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The Curious Reason Some Icebergs Are Green

Popular Mechanics logo Popular Mechanics 3/7/2019 David Grossman
a large body of water with a mountain in the background: An ocean oddity might actually be essential. © AGU/Journal of Geophysical Research: Oceans/Kipfstuhl et al 1992. An ocean oddity might actually be essential.

Green icebergs are a part of Antarctica's mythology. Since the 18th century, explorers have noted strange verdant outliers at the bottom of the world. The mystery of green ice has been with science for more than 200 years, but now a team the University of Washington believes they have solved the riddle.

Ice usually appears white or blue because water absorbs the other colors of the visible spectrum. Scientists have had their pet theories about green ice, but the study from glaciologist Stephen Warren offers a new theory. Iron oxides, the same compounds that create brown and red rust, are turning icebergs emerald.

If the theory holds, it might do more than solve a riddle. The ice-trapped iron could represent a crucial missing link in the food chain. The icebergs would get the iron during their formation, found in Antarctica's rock dust. Then the icebergs carry that iron dust out to the ocean, where they could feed phytoplankton, the microscopic organisms that provide nourishment for types of whales, jellyfish, krill, zooplankton, and a variety of other underwater species.

“It’s like taking a package to the post office," Warren says in a press statement. "The iceberg can deliver this iron out into the ocean far away, and then melt and deliver it to the phytoplankton that can use it as a nutrient. We always thought green icebergs were just an exotic curiosity, but now we think they may actually be important.”

Researchers studying a green iceberg in 1996. © Collin Roesler Researchers studying a green iceberg in 1996.

For Warren, the journey towards an understanding of green ice began in 1988. Back then, he took a core sample from a green iceberg near the Amery Ice Shelf, near the coast of East Antarctica.

“When we climbed up on that iceberg, the most amazing thing was actually not the color but rather the clarity,” Warren says, remembering the ice. “This ice had no bubbles. It was obvious that it was not ordinary glacier ice.”

No bubbles makes for an odd glacier. Common glaciers form over long periods of time as layers of snow build up and solidify, almost always leaving reflective air pockets. But some Antarctic glaciers are different: they have a layer of ocean water frozen to the underside of an ice shelf. This layer is known as marine ice. Marine ice, lacking in air pockets, doesn't reflect light and has a darker hue.

That still doesn't account for the green. At first, Warren thought it might be dissolved organic carbon, which is yellow. Yellow and blue can form to make green, after all. But tests showed the same levels of organic carbon in the green marine ice as any other type of glacier.

In 2016, however, a study led by Laura Herraiz Borreguero of the University of Southampton offered a breakthrough. Borreguero's examination of marine ice showed that it held 500 times more iron than typical glaciers.

That discovery led Warren back with a new problem: Where was the iron coming from? He now suspects it to be glacial flour, sediment finer than sand capable of being carried by water. Glacial flour is frozen in ice around the world, like the Icy Bay at Wrangell-St. Elias National Park in Alaska. But the iron from Antarctica could be the special sauce that makes the glacial flour glimmer green.

Source: AGU

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