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Why is sea water salty?

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Why is sea water salty?

Why is sea water salty?

Many people wonder why sea water is actually salty, while the water from streams, rivers and lakes usually does not taste like salt. A classic everyday question with a simple answer that consists of two parts.

From a chemical point of view, there is salt water in all streams, rivers and lakes. Only in most cases is the salt concentration much lower than in the oceans, which means that you can often not taste the salt in smaller bodies of water.

But why is the water of the oceans so much saltier than the water of most other bodies of water, which in almost all cases flow into the sea at some point? Sea water is salty for two reasons:

  1. Since the formation of the primeval ocean, salt has been loosened from rocks on the sea floor.
  2. Streams and rivers carry salts into the sea. If sea water evaporates, the salt remains.

Oceanographers describe the salinity with the unit Practical Salinity Units , PSU for short , which defines the salt in grams per kilogram of seawater. The average salt content of the oceans today is 34.7 grams of salt per kilogram of seawater (3.47 percent or 35 PSU rounded). To put it more figuratively, this is about three tablespoons of salt per liter of water.

However, this is the global average. The regional differences vary enormously! The salt concentration is higher in bodies of water near the equator, where a lot of water evaporates. In the oceans, where larger amounts of freshwater enter the oceans through rivers rich in water and less water evaporates, the salinity is lower.

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The Baltic Sea is a good example of this. The salinity of this body of water is between 0.3 and 2 percent (3 to 20 PSU) – up to 1 PSU is called fresh water . This low salt content is due to the fact that relatively little water evaporates in the Baltic Sea and also many water-rich, but low-salt rivers flow into the Baltic Sea.

Why only a few rivers carry salt water

In contrast to the oceans, the water in rivers only lingers for a relatively short time. So the water can only loosen a few salt ions from the subsoil on the way to the sea. Even slightly acidic rainwater, which dissolves mineral salts from the soil and then flows into streams and rivers, cannot enrich these rivers with salt so much that one can taste it.

Salt lakes with up to 440 PSU

It looks a little different with lakes. In addition to huge and beautiful freshwater lakes, there are also numerous salt lakes on earth. These lakes are usually fed by salty springs or they are mostly large lakes with no runoff in very warm regions. Here, as with the seas, there is higher salt accumulation due to evaporation.

An example of a large salt lake would be the Dead Sea with a salt content of 28 percent (280 PSU), whereby the water in this 810 square kilometer lake (as of 2020) at a depth of around 50 meters has a salt content of up to 33 percent (330 PSU ) can reach. Some lakes in the Antarctic dry valleys are even more extreme, such as Don Juan Lake, which has a salinity of 44 percent (440 PSU).

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