Sometimes the simplest question is able to confuse a sage. While resting at the sea in summer, some people wonder why the water here is salty (with a characteristic bitter-salty taste), and in the river that flows through my city, it is fresh and good for drinking. The answer seems simple, but when you think about it more, the simplicity starts to disappear somewhere. Now in the world there are 2 main theories explaining the salinity of sea water. Let's try to consider both, but first let's touch on the concept of water salinity in general.
How is water salinity determined?
It is customary to measure the concentration of any substance in a liquid in percent (1/100 fraction) or in ppm (1/1000 fraction). To determine the salinity of water, it was decided to use ppm, and the concentration of salts in the Bay of Biscay was chosen as the starting point - there it is 35 ppm. This means that 35 grams of various salts are dissolved in a liter of Biscay water.
The boundary between fresh and salt water was determined at 1 ppm - all water where salts are dissolved less than 1 gram is fresh, more salt. It is not very easy to feel salinity in 1 ppm, but to feel the difference in the taste of the waters of different seas is very real.
The Dead Sea has the maximum salinity, where it can reach 350 ppm (a liter of water contains up to 350 grams of salt), although this is not a sea, but an endless lake. The minimum salinity is 7 ppm in the waters of the Baltic Sea. The average salinity of the World Ocean is 35 ppm.
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Conventional theory
Even while sitting at a school desk, almost everyone heard an explanation of the salinity of sea water from the mouth of a geography teacher. The teacher said that water evaporates from the surface of the oceans and seas very pure - almost distilled. There are really few different substances in it, so there are almost no salts. And falling to the surface of the earth (and almost all rainwater came from sea fumes), it is absorbed into the soil. Then underground water is collected in streams, which on the surface merge into rivers and flow into the seas. This is how the hydrological cycle or water cycle occurs in nature.
Diagram of the water cycle in nature. Image Source: United States Geological Survey (USGS) - Agency of the US Department of the Interior.
On its way to the sea, river water washes away mineral rocks and dissolves some of the salts in it. The concentration of these salts is low, so the river water remains fresh throughout its entire length. The salts accumulated along the way are dumped into the World Ocean, and there is nowhere to go from it - after all, pure water evaporates, without impurities. Considering that such a cycle lasts for billions of years, it becomes quite clear why sea water tastes salty.
This theory explains well the salinity of the World Ocean and the high concentration of salts in some closed water bodies. But sea water contains mostly common sodium chloride salt, the concentration of other compounds is much less. And the fresh water of rivers is most saturated with carbonates, that is, salts of carbonic acid. It would be logical if there were a lot of carbonates in seawater, but this is just not the case.
New theory
Not so long ago, scientists put forward a new theory that quickly gained popularity in the scientific community. There is no longer any doubt that in the days when the Earth was young, violent volcanic activity flowed on its surface. Large eruptions were the daily norm.
The photo shows the eruption of the Villarrica volcano in Araucania, Chile (2015).
A very fair amount of volcanic gases entered the atmosphere, they were saturated with chlorine, fluorine and bromine in an unbound form. These gases quickly combined with water vapor to form various acids. Then, these resulting liquids fell out with acid rains, including into the originally formed ocean, so that the water in it was always acidic at first.
The acids of the World Ocean had high chemical activity and entered into compounds with the metals of the bottom surface (sodium, potassium, magnesium). And the compounds of the acid with the metal lead to the formation of the corresponding salts. Hence, the familiar taste of sea water appeared. Over time, the planet's volcanic activity dropped. About half a billion years ago, the acids were neutralized and the ocean water got its current taste.
Which theory is correct?
Which of the above theories is true is up to the reader to decide, each of them has the right to life. It seems to the author of the article that both scientific statements are true to varying degrees. Probably, the first salts in the World Ocean were obtained as a result of continuous volcanic eruptions. And now the salinity of the seas is maintained due to their removal from the land in dissolved form by fresh river water. Salinity can also be maintained by marine life that need solutes to build their organisms.
Dead sea salt.
