Green icebergs have always been a sort of myth. Explorers since the 18th century have been perplexed by the oddly colored ice formations, but now a team from the University of Washington believes they have the answer. More often than not, ice appears to have a white or blue color because water absorbs the other colors of the visible spectrum. Scientists have had their 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 a lovely shade of green.
Amazingly, this environmental anomaly may actually serve a vital purpose to the marine world. It turns out that the ice-trapped iron could essentially be a delivery service to the micro-organisms living in the sea. 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, which is responsible for providing nourishment to whales, jellyfish, krill, zooplankton, and a variety of other underwater species.
"It's like taking a package to the post office," Warren says. "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."
Warren has been studying green icebergs for a while now, since 1988 to be exact, when 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, "This ice had no bubbles. It was obvious that it was not ordinary glacier ice."
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. However, that doesn't explain the greenish tint of certain icebergs.
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. That raises another question; where is the iron coming from? Warren 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.
