Bather floating in the extremely salty water of the Dead Sea, off the coast of Israel
If the oceans are fed mainly by freshwater rivers, why is their water salty? No, it’s not because they have a lot of cod in sauce…
At first glance, it may seem strange that the sea is salty, especially if we take into account that the water that flows into the ocean comes from rivers and streams, which are freshwater.
So, why is the ocean sea salty?
The answer to this question leads us to explore the complex natural processes that have shaped our planet over millions of years, explains professor Antonio Figueras Huerta, researcher at the Instituto de Investigaciones Marinas, in Spain, in an article in .
Fresh water reaching the ocean is not completely free of salts and minerals. These are dissolved in earth’s rocksa process that begins with rain, which contains small amounts of carbon dioxide from the airwhich makes it a weak acid.
When this rain falls on the rocks, the acid dissolves small particles of minerals and salts. These are transported by rivers and streamseventually reaching the ocean.
However, we don’t taste the salt in the riversstreams and lakes. This is due to the fact that fresh water, constantly renewed by rain, dilutes the amounts of dissolved salts. The volume of fresh water is much greater than the volume of dissolved minerals.
When fresh water from rivers flows into the ocean, it carries these minerals with it. But the ocean, unlike lakes and rivers, it is a cumulative system. Salt and minerals are not easily removedaccumulating over time.
Furthermore, there is another fundamental process that adds minerals to the oceans: hydrothermal vents at the bottom of the sea.
At the bottom of the sea, water seeps through fissures in the Earth’s crust, where it is heated by underground magma. Hot water (up to 400°C) acts on rocks, dissolving salts and minerals to formsimilar to sugar in a glass of milk warm.
This mineral-enriched mixture is released back into the ocean through vents, adding even more dissolved components to the seawater.
Underwater volcanoes also play an important role in salinity of the oceans. During eruptions, release a mixture of dissolved minerals and gases that enrich seawater with a variety of ions.
A perfect molecule
The sodium ion (Na) and the chloride ion (Cl) are the two most abundant ions in water of the sea: together they represent around 85% of all dissolved salts in the ocean. Their combination forms sodium chloride (NaCl), or common salt.
In addition to being the main responsible for ocean salinitythese ions help regulate essential properties of the sea, such as density and conductivity.
An interesting feature is that the relationship between sodium and chloride in seawater remains practically constant at a ratio of 1:1which makes it unique compared to other ions.
On average, the sea contains about 3.5% saltwhich means that each liter of seawater contains about 35 grams of salt. There is also some goldbut in such small quantities that its exploitation is not profitable.
On the other hand, the salinity of the sea is not uniform across the globe. It tends to be lowest near the equator and at the poles, where there is more precipitation and ice melt, respectively. what dilutes the salt.
Nonetheless, in mid-latitude areaswhere evaporation is greater, salinity tends to increase.
An extreme example is the Mediterranean Sea, which is saltier than the Atlantic Ocean due to strong evaporation and a reduced supply of fresh water.
Salt lakes, such as Mono in California or the Caspian Sea in Asia, have an even higher salinity than the ocean. They are located in arid regions where freshwater input is limited and evaporation is high. As a result, the salts become increasingly concentrated.
An extraordinary example is the Dead Sea, located between Jordan and Israel. It is a salt lake with one of the highest salinity levels in the world: around 340 grams of salt per liter, almost ten times more than the ocean.
In these locations, salt accumulation is a constant process, and over time the salt level continues to increase.
Ancient oceans, not so salty
Nos primitive oceansthe salinity was probably much lower. Billions of years ago, when the Earth was covered by large, young oceans, rain began to fall and erode the rocks of the newly formed continents. These minerals were transported to the seastarting the accumulation process.
Currently, scientists estimate that rivers and streams around the world annually transport approximately four billion tons of dissolved salts.
Despite this enormous amount, the salinity level of the oceans remained relatively constant over the last 200 million years. How is this possible?
The ocean reached a dynamic balance. The amount of salt that enters is equal to the amount that is removed.
Some of the dissolved salts precipitate and deposit as sediment at the bottom of the sea, forming new minerals. This process, along with the absorption of minerals by marine organisms, helps maintain stable salinity levels.
In other words, although rivers and hydrothermal vents continue to add salts to the ocean, a similar amount of these salts is removed through precipitation and other natural processes.
Next time you dive into the ocean and feel the salty taste on your lips, remember that you are experiencing the result of billions of yearss of geological and chemical processes.
The salinity of the sea is tangible proof of ongoing interaction between the atmosphere, land and ocean, a balance that has been essential to life as we know it.
