The water in the sea is salty. The water in lakes and rivers, on the other hand, usually contains very little salt. The causes are related to the sources and sinks of sea salt. In fact, water in some lakes can be even saltier than sea water. But why is sea water salty? Let’s take a look at the reason.
Sea salt, which is dissolved in water, consists of over ninety percent of ions of sodium and chlorine. When water evaporates, these ions are deposited as solid sodium chloride. In addition, dissolved sea salt also contains magnesium, calcium, potassium, and sulfate ions. Oceanographers describe the salinity with the unit Practical Salinity Units or PSU for short, which is defined as grams of salt per kilogram of sea water. Sea water has an average salinity of 35 PSU. The water in rivers and lakes, which we call freshwater, is, of course, not sweet, but just very low in salt. It contains significantly less salt than the oceans, which is less than one PSU on average. Rivers are fed directly or indirectly by rainwater or meltwater.
Because rainwater contains small amounts of carbon dioxide, it is slightly acidic. That is why it can release mineral salts from the soil or rock when it flows over or through it. Rivers then transport the salt into the sea. This way, it is constantly replenished. In addition, two other processes that take place at the bottom of the oceans, also supply salt. Hot, saline water emerges from hydrothermal vents on the seafloor on the mid-ocean ridges. Undersea volcanic eruptions also lead to the release of salt into the water. It is because seawater releases mineral salts from the surface of lava that has already cooled down.
Since the sea is constantly replenished with salt, the water should actually be getting saltier and saltier. But that doesn’t happen. Geological surveys have shown that the oceans have had an almost constant salinity for hundreds of millions of years. It is due to the salt sinks at the bottom of the oceans where sea minerals and other substances are constantly producing new minerals. In addition, tiny marine organisms use calcium ions to form their lime shells. These processes remove the salts from the sea again.
The salt concentration also changes slightly if the amount of water in the oceans increases or decreases. During the last ice age, a huge amount of water was frozen, forming kilometers of ice sheets and the sea level dipped by up to 120 meters. But the change in water volume was small compared to the total volume of the oceans. On average, the world’s oceans are 3700 meters deep. That is why the lowering of the sea level during the ice age, hardly had any effect on the salinity. On average, the values were only one or two PSU higher than in the present.
The salt concentration in the sea varies considerably. In many places, the value deviates greatly from the average 35 PSU. Where the evaporation is high, but little fresh water from rivers or precipitation arrives, the concentration increases like in the Eastern Mediterranean, the Red Sea and the Persian Gulf, the researchers measured salinity of over 38 PSU. On the other hand, if it rains a lot or rivers dump large amounts of freshwater, and only a little water evaporates, it lowers the salinity significantly. Low concentrations between 6 and 15 PSU are measured in the Baltic Sea.
Lakes without runoff, on the other hand, can have a similarly high salinity as the oceans. Some are even much saltier. Tiny Don Juan Lake, in an arctic valley in Antarctica, is considered to be the saltiest lake on earth with a salt concentration of 400 PSU. This lake does not freeze even in the coldest of winters.