted, and its density of about 1,240 kg per cubic metre is why swimming in it is like floating. The Great Salt Lake is similar: Its salinity rises and falls with its water level. The NCERT list also places Lake Van in Turkey at 330‰, but measurements put it near 23‰, less salty than the ocean; the lake is known instead for its strongly alkaline water, rich in sodium carbonate, with a pH of 9.7 to 9.8.

Vertical Distribution of Salinity and the Halocline

The vertical distribution of salinity is the way it changes with depth, and that change depends on where the sea is. At the surface, salinity rises when water is lost to ice or evaporation and falls when fresh water arrives from rivers or rain. Deep water barely changes in salinity, because no evaporation, rain or river inflow reaches it. So there is a marked difference between the surface zones and the deep zones of the oceans.

Where the surface is freshened, the lower-salinity water rests above the higher-salinity, denser water. Salinity then generally increases with depth, and a distinct zone where it increases sharply is called the halocline. Other factors being constant, saltier seawater is denser and sinks below less salty water, so the ocean becomes layered, or stratified, by salinity.

• Subtropics: An excess of evaporation over precipitation can make surface waters saltier than deep waters, the reverse of the usual pattern.
• Arctic Ocean: A distinct low-salinity surface layer, with the halocline between 50 and 250 m deep, keeps warm, saline Atlantic water from mixing with the cold surface layer, which allows sea ice to grow.
• Baltic layering: Saltier water flowing in from the North Sea mixes only slowly with the fresh upper layer, so most of the salt water stays below 40 to 70 m.
• Black Sea layering: Denser, saltier Mediterranean water entering through the Bosporus flows in underneath the fresher surface water, creating a permanent deep layer that does not mix and has no oxygen.

Effects of Ocean Salinity: Density, Currents, Climate and Life

Density-Driven Deep Currents, Gibraltar and the Bosporus Undersea River

The most important effect of salinity is on density. Water with high salinity is denser than water with low salinity, just as cold water is denser than warm water, and denser water tends to sink while lighter water tends to rise. Differences in water density therefore affect the vertical movement of ocean currents. The density of surface seawater ranges from about 1,020 to 1,029 kg per cubic metre, depending on its temperature and salinity.

Most of the ocean's water is moved by differences in density, not by wind. Surface currents make up about 10 per cent of all the water in the ocean, in the upper 400 m; deep water currents make up the other 90 per cent and move around the ocean basins because of variations in density and gravity. Deep waters sink into the deep ocean basins at high latitudes, where the water is cold enough for its density to increase. This slow, density-driven circulation is the thermohaline circulation.

• Salty water sinks: Atlantic water entering the Mediterranean warms and grows saltier as it flows east; by the Levantine Sea it is dense enough to sink and form the Levantine Intermediate Waters.
• A river under the sea: Saline Mediterranean water, reaching the Black Sea through the Dardanelles, the Sea of Marmara and the Bosporus, flows along the bed of the Black Sea beneath the fresher water above. The channels it cuts in the Black Sea floor were mapped in 2010.
• Exchange at Gibraltar: Saltier Mediterranean water sinks below the incoming Atlantic water and flows out as a deep current, while less saline Atlantic water flows in at the surface.
• Sea ice and deep water: Brine rejected by forming sea ice sinks in the water below, and in coastal polynyas this helps form the dense water masses of the Arctic and the Antarctic.

Salinity and Climate Change, Marine Life, Salt and Minerals

Salinity reaches beyond the water itself into climate and life. Because it helps govern the density and heat capacity of seawater, it shapes the currents that carry heat around the planet and the exchange of heat between ocean and atmosphere, which links it to the monsoon and El Niño.

• Climate change: Human-caused climate change has contributed to observed changes in surface and subsurface salinity since the 1950s. Projections for the 21st century show fresh ocean regions getting fresher and salty regions getting saltier. Over the Mediterranean, falling rainfall is projected to raise salinity, and with warming the sea is likely to become more stratified by the end of the century.
• Dissolved gases: The solubility of oxygen, carbon dioxide and other gases in seawater depends on its temperature and salinity, so salinity also affects how much oxygen marine life can draw on.
• Marine life: Most fish are stenohaline, restricted to either salt or fresh water. Euryhaline species such as salmon can live across a broad range of salinities, and river estuaries serve as staging points where salmon, shad and eels adjust to the change in salinity during migration.
• Dead zones: In the Baltic Sea, denser saline water stays on the bottom, isolated from the surface and the atmosphere, which lowers its oxygen and leaves about a quarter of the seafloor as a variable dead zone.
• Salt and minerals: Sea salt is produced by evaporating seawater, and where seas dry out completely they leave evaporite deposits of gypsum and halite. The hot, saline brines found in the 1960s on the floor of the central Red Sea, at about 60 °C, carry metalliferous muds.

Sources

• NCERT: Fundamentals of Physical Geography (Class XI), Water (Oceans)
• NCERT: Fundamentals of Physical Geography (Class XI), Movements of Ocean Water
• Wikipedia: Sea salt
• NOAA National Ocean Service: Why is the ocean salty?
• IPCC: Sixth Assessment Report, Working Group I, Summary for Policymakers
• NASA Science: Aquarius mission
• Wikipedia: Salinity
• Wikipedia: Seawater
• Wikipedia: Ocean
• Wikipedia: Halocline
• Wikipedia: Brine rejection
• Wikipedia: Brackish water
• Wikipedia: Thermohaline circulation
• Wikipedia: Aquarius (SAC‑D instrument)
• Wikipedia: Soil Moisture and Ocean Salinity
• Wikipedia: Red Sea
• Wikipedia: Mediterranean Sea
• Wikipedia: Black Sea
• Wikipedia: Baltic Sea
• Wikipedia: Dead Sea
• Wikipedia: Lake Van
• Wikipedia: Great Salt Lake
• Wikipedia: Caspian Sea
• Wikipedia: List of bodies of water by salinity
• Wikipedia: Hydrothermal vent
• Wikipedia: Euryhaline
• Wikipedia: Evaporite
• Wikipedia: Phytoplankton
• UPSC: Civil Services Examination question papers