At first glance, it might seem strange, especially when we consider that the water flowing into the ocean It comes from rivers and streams, which are freshwater. The answer to this question leads us to explore the complex natural processes that have shaped our planet for millions of years.
The fresh water that reaches the ocean is not completely free of salts and minerals. These dissolve from rocks on land, a process that begins with rain, which contains small amounts of carbon dioxide from the air, turning it into a weak acid.
When this rain falls on rocks, the acid dissolves small particles of minerals and salts. These are transported by rivers and streams, finally taking them to the ocean.
However, we do not notice a salty taste in rivers, streams and lakes. This is because fresh water, constantly renewed by rain, dilutes the amounts of dissolved salts. The volume of fresh water is much greater than that 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, is a cumulative system.
Salt and minerals are not easily removed, they have accumulated over time. Additionally, there is another fundamental process that adds minerals to the ocean: hydrothermal vents on the seabed.
Deep down, water seeps through cracks in the Earth’s crust, where it is heated by underground magma. Hot water (it can reach 400ºC) acts on the rocks, dissolving salts and minerals in a similar way to sugar in a glass of hot milk.
This mineral-enriched mixture is released back into the ocean through the vents, adding even more dissolved components to the seawater.
Underwater volcanoes also play an important role in ocean salinity. During eruptions, they release a mixture of dissolved minerals and gases that enrich seawater with a variety of ions.
He sodium ion (Na) and chloride ion (Cl) are the two most abundant ions in sea water: together they represent approximately 85% of all dissolved salts in the ocean. Their combination forms sodium chloride (NaCl), or common salt.
In addition to being the main contributors to ocean salinity, these ions help regulate essential properties of the sea, such as density and conductivity.
An interesting feature is that the ratio of sodium to chloride in seawater remains approximately constant at a ratio of 1:1, making it unique compared to other ions.
On average, the sea contains about 3.5% of sal meaning that each liter of ocean water contains approximately 35 grams of salt. There is also some gold, but in such low quantities that it is not profitable to exploit.
On the other hand, sea salinity is not uniform throughout the planet. tends to be lowest near the equator and at the poles where there is more precipitation and ice melting, respectively, which dilutes the salt.
Meanwhile, in mid-latitude areas, where evaporation is greater, salinity tends to increase. An extreme exponent is the Mediterranean Sea, which is saltier than the Atlantic Ocean due to strong evaporation and the limited supply of fresh water.
salt lakes
Salt lakes, such as Mono Lake in California or the Caspian Sea in Asia, have even higher salinity than the ocean. They are located in arid regions where the supply of fresh water is limited and evaporation is high. As a result, the salts become more and more 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: about 340 grams of salt per liter, almost ten times more than the ocean.
In these places, the accumulation of salts is a constant process and, over time, the salt level continues to increase.
Ancient oceans, not so salty
In early oceans, salinity was probably much lower. Billions of years ago, when the Earth was covered by large young oceans, rains began to fall and erode the rocks of the newly formed continents. These minerals were transported to the sea, beginning the accumulation process.
Currently, scientists estimate that rivers and streams around the world transport approximately four billion tons of dissolved salts to the ocean each year.
Rivers in the United States alone contribute about 225 million tons of dissolved solids and 513 million tons of suspended sediments annually.
Despite this enormous amount, the ocean’s salinity level has remained relatively constant over the past 200 million years. How is it possible? The ocean has reached a dynamic balance. The amount of salt that enters is equal to the amount that is removed.
Part of the dissolved salts precipitate and are deposited as sediments on the seabed, forming new minerals. This process, along with the absorption of minerals by marine organisms, helps keep the salinity level stable.
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.
The 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 years of geological and chemical processes.
Sea salinity is tangible evidence of the continuous interaction between the atmosphere, land and ocean, a balance that has been essential for life as we know it.
*Antonio Figueras Huerta is a research professor at the Higher Council for Scientific Research, Institute of Marine Research (IIM-CSIC), Spain.