Bizarre Green Icebergs Have Baffled Scientists for Decades—but Mystery of Emerald Color Might Soon Be Solved

For over a century, sailors and explorers have reported seeing bizarre emerald green icebergs around Antarctica. Their unusual color has long perplexed researchers, and they have been scrutinized by scientists for decades.

Now Stephen Warren, a glaciologist at the University of Washington, and his colleagues may have finally solved the mystery. In a paper published in Journal of Geophysical Research: Oceans, researchers found the icebergs are green because of iron oxides from rock dust found on Antarctica's mainland.

"Green icebergs occur only in the Antarctic Ocean, so they would not have been seen before Cook's expedition in 1774, if then," Warren told Newsweek. "Coleridge's poem The Rime of the Ancient Mariner mentions 'ice, mast high, came floating by, as green as emerald.' Maybe that was just poetic imagination. Green icebergs entered the scientific literature in 1921 with Drygalski's report of the German expedition of 1903, and since then there have been occasional reports of sightings by ship captains."

Normally, icebergs form when they break away from glaciers and ice shelves. This glacier ice is formed of layers of snow that builds up over time and solidifies, meaning it has air pockets that reflect light. This ice appears blue because it absorbs more red light than blue.

However, in Antarctica some icebergs have a layer of marine ice, which is ocean water that has frozen to the underside of the ice shelf. Because it lacks air pockets, marine ice appears clearer and darker than normal ice. Warren said about one percent of icebergs contain visible marine ice.

Initially, scientists thought the green icebergs were made from marine ice rather than glacier ice. The color was believed to be the result of dead marine plants and animals that had become trapped when the water froze—as the organic material broke down, it turned into dissolved organic carbon, which has a yellow color. When the blue glacier ice mixed with the yellow marine ice, theory hypothesized, a green color would be produced.

However, in the 1990s, Warren discovered this could not be the case. During an expedition, his team found that green ice contained the same amount of organic material as blue—meaning something else must be responsible for the emerald coloring.

green iceberg wendell sea
A green iceberg floats in the Wendell Sea in 1985. Green icebergs’ unusual color has long perplexed researchers. AGU/Journal of Geophysical Research: Oceans/Kipfstuhl et al 1992

Recently, researchers in Tasmania tested an ice core from Antarctica's Amery Ice shelf. They discovered the iron content of the ice at the base of the core was about 500 times higher than what was found in the glacier ice above.

Because iron oxides in soil and rocks tend to have yellow, orange and red hues, Warren thought this could be what was turning the icebergs green. In their paper, researchers suggested iron in the Antarctic mainland rock mixed with the ocean water, which then froze as green marine ice. If and when it capsized after breaking away from the glacier or ice shelf, you would get a green iceberg.

This, the team said, was important as these icebergs could be playing a vital role in delivering the iron as a nutrient to the Southern Ocean. Iron is an important nutrient for phytoplankton—tiny marine plants that form the base of many food webs.

"It's like taking a package to the post office. 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," Warren said in a statement. "We always thought green icebergs were just an exotic curiosity, but now we think they may actually be important."

The team hoped to sample icebergs of different colors for their iron content to see if the hypothesis holds up.

"I have formed a collaboration with scientists at the Antarctic Center in Hobart, Australia, who are experts at measuring iron in sea ice and seawater," Warren told Newsweek. "If our project is approved, we will travel by ship to the Australian Antarctic stations close to the Amery Ice Shelf, where green icebergs are commonly found. The iron analyses will be done in Australia. Spectral reflectance of icebergs will be measured by others of our group. We have submitted a proposal to do this work."

This article has been updated to include comments from Stephen Warren.