If you've ever swum in the ocean, you've probably tasted a mouthful of seawater. It's pretty salty. In fact, it's 220 times more salty than freshwater. Th t's a lot of salt! And scientists have wondered for a long time why the oceans contain so much salt. After all, freshwater pours into the oceans from rivers all over the world 365 days each year, and millions of gallons of water fall on the oceans as rain all the time. But that's actually part of the reason why the sea got so salty in the first place, and stays salty now.
All the water that hits the earth, either in the form of rain hitting the ground or running over river beds, causes erosion, which brings lots of minerals into the oceans, including salt. When the sun's heat causes seawater to evaporate, the salt and minerals in the oceans are left behind.
Another reason that the oceans contain a lot of salt is because of the way the earth is always changing. When oceanographers discovered superhot deep-sea vents, they learned that ocean water cycles through the earth's crust. Th e water flows through cracks in the sea floor until it hits very, very hot rock. Th e water gets superheated and dissolves minerals from the rock, then shoots back up to the surface of the seafloor through thermal vents. Th e minerals that dissolve into the water contain lots of salt.
So the combination of new salt being brought into the oceans and water being removed through evaporation means that the salt stays behind even when water leaves. In fact, over time—long, long periods of time—the oceans have become saltier.
Of the five ocean basins, the Atlantic Ocean is the saltiest. On average, there is a distinct decrease of salinity near the equator and at both poles, although for different reasons. Near the equator, the tropics receive the most rain on a consistent basis.
JUST HOW SALTY IS THE OCEAN?
You'd be surprised at how sensitive you are to salt—and just how much salt the ocean has. Fill three cups with water from your faucet. Leave one alone. Add a pinch of salt to the second one. Add a teaspoon of salt to the third one. Taste the first cup: this is freshwater, and even though there are dissolved salts and minerals in the water, there is too little for you to taste. Try the second cup: you may or may not be able to taste the salt in this, depending on your taste buds and how much you think of as a “pinch” of salt. This is called brackish water and is similar to the salt content of water in places where freshwater rivers flow into oceans. Now taste the third glass: it will taste really salty. This is about the same salt content as a glass of sea water.