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“The sun’s energy causes pure water (H2O) to evaporate from the sea. After forming clouds in the atmosphere, it falls as rain – partly over land – and flows back into the sea, where it evaporates again as pure water.
A global water cycle
This process – described here in greatly simplified terms – is known as the “global water cycle”. Once the water has rained off over land, it comes into contact with solids such as rock. The rock is dissolved by the water – very slowly but steadily. It can take more than a million years for one grain of sand to dissolve in water. The “dissolved” rock elements are transported to the sea, by rivers for example, in the form of solutes (known in layman’s terms as “salts”). Roughly the same amount of water flows into the sea as evaporates. The incoming water carries salts with it that are left behind when the water evaporates. As a result, the solutes accumulate in the sea: some are used and removed from the water again, like calcium, which among other things goes into building coral reefs. The concentration of the unused solutes increases, on the other hand – such substances include sodium and chloride, which together make up cooking salt.
Salt is in the sea for many reasons
Apart from being transported by rivers, there are other ways in which solutes find their way into the sea, including via the famous “black smokers”, which are thermal springs on the seabed. My working group at the Leibniz Center for Tropical Marine Ecology (ZMT) is researching the effects of submarine groundwater discharge. This is groundwater which flows directly into the sea. It can also transport large quantities of the salts into the sea and have consequences for coastal ecosystems.
What role is played by salt in the sea?
Why is salt so important in the sea? It has always been there, and all life in the sea has adapted itself to the salt. As such, the salt content of the sea has had a key bearing on evolution. Furthermore, the salt affects our climate: the Gulf Stream only functions because of the higher density of the saline sea water. If salts had not accumulated in former oceans in the Earth’s history, we would also not have the salt deposits from which we derive our cooking salt and which we use for underground storage. Without salt in the oceans, our world would certainly have evolved in a very different way.”