- Science: Explained
Why is the Ocean salty?
Everyone knows the Ocean is salty. But how did it get salty? Where did the salt come from? Is it getting more salty?
These are all great questions to bring up as you ask for the salt over dinner. And after this explainer, you can answer them.
Why is the Ocean salty: Explained
Imagine a bowl. Now pour some slightly salty water into the bowl and put it into the sun on a hot day. Eventually, the water will evaporate, leaving that little bit of salt behind.
Now, add some more slightly salty water to the bowl. The left-behind salt dissolves and mixes, making saltier water. Leave it in the sun again, the water will evaporate and leave salt behind. If you now attach a constant stream of slightly salty water into the bowl, you have a little model of our Ocean.
Just like in the above example, rivers (the stream) bring tiny amounts of salt into the Ocean (the bowl) and the sun evaporates the water, leaving behind the salt. These amounts have built up over huge periods of time.
Where does the salt in the Ocean come from?
Most of the salt in our Ocean comes from rocks on land. Carbon dioxide in the atmosphere dissolves into rainwater, making it slightly acidic. When it rains, this slightly acidic water can dissolve the rocks it falls on and over, in the form of ions (charged molecules).
These ions, mostly sodium and chloride, are carried into rivers, which carry them into the Ocean.
Some salt also comes from volcanic activity, where elements from the Earth’s core can be released into the Ocean through underwater volcanoes and hydrothermal vents.
Why aren’t rivers salty?
Rivers are, very slightly salty. Depending on what the water has run over on its journey (rocks, decomposing plants, your ex’s belongings etc.), the contents of each river, including salt levels, varies.
For water to be considered freshwater, its salt content must be less than 0.05% salt (saltiness is also commonly shown as parts per thousand or ppt: 0.05% is 0.5ppt). This means freshwater can still have a little salt in.
Between 0.05 and 3% salt content is brackish water, and saltwater is between 3 and 5% salt. Above 5% (or 50ppt) salt is brine.
What are the saltiest bodies of water in the world?
One of the saltiest bodies of water is the Dead Sea. It was cut off from the river Jordan by damming in the 1950s, so there is no significant freshwater input. This means the water is gradually disappearing, as the water level drops close to 1.21 metres (4 feet) every year.
Think back to our bowl of water example – the water evaporating leaves its salt behind, so the sea is getting more salty. Salinity is roughly 337ppt (33.7% salt) – ten times the average of the Ocean.
But it isn’t the saltiest – that title goes to the Gaet’ale Pond in Ethiopia. It is a volcanic spring, with a salinity of 433ppt. You wouldn’t want to swim there – CO2 bubbling up presents the risk of suffocation and the hot, acidic water could leave painful burns.
Does the Ocean vary in saltiness?
Yes. Where there is more freshwater entering the Ocean, it’s less salty. This can be in places that rain a lot, have lots of rivers entering, or have ice melting. On average the Ocean has a salinity of 35 ppt.
Have a look at the picture below. The red areas show high salinity, purple areas are low salinity. Try and work out why each area is the colour it is.
The Baltic Sea is very enclosed, has lots of river input and rain, and little evaporation, so the Ocean surface can be around 10ppt. The Red Sea is much higher, 40ppt. This is due to very little rain or river input, and high evaporation.
Is the Ocean getting saltier?
The Ocean is now more in balance. If we go back to our bowl example, there is a bit we didn’t tell you about. Salt can mineralise at the bottom of the bowl – solidifying into rock, leaving the water. The amount of salt that mineralises is the same as the amount entering, so the Ocean stays the same level of salty.
So, with the next salty mouthful of Ocean water you get – thank the rocks and the rivers (and the rain and the sun).
