Stephen Warren, study leader at the University of Washington, has been on the case of the green icebergs for more than 30 years. He first took samples from one of these green hunks of ice in 1988, near the Amery Ice Shelf, East Antarctic. Most glacial ice occurs in shades of white to brilliant blue. The bluer the ice, the older it is. Typically, compression (压缩) from accumulating layers of snow pushes air bubbles out of the ice, reducing the scattering of white light. The compressed ice absorbs most of the light except for blue, creating the blue color seen in the hearts of icebergs and glaciers.
The green ice was similarly bubble-free, and yet it looked green instead of blue. Warren and his team soon found that the green ice came not from glaciers, but from marine ice. That's the ice from the undersides of floating ice shelves.
At first, the team thought that organic material in the marine ice was causing the green color. But further research showed that the green marine ice didn't have a higher — than — usual amount of organic impurities. Now, a new study finds that a different sort of impurity may be the root cause of the green ice. Warren and his colleagues report that the marine ice at the bottom of the Amery Ice Shelf has 500 times more iron than the glacial ice above.
This iron comes from the rocks under the Antarctic Ice Sheet, which are turned into fine powder as glaciers move over them. The icebound iron oxidizes (氧化) as t contacts seawater. The resulting iron oxide particles take on a green color when light scatters through them. When icebergs break off the larger ice shelf, they carry this iron-rich ice with them. It's like taking a package to the post office. The iceberg can deliver this iron into the ocean far away, and then melt and deliver it to other living things that can use it as a nutrient.
