Tiny bits of iron may explain why some icebergs are green

The emerald ice may help shuttle an essential nutrient around marine food webs

green icebergs

GREEN SCENE  Maybe only 30 out of 1,000 icebergs have a green hue, earning them the nickname “jade bergs.” Now scientists may know why the ice has this unusual color.      

Steve Nicol

Scientists may have finally figured out why some icebergs are green. Iron oxides could create the emerald hue.

Icebergs often appear mostly white because light bounces off air bubbles trapped inside the ice. But pure ice — ice without air bubbles that often forms on a berg’s underside — appears blue because it absorbs longer light wavelengths (warm colors like red and orange) and reflects shorter ones (the cooler colors).  

Since the 1930s, though, mysterious capsized icebergs with green undersides, nicknamed “jade bergs,” have been spotted around Antarctica.

In the early 1990s, glaciologist Stephen Warren of the University of Washington in Seattle and his colleagues proposed that the green came from microscopic carbon particles from dead organisms. When integrated into ice, these yellow carbon particles would absorb blue light leaving green to be reflected. Later experiments, though, found that the amount of carbon in green icebergs was too low to create the emerald hue.

“So we were left with this disturbing result,” Warren says.  

Then in 2016, researchers discovered iron oxides in a decades-old preserved green ice sample taken from the Amery ice shelf in Antarctica. Iron oxides such as rust reflect reds and oranges but absorb blue light. If these particles, possibly picked up from rocks crushed by the weight and friction of glaciers flowing toward the ocean, get incorporated into ice forming underwater, the result would be a vibrant green, Warren and his colleagues report online February 7 in the Journal of Geophysical Research: Oceans.

Warren hopes to return to Antarctica to collect samples to see if jade bergs are rich in iron. If so, that could both solve a mystery and suggest a previously unknown role for this unusual ice: ferrying a scarce but essential nutrient to the microscopic plankton that the entire ocean food web relies on.

“I don’t know how important [green icebergs] are,” Warren says. “I guess we’ll find out.”

Editor’s note: This story was updated March 7, 2019, to correct that the study in the Journal of Geophysical Research: Oceans was reported online February 7, not March 4.

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