How is the salinity of the oceans measured

Why is sea water salty?

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 lakes can be even more salty than sea water.

Sea salt dissolved in water consists of over ninety percent ions of sodium and chlorine. As soon as the water evaporates on the beach, these ions are deposited as a solid crust made of sodium chloride - i.e. table salt. In addition, the dissolved sea salt also contains magnesium, calcium, potassium and sulfate ions. Oceanographers describe the salinity with the unit “Practical Salinity Units”, PSU for short, which is defined as grams of salt per kilogram of seawater. Seawater has an average salinity of 35 PSU. The water in rivers and lakes, which we call fresh water, is of course not sweet, but simply very low in salt. It contains significantly less salt than the oceans, namely less than one PSU on average. Rivers are fed directly or indirectly by rainwater or meltwater. But the salt does not come from the clouds.

Salinity on the sea surface

Because rainwater contains small amounts of carbon dioxide, it is slightly acidic. This is why it can dissolve mineral salts from the soil or rock when it flows over or through it. Rivers then transport the salt into the sea. In this way, it receives constant supplies. In addition, two other processes that take place in the ocean itself replenish salt: hot, salty water escapes from hydrothermal springs on the sea floor - for example on mid-ocean ridges. Undersea volcanic eruptions also lead to the release of salt into the water. Because sea water dissolves mineral salts from the surface of already leaked, cooling lava.

Since the sea is constantly being supplied with salt, the water should actually become increasingly salty. But that doesn't happen. It is known from geological studies that the oceans have had an almost constant salinity for hundreds of millions of years. The budget is balanced. This is due to the salt sinks: At the bottom of the oceans, new minerals are constantly being created from sea salt and other substances. In addition, tiny marine organisms use the calcium ions to form their calcareous shells. These processes remove the salts from the sea.

Of course, the salt concentration can also change if the amount of water in the oceans increases or decreases. During the last ice age, large amounts of water were stored on the continents as ice sheets that were kilometers thick. The sea level fell by up to 120 meters. But the associated 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 had hardly any effect on the salinity: On average, the values ​​were only one to two PSU higher than at present.

Salt production in Lanzarote

In spatial terms, however, the salt concentration in the sea varies considerably - in many places the value deviates significantly from the average 35 PSU. Where there is high evaporation but little freshwater from rivers or precipitation, the concentration is increased: In the eastern Mediterranean, the Red Sea and the Persian Gulf researchers measure a salinity of over 38 PSU. On the other hand, if it rains a lot or rivers deliver large amounts of fresh water while only a little evaporates, this lowers the salinity. For example, significantly lower concentrations between 6 and 15 PSU are measured in the Baltic Sea.

Lakes without a runoff, on the other hand, can have a salt content similar to that of the oceans. Some are even more salty. The Dead Sea and the Great Salt Lake in the US state of Utah contain ten times as much salt as the oceans. Tiny Don Juan Lake in a dry valley in the Antarctic is considered to be the saltyest lake on earth: a salt concentration of 400 PSU was measured there. This lake does not freeze over even in the dead of winter.