Sea water is salty because it contains a significant amount of mineral salts, especially sodium chloride, resulting from the erosion of rocks and continents, as well as from inputs from underwater volcanoes.
It all starts from the earth rocks: rain falls, it gently flows and wears down these rocks along the way, carrying away minerals. These minerals contain salts—primarily sodium chloride (yep, the one we put on our fries!). They travel with the rivers calmly until they flow into the oceans. Once they reach the sea, the salts accumulate there because the water evaporates, but not them. As a result: over time, the seas become increasingly loaded with dissolved salts. Additionally, volcanic activity underwater and hydrothermal vents add their bit of salt by releasing a lot of minerals directly into the oceans. That's why seawater is so salty today, and has been for millions of years!
Oceans become more or less salty due to certain natural processes that are easy to grasp. For example, when it is hot, evaporation removes freshwater but leaves the salt behind, boosting salinity. Conversely, when it rains heavily or rivers discharge abundantly into the sea, they bring freshwater and dilute the salts, thus reducing salinity concentration. In cold regions, when water freezes, it loses a good part of its salt, leaving it in the surrounding water and consequently increasing local salinity. Ocean currents can also carry salty water around, creating more or less salty areas across the globe. Finally, geological activity such as underwater volcanoes can release salt directly into the water, thus locally influencing the amount of salt found there.
Salinity is a bit like underwater weather: some marine organisms thrive when it gets salty, while others do not! The concentration of salt largely determines the species capable of living there, influencing their ability to maintain their internal balance of water and minerals (their osmoregulation, so to speak). For example, species like clownfish or corals are quite demanding: a small variation in salinity can stress their organism and even weaken them in the long run. In contrast, other creatures are more comfortable with these fluctuations, allowing them to occupy specific ecological niches, such as estuaries, where salinity is often unstable. Overall, the more abruptly salinity changes, the more it disrupts the entire marine ecosystem, as food balances and trophic chains rely on this relative stability. A true balancing act in the marine world!
Depending on where you are sailing at sea, salinity can change quite a bit, and there’s an explanation for that. Warm places with a lot of evaporation, like the Red Sea, are very salty. In contrast, regions close to the equator experience heavy rain that further dilutes the water, reducing salinity. Near the poles, with the melting of glaciers and snow, it’s the same; the water becomes less salty because there is more freshwater input. Estuaries, where rivers meet the ocean, are also fresher, precisely due to the massive influx of freshwater from the rivers. Ocean currents also play an important role as this great natural mix transports salty water from one place to another, regularly altering salinity.
If all the water from the world's oceans were to completely evaporate, the salts it contains could cover the continents with a layer of salt approximately 150 meters thick!
The Dead Sea, located in the Middle East, has a salinity almost ten times higher than that of the oceans, allowing swimmers to float very easily on the water.
On average, each liter of seawater contains about 35 grams of dissolved salts, primarily sodium chloride (table salt commonly used in cooking).
Salinity directly influences the density of seawater, which initiates ocean currents such as the Gulf Stream, having significant climatic effects on Europe and North America.
The differences in salinity influence the density of ocean waters. Highly saline water is denser and tends to sink to the depths, pushed by less saline water at the surface. This difference generates movements of water masses and contributes to the formation of major ocean currents.
The difference in salinity is mainly explained by evaporation and the input of freshwater. Regions with high evaporation and little influx of freshwater, such as the Mediterranean Sea or the Red Sea, have saltier waters, unlike regions where major rivers supply freshwater.
The melting of glaciers adds large amounts of freshwater to the ocean, which locally dilutes the concentration of salts and thus temporarily decreases salinity in these areas. This phenomenon has consequences for ocean currents and the global climate.
Yes, some animals known as extreme halophiles, such as the shrimp Artemia salina or certain microorganisms, tolerate or even prefer to live in environments with very high salinity levels.
No, drinking seawater worsens dehydration due to its high salt concentration. The body will try to eliminate this excess salt, leading to further loss of water. This physiological reaction can quickly become dangerous.
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