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Why is seawater salty?

4 may 2023

The water in our lakes and rivers is not always necessarily all that fresh. But it is definitely not as salty as the water that gets in our mouth when we go swimming on beach holidays. That being said, why is seawater salty in the first place, when other water bodies are not?



What we now buy cheaply in the supermarket once used to be treated like “white gold”: Salt. This was because mining salt or transporting it inland from the sea was an expensive business. In geological terms, however, salt goes in the opposite direction: Rainfall releases the salt locked into rocks, which is then washed by rivers into the ocean. The oceans also have their own immense sources of salt, of course: Deep down on the seabed, salt is released from hot springs or by submarine volcanic eruptions. And once seawater has evaporated, what is left behind is salt. Since the oceans have been fed with salt since time immemorial, one might think that they should be getting ever saltier. But this isn’t the case. Geological surveys show that the salt content of the oceans has remained virtually constant for hundreds of millions of years. This is due to what are known as salt sinks: Down at the bottom of the ocean, new minerals are constantly being formed out of sea salt and other dissolved substances. Mussels and other marine organisms, for instance, extract calcium from the water to form their calcareous shells or skeletons. The brightly coloured coral reefs would not be possible without marine salt.


Salt in ocean water

Three dessert spoons of salt on average

The average salt content of the oceans is 34.7 grammes per litre. This amounts to roughly three dessert spoons of salt. There are significant differences, however: In hot regions, where a lot of water evaporates and rainfall is scant, the salt content is higher. Examples include the Persian Gulf, the Red Sea and the eastern Mediterranean, which extends from Italy to Israel and Egypt via Croatia and Greece. By way of contrast, in the estuaries of major rivers, where a lot of freshwater flows into the sea, the salt concentration is low. Around the estuary of the Amazon, for instance, the salt content of the Atlantic is significantly lower for over one thousand kilometres. The salt concentration also tends to be lower in colder and wetter regions. This is because more fresh water flows into the ocean here, and little evaporation takes place, both in the ocean and in the rivers, which therefore flush less salt per litre into the sea. In the Baltic, for instance, the salt content does not exceed two percent. Between Finland and Sweden, the measurable amount is only 0.3 to 0.5 percent. This is only a little more than the typical freshwater level.

Fresh water = water with less salt

There’s more to fresh water than meets the eye – or the taste buds: Salt is also to be found in our rivers. Albeit in such low concentrations that we can’t taste it. The reason for this low salt content is that river water has a relatively short journey and is always in motion. This means that less salt is released from the rocks while, at the same time, less evaporation takes place. Concentrations of salt as high as those of the oceans are therefore excluded. The same thing applies to lakes – at least when they are in motion. But if there is no or only very little replenishment by river water, no drainage and a lot of evaporation, and if there are sources of salt, the salt content of lakes can increase. And it can do so dramatically. The Dead Sea is a good example, although it is anything other than dead, as its waters are home to a wide range of microorganisms. Nor is it a sea: it is in fact the lowest altitude lake in the world. But with a salt content of up to 33 percent, water from the Dead Sea makes any real ocean taste pretty fresh in comparison.

Saltwater champions in the desert

One of the world’s saltiest water bodies is the Don Juan lake in the Antarctic. This tiny pond, which is only ankle deep, does not freeze even at temperatures of minus 40 degrees – thanks to its salt content of over 40 percent. Only the Gaet’ale in Ethiopia is saltier. This lake is in the hottest and driest region of the globe and is fed by a thermal spring. With its water temperature of up to 55 degrees and a salt content of 43 percent, freezing is simply never going to happen here.


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