Why are our oceans salty?

What is salinity?

Ocean salinity, defined as the salt concentration in the seawater, is measured in unit of PSU (practical salinity unit), which is a unit based on the properties of seawater conductivity. It is equivalent to per thousand or (o/00) or to g/kg.

The commonest way to record salinity is to measure the amount of salt in 1000g of water, so it is referred to as ‘parts per thousand’ or ppt. Most of the ocean has a salinity of between 34ppt and 36ppt. (www.goo.gl/4AvV6G)


Global salinity patterns are linked to rainfall and evaporation. Salt in the ocean comes from rocks on land. The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid (carbon dioxide and water). (www.goo.gl/vwa6NB)

As the rain erodes the rock, acids in the rainwater break down the rock. Thus, ions or electrically charged atomic particles are produced. These ions are carried away by the runoff to streams and rivers and, ultimately, to the ocean.

Two of the most prevalent ions in seawater are chloride and sodium. Together, they make up over 90 per cent of all dissolved ions in the ocean. Sodium and chloride are ‘salty.’ Many of the dissolved ions are used by organisms in the ocean and are removed from the water. Others are not used up and are left for long periods of time where their concentrations increase over time. (www.goo.gl/vbKbKL)

Why some places have higher salinity than others?

During evaporation, water is evaporated and salt is left behind, making the concentration strong. Isolated bodies of water produce higher salinity levels as more water evaporates out as a percentage of the solution than larger bodies.

Therefore, a more isolated sea like the Red Sea or Dead Sea has a higher salinity level.

In fact, geologists have been able to confirm theories that suggested that the Mediterranean Sea was once completely cut off from the Atlantic. The salinity of this Sea is unusually high, given the amount of mixing occurring between the Mediterranean and the Atlantic through the Strait of Gibraltar.

Most seawater has about 35 g (7 teaspoons) of salt in every 1000 g (about a litre) of water. In fact it would take three 6 million shipping containers full of salt to make an Olympic-size swimming pool as salty as the sea. (www.goo.gl/4AvV6G)


Places of higher salinity
There are parts of the ocean where there is little rain. In addition, warm dry winds cause lots of evaporation. This evaporation removes water. When water vapour rises into the atmosphere, it leaves the salt behind, so the salinity of the seawater increases. This causes the seawater to become denser. North and south Atlantic have high salinity because these are areas experience strong winds and not much rain.

The Mediterranean Sea in Europe has very high salinity (38 ppt or more). It is almost closed from the main ocean, and there is more evaporation than there is rain or extra freshwater added from rivers.

Regions of high salinity seawater are centred around 30o north and 30o south of the equator because of excess evaporation in these regions.

The most saline water body on earth is Don Juan Pond, Antarctica with 40 per cent of its water composed of salt. Also known as the Salt Sea or hypersaline lake, the Dead Sea has 33.7 per cent salinity. (www.goo.gl/OgvUNr)

Places of lower salinity
Some parts of the ocean experience a lot of rain. The freshwater added at the surface dilutes the seawater, reduces the salinity and therefore makes the seawater less dense. Seawater can also be less saline near land, where rivers add freshwater.

The ocean around Antarctica has a low salinity of just below 34ppt, and around the Arctic it is down to 30ppt. Icebergs that have broken off ice sheets formed over land do not contain salt, and the freezing of seawater into ice floes removes more salt. Thus, icebergs add freshwater.

The Baltic Sea, almost enclosed by northern Europe and Scandinavia, has a very low salinity of about 10 ppt. This is mainly due to the huge amount of freshwater added from hundreds of rivers.

Low salinity water (10o-15o north) lies in the polar and subpolar regions and near the equator. (www.goo.gl/4AvV6G)

The difference between 34ppt and 36ppt salinity doesn’t sound very much, but it is enough to cause a difference in density. Even slightly denser seawater sinks below less dense water.

However, the effect is greater if the salty water gets cold, as temperature has a greater effect on density than salinity does. A combination of high salinity and low temperature makes seawater so dense that it sinks to the bottom of the ocean and flows across ocean basins as deep, slow currents. (www.goo.gl/4AvV6G)

Measuring Salinity
There are two main methods of determining the salt content of water: Total Dissolved Salts (or Solids) and Electrical Conductivity.

Total dissolved salts (TDS) is measured by evaporating a known volume of water to dryness, then weighing the solid residue remaining. Electrical conductivity (EC) is measured by passing an electric current between two metal plates (electrodes) in the water sample and measuring how readily current flows (ie conducted) between the plates. The more dissolved salt in the water, the stronger the current flow and the higher the EC. Measurements of EC can be used to give an estimate of TDS.

Measurement of TDS is tedious and cannot be carried out in the field. EC measurement is much quicker and simpler and is very useful for field measurement. There are however a few simple precautions to note in doing so and these are outlined here.

In future years, one of the main goals is to fine-tune the readings and retrieve data closer to the coasts and the poles. Land and ice emit very bright microwave emissions that swamp the signal read by the satellite. At the poles, there’s the added complication that cold polar waters require very large changes in their salt concentration to modify their microwave signal. Another factor that affects salinity readings is intense rainfall. Heavy rain can affect salinity readings. (www.goo.gl/w4qcLS)

Coral Reef Survival
Globally, corals occur in a salinity range between 32 and 40 PSU (practical salinity units), although coral reefs also thrive in high and low salinity conditions.

More important than average salinity concentrations, however, are sudden decreases in salinity due to high freshwater input. Decreased salinity is seen as a second factor for the disappearance of massive coral reefs in front of large river mouths. Generally, corals prefer water of normal salinity (3.5 per cent) with an annual maximum temperature above 22° C but below 28° C.

Winds and currents are also important in shaping individual reefs and in determining the orientation, shape, and position of the coral sand cays, or “low islands,” that develop on reefs. (www.goo.gl/YkpNcQ)

Indian Ocean and Salinity
The sea surface salinity (SSS) in the Bay of Bengal is a major signature of the monsoon in India. The freshwater fluxes come from the delta of the Ganges and the Brahmaputra. In May the salinity in the Indian Ocean is at the highest level. The maximum debit is in October, two months after the peak Indian monsoon. The values range from 28 to 38 psu.



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