By Adelle Rodda
“Water, water, every where, nor any drop to drink.” Laments a thirsty sailor aboard a vessel lost at sea in Samuel Taylor Coleridge's The Rime of the Ancient Mariner. A fair enough complaint too, because not only does seawater taste terrible, it can kill you. Drinking just a pint of the salty brew can cause kidney failure and without functioning kidneys you die – which is why drinking seawater is a terrible idea.
So why is the sea so salty? This week’s sci non fi explores the mystery of the briny deep.
The world’s oceans – that cover around 70% of the planet’s surface and hold 98% of all the water on Earth – haven’t always been here. In the beginning, Earth was a barren, volcanic wasteland, devoid of any liquid water. Over time, as the planet cooled, water vapour and other particles, released from molten rock and volcanos on Earth’s surface, rose into the atmosphere forming clouds. These clouds eventually rained and filled the rocky caverns and valleys of Earth with liquid water, forming oceans, lakes, and rivers.
So if all the bodies of water on Earth were originally filled with non-salty rainwater, how did seawater get salty, while lakes and rivers remain fresh?
The salt in the ocean comes from rocks on land, as well as gas and solid debris ejected from volcanic eruptions and hydrothermal vents.
“Salts” are ionic (having a positive or negative charge) compounds that are formed when an acid reacts with a mineral. The Earth’s crust and surface is made up of rocks that are composed of a variety of minerals. When rainwater – which is slightly acidic – comes into contact with rock it erodes, releasing salts, which run off into streams, rivers and eventually the sea.
Lakes, rivers, and streams all contain salts too, but the amount is so negligible we can’t taste it – with the exception of the deceivingly named Dead Sea, which is actually a lake and the Great Salt Lake in Utah. Normally, bodies of “freshwater” are regularly drained and then replenished with fresh, unsalted rainwater. The ocean, on the other hand, is the end of line. All the water and salt contents of every lake, river, and stream eventually end up in the sea. The sea becomes saltier still, when the sun evaporates water from the ocean leaving the salt behind – concentrating the salinity of seawater even more.
Rivers transport about 4 billion tons of dissolved salts to the ocean each year. There are more than 70 chemical elements in seawater, but most occur in extremely small amounts. Sodium and chloride – the elements that make up table salt – are the most abundant, constituting 85% of the total amount of “salts” in the ocean and giving seawater its characteristic salty taste.
Many of the other salts undergo chemical or physical reactions that remove them from the sea. Or, they are utilised by marine organisms – such as diatoms and molluscs that use silica and calcium to make their shells. Apparently table salt is not as appetising to sea creatures as it is to us.
Salts also enter the ocean through hydrothermal vents and underwater volcanos on the sea floor. These vents spew steaming hot water along with dissolved minerals, or salts, from the Earth’s crust into the ocean.
The average saltiness of seawater – or salinity, in science speak – is 35 parts per thousand. Which equates to around 2 teaspoons of salt in one cup of water. It may not seem like that much, but the ocean is full of many cups of water. If you took all the salt out of the ocean and spread it evenly over the Earth’s surface it would form a layer 166 metres high, which is the height of a 40-storey building.
No wonder sailors got the nickname “salty dog”, which also happens to be the name of a delicious, highly drinkable cocktail that would require consuming copious pints of – in order to cause kidney failure. Phew!
Comments