We all know that the sea is salty, but have you ever questioned why?
Why is the sea salty?
The majority of salt gets to the sea from rocks. When rain – which is slightly acidic – falls to the ground, it erodes rock. This, in turn, releases salt into the water. This ever-so-slightly salty water then flows into rivers, which eventually make their way to the sea.
Why is the ocean saltier than rivers?
The sea is a whole lot saltier than rivers and lakes – about two hundred times saltier, in fact. This is because the sea is the “bottom” of the chain for salt. While freshwater evaporates from the ocean to become rain, once salt is in the ocean, it has nowhere else to go. This means that salt levels become concentrated, resulting in our salty seas.
Interestingly, the same thing can happen in lakes. For example, the Dead Sea (which is technically a lake), does not have any ‘outflow’ of water, meaning that salt gets trapped there just as it would in the sea.
The Dead Sea is actually about ten times saltier than the oceans, with salt levels that mean hardly any organisms can survive in or on its waters.
How much salt is there in the ocean?
The oceans hold a remarkable amount of salt. In one cubic mile of sea, there are 120 million tonnes of the stuff. It’s difficult to visualise an amount so vast. Perhaps it would help to think of it as 20 million elephants-worth of salt. Or 6 million double decker buses.
Although all seas are salty, salt levels vary across regions. At the equator and at the poles, the sea is actually less salty than other areas.
At the equator, this is because of the high volume of rainfall. At the poles, it is because of the freshwater released by melting sea ice.
The Mediterranean Sea, on the other hand, is one of the saltiest seas on the planet.
Why isn’t the ocean getting saltier?
It would make sense to assume the sea is getting saltier, if salt keeps getting deposited into the ocean but cannot escape. But actually, our seas have quite stable salt levels. This is because there is a balance between the amount of salt coming into the ocean, and the amount of salt that is being deposited on the sea floor.
