Seen in stunning pictures from either of Earth’s Poles, icebergs are most often white-blue objects. But like an artist willing to experiment with whatever resources are available, nature is also capable of creating startlingly green icebergs, and they can be found only in Antarctica.

Although the scientific literature is full of reports of these emerald ice blocks going back more than a century, no one could adequately explain where they were coming from. Now, a team of researchers may have finally cracked the case.

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According to their work, the unusual hue seems to be a combination of two distinct processes. First, bubble-free icebergs need to form at the bases of ice shelves jutting out into the Southern Ocean. At the same time, ground-up yellow-red glacial dust from Antarctica’s bedrock has to be brought along for the ride.

“It’s pretty much Antarctica’s version of mixing blue and yellow paint together to get green,” says James Lea, a glaciologist at the University of Liverpool who was not involved with the work.

True colors shining through

Many U.S. journalists like to refer to these curiosities as emerald icebergs, says Steve Warren, emeritus professor at the University of Washington and long-time Antarctic aficionado. These people are perhaps recalling the 1798 epic The Rime of the Ancient Mariner, in which poet Samuel Taylor Coleridge describes an Antarctic sailor seeing “ice, mast high … as green as emerald.” Scientists who regularly spot the oddities in Antarctic waters instead call them as jade bergs.

Whatever the moniker, scientists knew that the color must have something to do with their physical and chemical properties. Glacial ice tends to have a blueish hue because it absorbs the redder, longer wavelengths of light, leaving the bluer, shorter wavelengths free to be scattered back into our eyes.

But if the ice contains air bubbles, the light going through it is constantly being forced to change direction, making it more likely to reappear at the surface after just a short travel time. This reduces absorption and leads to a whitening effect. By contrast, in some parts of Antarctica, the ice is so compressed that it lacks any bubbles whatsoever. This creates a longer pathway through the ice so that, to our eyes, it looks incredibly clear and vibrantly blue.

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In many accounts, green icebergs also appear to be strikingly translucent. This suggested to researchers that they lacked bubbles, which gave a clue to their origin.

Although melting is a more prominent concern, seawater can sometimes freeze onto the underbellies of Antarctic ice shelves, too. This creates a thick layer of what is known as marine ice. Unlike the air-trapping ice on the surface world, marine ice forms under higher pressures where air is more soluble and consequently tends to lack bubbles.

Those clear, vibrantly green icebergs, then, must be forged from marine ice. This was all but confirmed in the 1980s, thanks to cores samples taken by Warren and his colleagues from East Antarctica’s Amery Ice Shelf. The green coloration, however, remained puzzling.

Reopening a cold case

Warren’s team suspected that the culprit was dissolved organic carbon, the microscopic remains of marine life. This material is yellow-ish, and mixing it with pure blue ice would certainly make a green shade. At the time, they weren’t able to quantify the dissolved organic carbon in the ice cores, but they used techniques to make it fluorescence to confirm its existence.

The results of their 1993 paper were widely reported on at the time, but their victory lap was short-lived. In 1996, during Warren’s second sea-ice voyage with the Australian Antarctic Program, his team found plenty more green-tinted marine ice. This time, they could properly sample the dissolved organic carbon, and found that the amounts in the blue and green marine ice were not only similar, but also too small to have any effect on the apparent color.

“We weren't sure what to make of this information, and we didn't have enough material to recheck the analyses,” Warren says. “So we sat on it, not publishing anything.”

Then, in 2016, a study led by physical oceanographer Laura Herraiz-Borreguero–then at the University of Tasmania and now at the University of Southampton–serendipitously found the missing piece of the puzzle. Within several Amery Ice Shelf cores, the marine ice contained up to 500 times more iron compounds than the glacial ice sitting above it.

Iron oxides are a common component of soils and rocks, and their work suggests that it is being eroded away as ice inexorably flowed over East Antarctica’s bedrock. The resulting powdery sediment, so-called glacial flour, eventually made its way into the sea, where the rusty substance occasionally froze to the base of ice shelves and found itself locked into marine ice that ultimately broke off and formed icebergs. As it happens, iron oxides tend to be yellowy-red.

“Ice filters out the red light, and iron oxides filter out the blue light, so green is what's left to escape as refracted sunlight re-emerges from the iceberg,” Warren explains.

In his recent study, published in the Journal of Geophysical Research: Oceans, his team calculated how much iron you would need to shift the color of ice from blue to green, and they found that the amount in the Amery marine ice was sufficient.

Artists of the Southern Ocean

Although this is somewhat indirect evidence and more fieldwork is needed to shore up this model, Lea reckons that this mechanism “is a really neat idea which explains the available observations.”

This conveyor belt of green icebergs has some important consequences for the environment down there, Lea adds. Glacial grounding lines, where glaciers go from sitting on bedrock to floating on the sea, have been retreating all around Antarctica in recent times. This migration can cause the ice shelves in front of them to destabilize and fragment, potentially unleashing armadas of jade bergs.

While this unfortunately permits the terrestrial ice behind them to fall into the sea and contribute to sea level rise, this study suggests that the creation of this iron-rich emerald fleet “could provide a feast for the algae that crave this essential nutrient to survive,” he says.

In other words, algae hanging out near the Amery Ice Shelf might be getting the best takeaway food service in the entire Southern Ocean.