Since the 1900s, maritime explorers have wondered about an unusual phenomenon in certain parts of Antarctica: emerald icebergs, tinged deep green rather than the typical white or blue. Finally, scientists are beginning to understand why these remarkable structures are so curiously hued — and the answer reveals that they’re essential components of the ocean.
Stephen Warren, Ph.D., professor emeritus of the University of Washington, first saw a green iceberg on an expedition nearly three decades ago and has worked to solve the mystery of their weird hue ever since. In a recent study published in JGR Oceans, he and his colleagues present a new theory: The color is due to green iron oxide minerals that get incorporated into the icebergs, which subsequently deliver iron through the Southern Ocean as they travel away from their origin, melting along the way.
“My personal interest in green icebergs started in 1988 when I was on an Australian expedition in the Antarctic Ocean to measure the reflection of sunlight by sea ice, with my student Richard Brandt,” Warren tells Inverse.
“We had no expectation of encountering a green iceberg, but we were lucky. We were able to climb up on the iceberg, measured reflected sunlight to determine the true color, and took a core sample. That was the start.”
Warren and collaborating scientists previously theorized that the green color was due to the presence of marine-derived organic matter in the iceberg. Icebergs typically look bluish-white because glacier ice contains bubbles that scatter ice. What’s different about green icebergs is that they contain marine ice — frozen ocean water, devoid of air pockets, stuck to the underside of an ice shelf. Marine ice usually contains organic and inorganic particles from the ocean, left behind by long-dead marine plants.
It’s been established that ice can turn green when red and yellow particles from seawater combine with the blue of ice. Warren and his team previously thought that the red and yellow colors came from the particles from the marine plants.
But that explanation took a back seat when scientists from the University of Tasmania discovered an even more unusual characteristic of marine ice. When they tested an ice core from the Amery Ice Shelf, they discovered the marine ice near the bottom of the core had nearly 500 times more iron than the glacial ice above it.
Iron, it should be noted, also casts off the necessary yellows and reds necessary to create the green.
Warren suspects that the iron oxides discovered in that core came from the rocks on Antartica’s mainland. As glaciers flow and slide, they erode bedrock, which are typically “a few percent iron.” The eroded material is a fine powder scientists call “glacial flour,” which could become incorporated into an iceberg as it forms near the mainland, turning it green.
Warren has formed a collaboration with the Australian geologists who tested the ice core, and they’ve written a proposal to study more icebergs near one of the Australian Antarctic stations. They plan to measure the spectral reflectance of blue and green icebergs to quantify their color as well as drill short cores in the icebergs to measure levels of iron, salts, isotopes, dissolved organic carbon, and more.
“If our proposal is approved by the Australian Antarctic Science Program, we will have results two years from now,” Warren says.
If the iron theory proves to be true, it will mean that green icebergs aren’t just exotic curiosities but also a crucial aspect of the marine ecosystem. When green icebergs break off of the ice shelf, they may be transporting much-needed iron from Antartica to the open sea.
This could be a boon for phytoplankton, which need both iron and sunlight to carry out photosynthesis. Phytoplankton are the foundation of the aquatic food chain and consume carbon dioxide on a scale equivalent to forests — helping the Earth stay as green as its icebergs.
Ice crystals form in supercooled seawater beneath several Antarctic ice shelves; as they rise to the ice‐shelf base they scavenge particles from the water and incorporate them into the growing basal ice. The resulting marine ice can be ~100 m thick; it differs from sea ice in that it is clear, desalinated, and bubble‐free. Icebergs of marine ice vary in color from blue to green, depending on the nature and abundance of foreign constituents in the seawater that became trapped in the ice as it grew. A red or yellow material (i.e., one that absorbs blue light), in combination with the blue of ice, can shift the wavelength of minimum absorption to green. Previously, dissolved organic carbon (DOC) had been proposed to be responsible for the green color. Subsequent measurements of low DOC values in green icebergs, together with the recent finding of large concentrations of iron in marine ice from the Amery Ice Shelf, suggest that the color of green icebergs is caused more by iron‐oxide minerals than by DOC. These icebergs travel great distances from their origin; when they melt they can deliver iron as a nutrient to the Southern Ocean.