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Salinity Distribution of Seawater

Last updated date: 30th May 2024
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Salinity of Ocean Water

Salinity means the saltiness or the amount of salt which is dissolved in a water body. The water body which has salt content in it is called the saline water body.

Salinity is considered to be an important factor that determines many aspects of the water. The chemistry of natural waters and that of the biological processes present within it, is a thermodynamic state which is variable along with the temperature and the pressure that governs the physical characteristics of water – density, heat capacity, and other characteristics. 

In this discussion, we will know more about the salinity distribution, the salinity of the ocean, also we will know about the salinity content in the red sea.  

Salinity Distribution

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A discussion of salinity that means the salt content of the oceans requires an understanding of these important concepts: 

  • The present-day oceans are considered to be in a state where it receives as much salt as they lose

  • The salty oceans have been mixed with each other over such a long time period that their composition of sea salt is actually the same everywhere.  

This uniformity of salt content is the reason for the little variation of salinity over space or over the time period. The range of salinity that is being estimated is from 33 to 37 grams of salt per kg. Majorly, the observed departure of this mean value of approximately 35 PSU, is caused by the processes at the earth’s surface which locally add or remove the freshwater content in it. The regions which have high evaporation are of elevated surface salinities, while those regions where there is higher precipitation have depressed surface salinities limit within themselves. Near the shore the regions that are close to large freshwater sources, there the salinity may be lowered by the process of dilution. This is very true in those areas where the region of the ocean which receives the freshwater is isolated from the open ocean by the geographical land.

The Salinity of the Ocean 

Salt in the ocean originates two sources:

  • runoff from the land 

  • openings in the seafloor.

The rocks on the land surface are the major source of salts that are dissolved in seawater. The rainwater which falls on the land is a little acidic in concentration, so it erodes the rocks. This erosion of rocks releases the ions which are being carried away to the streams and to the rivers which eventually flow into the ocean. These dissolved ions are used by the organisms living in the ocean and thus they are removed from the water. While, others are not removed, so their concentrations increase over the time period.

There is another source of salts present in the ocean called hydrothermal fluids. This salt comes from the vents on the ocean floor. The ocean water seeps into these cracks in the seafloor and thereby it is heated by magma from the Earth’s core. This heat causes a series of chemical reactions. The water here tends to lose the oxygen, magnesium, and sulfate concentration and it picks up metals like iron, zinc, and copper from the surrounding rocks. The heated water is then released through the vents in the seafloor, thereby carrying the metals with it. The ocean salts which come from underwater volcanic eruptions, directly release the minerals into the ocean water.

Prevalent most ions present in the seawater - chloride, and sodium. Together these ions make up approximately 85 percent of all the dissolved ions in the ocean. While magnesium and sulfate make up for another 10 percent of the total. Other ions are found in very minute concentrations.

Red Sea Salinity 

The Red Sea is actually a sea-water inlet of the Indian Ocean which lies between the continents of Africa and Asia. 

Talking about this sea salinity, the Red Sea is known to be one of the saltiest bodies of water in the world, this owes to higher evaporation and lower precipitation rate, no significant rivers or streams drain into this sea. The southern connection to the Gulf of Aden is an arm of the Indian Ocean, which is quite narrow.

Vertical Distribution of Salinity 

  1. The saltiness of ocean water changes with depth, but the way the salinity changes rely on the position of the sea.

  2. Salinity which is present at the surface of the sea gets decreased by the input of fresh waters or it gets increased by the loss of water, as it forms ice, or by the process of evaporation.

  3. Salinity present at the depth is fixed as neither water nor as salt which can be added to it.

  4. There is a major difference in the salinity occurring between the surface zones and between the deep zones of the oceans.

  5. The lower saline water saturates quite above the higher saline dense water.

  6. The salinity usually increases with the depth and there is a distinct zone known as halocline, where the salinity increases too abruptly.

  7. The increase in salinity of seawater causes an increase in the density of the water.

  8. High salinity seawater, which usually, sinks below the lower salinity water leads to stratification by the amount of salinity.

What is the Salinity of Seawater?

The seawater salinity is about 35 parts per thousand. 35 parts per thousand are the average salinity which can be stated in another way, that is about 3.5 percent of the weight of the seawater comes from the dissolved salts. 

FAQs on Salinity Distribution of Seawater

1. What do You mean by Dilution?

Ans. Dilution is the process whereby it decreases the concentration of a solute present in a solution. By mixing another solvent like adding more water to this solution. In order to dilute a solution, this means adding more solvent without adding more solute content in it.

The reaction of acid or base with the water leads to the decrease in the concentration of ions (H3O+/OH) measured per unit volume. This is known as the process of dilution. Here, the acid needs to be added slowly to the water with constant stirring the content.

2. Why Can't We Drink Seawater?

Ans. Seawater is really toxic for human beings. Our body is unable to get rid of the salt which comes from seawater, hence later it becomes harmful for the body. Normally, the human body gets rid of the excess salt through the kidneys in the form of urine, but this also needs fresh water to dilute the salt content in the human body for the kidneys to function properly.

3. How Salinity Affects Ocean Water?

Ans. Salinity can majorly affect the density of ocean water. The Water that has more saline concentration is denser and heavier, this will sink underneath the less saline and warmer water. This also affects the movement of the ocean currents. The salinity of the water also affects marine life, which is regulated by the intake of salt water.