Imagine that all of the water in the oceans evaporated, just leaving the salt (sodium chloride) behind. Now imagine that you gathered up all that salt and spread it equally over the Earth’s land surface (perhaps with a little help from a few million other people). How deep do you think that layer of salt would be? One estimate is 500 feet, or just over 150 meters!

Where did all this salt come from? The ocean’s “saltiness” comes from dissolved minerals (ions), so the real question is where did all these dissolved minerals come from? Part of the answer lies with rocks on land. As rain water falls on rocks, some of the minerals dissolve into the water*. The rainwater runs into rivers, and eventually into the oceans, carrying the minerals with it. Another source is from underwater volcanoes, where volcanic gases, which are especially rich in chlorine, bromine and sulphur, dissolve into seawater. A final source is from hydrothermal vents, where seawater flows through rocks in the sea bed at very high temperatures, stripping  minerals from them in the process.

However, that isn’t the end of the story as biology has played a large role in shaping which minerals dominate seawater. Life forms take up many of the biologically useful ions, such as calcium, magnesium, and iron, as fast as they enter the ocean (when they die, they sink to the seabed where these minerals will eventually form new rocks); subsequently, the concentrations of these minerals has remained fairly low. Other ions, such as sodium and chloride, are only scarcely used by organisms in the ocean – because of this their concentrations have increased over millions of years. Sodium and chlorine now make up over 90% of the ions in the ocean, and when water evaporates they form the most famous salt on Earth… sodium chloride, also known as table salt.

Is the ocean becoming more salty even today? The answer seems to be no. The rate at which sodium and chloride ions enter the ocean is now in balance with the rate at which they are removed. The average salinity of the ocean has not changed in many millions of years.

*This is why your shop-bought bottle of water often has a list of minerals on the label.

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This post is an excerpt from “Do Fish Sleep?: and 38 other ocean mysteries”, by David Aldridge. It is available to buy on kindle here for the reduced  price of $1.46 (99p) until the end of January. To read it, you do not need to own a Kindle device, just the Kindle App which is a free download for smartphones or tablets. It can also be read with“Kindle Cloud Reader” on a PC or Laptop.