Original photo by Mario_Hoppmann/ iStock
When seawater freezes, it undergoes a remarkable natural desalination process — known as brine rejection — that expels much of its salt content. So when that seawater ice melts, the result is almost pure freshwater. It may seem impossible, but it all comes down to what happens at the molecular level when the salty water freezes.
As ocean temperatures drop below freezing, water molecules begin forming ice crystals with a highly organized structure. That structure cannot incorporate salt ions, so those ions are largely excluded, pushing the salt out of the ice. Initially, not all of the salt is rejected; some of it is trapped in the ice, forming pockets of concentrated brine. The brine remains in a liquid state because it requires lower temperatures to freeze. So at that stage, the sea ice still has a high salt content, but over time, the ice continues to eject the brine.
Oops, incorrect!
It's a factThe massive Antarctic ice sheet contains a vast amount of frozen freshwater. More freshwater is added to the ocean as the ice sheet melts, making the water less salty and less dense — potentially disrupting global ocean circulation and affecting climate patterns worldwide.
Thanks to this phenomenon, sea ice contains significantly less salt than the water it came from. In liquid form, ocean water has an average salinity of 35 parts per thousand, while newly formed seawater ice has a salinity of between 12 and 15 parts per thousand. As the ice grows thicker and brine rejection takes place, the salinity decreases significantly: Arctic first-year ice has an average salinity of 4 to 6 parts per thousand, and sea ice four years or older is nearly free of brine.
When seawater that has been frozen for years eventually melts, the water released is dramatically fresher than the ocean around it. In these cases, when nearly all the brine is gone, the ice can be fresh enough to provide drinking water when melted — something that’s often done during polar expeditions.
In his 1911 book, Polar Exploration, the British polar scientist and explorer William Speirs Bruce describes how whalers and exploring ships in the Arctic extracted water from pools on the ice, which was often drinkable freshwater. Today, polar expedition members still take an occasional drink from these pools.
Numbers Don't Lie
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The largest moon in the solar system is ______, and its ocean may contain more water than all of Earth’s oceans combined.
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The largest moon in the solar system is Ganymede, and its ocean may contain more water than all of Earth’s oceans combined.
The world’s largest waterfall is underwater.
When we think of the world’s mightiest waterfalls, we normally picture them cascading majestically over cliffs to a turbulent plunge pool far below. But the world’s largest waterfall is actually located in the ocean. Known as the Denmark Strait cataract, it flows beneath the Denmark Strait, which separates Iceland and Greenland. At the bottom of that strait, a series of cataracts — beginning some 2,000 feet beneath the surface — plunge to a depth of 10,000 feet, a drop of nearly two miles.
This underwater waterfall exists due to density differences between the two water masses on either side of the Denmark Strait. When the southward-flowing cold water from the Nordic Seas meets the warmer water from the Irminger Sea, the cold, dense water quickly sinks below the warmer, less dense water, and plunges over a huge drop in the ocean floor. The resulting downward flow is estimated to well exceed 123 million cubic feet per second. By comparison, the discharge of the Amazon River into the Atlantic Ocean is just 7.74 million cubic feet per second.
Tony is an English writer of nonfiction and fiction living on the edge of the Amazon jungle.
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