Some icebergs are a glorious emerald green. Why?

A green iceberg.

IMAGE: AGU/JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS/KIPFSTUHL ET AL 1992

BY MARK KAUFMAN MAR 05, 2019

While traversing the seas off of eastern Antarctica in 1988, glaciologist Stephen Warren came upon green icebergs floating in the ocean. "We never expected to see green icebergs," said Warren, noting that a deep blue hue — not emerald green — is commonly observed in these chunks of ice.

Over three decades later, Warren and a team of researchers have put forward an explanation for these rarely seen icebergs' green hue. Their hypothesis, published Monday in the Journal of Geophysical Research: Oceans, argues that tiny iron-rich rocky particles, similar to flour or dust, are the culprits. Specifically, this finely ground-up rock, aptly named "glacial flour," gets trapped in the ice on the bottom of ice shelves — the ends of glaciers that float over the ocean — ultimately lending to the ice's deep green appearance. When the icebergs eventually snap off, the fresh bergs carry the verdant hue.

This irony-rich glacial flour, though, is reddish-yellow. So, why are the icebergs green? 

The answer is simple: light. Pure icebergs naturally reflect a blue color, as ice crystals reflect short blue wavelengths of sunlight while absorbing longer wavelengths of light like reds. But when masses of ice are infused with that yellowish-red glacial flour (which naturally absorbs blue light), the resulting iceberg ends up absorbing both blue and red, while reflecting what's left — a color that falls in the greenish spectrum.

"So what gets through is the green," said Warren, a professor emeritus at the University of Washington's Department of Atmospheric Sciences.

Pure blue marine ice, without iron glacial flour.

IMAGE: COLLIN ROESLER

Decades ago, Warren didn't suspect ground-up glacial dust was responsible for the greenery. Rather, he thought it was long dead sea life frozen in the ice. "We thought it was some dissolved, organic matter, bits of dead cells." But there just wasn't enough of this organic matter in the ice to account for the deep green color. The quandary lasted for years. Then, in 2016, researchers found that ice in the undersides of an Antarctic ice shelf contained nearly 500 times more iron than the ice above it, which rekindled Warren's curiosity and led him to this theory.

Now, Warren wants to return to eastern Antarctica to collect ice samples and see if their hypothesis holds true.

"It makes perfect sense," Ted Scambos, a senior research scientist at the National Snow and Ice Data Center who had no role in the research, said in an interview. "When you put it in a red-absorbing material, you're going to get green light coming back out," he explained.

A 1988  image showing dark green marine ice, at center.

IMAGE: WARREN ET. AL/JGR OCEANS 2019

A key element of green icebergs isn't just what they contain, but how they're made. Unlike ice atop glaciers, which is made of compressed snow, this ice is built from below in the ocean (hence the name "marine ice"). Small, plate-shaped ice crystals form in the frigid water below the ice shelf, and as these crystals float up, they "bump into" particles of glacier flour and carry them into the ice shelf. In a way, it's snowing from below, explained Warren.

Green icebergs aren't regularly seen. They're only visible in certain parts of Antartica where the glacial flour mingles with the ice, specifically the Amery Ice Shelf on the eastern side of the great continent. "The tourist ships don't go there," noted Warren. What's more, the green ice usually only becomes visible when an iceberg capsizes and flips over, exposing the green, iron-infused ice formed in the ocean. 

Marine ice freezes and forms under ice shelves.

IMAGE: AGU

So green icebergs haven't just been a mystery for decades, they're also not easy to spot unless you're on an expedition near the Amery Ice Shelf. 

A big question still remains about the green icebergs: "Why would anyone pay us to do that research?" Warren asked.

Green icebergs aren't just a natural curiosity, though. They may serve a critical purpose in the vast Southern Ocean, which is starved of iron. The phytoplankton that live here are the base of the food chain and need iron to grow. The green icebergs, then, might transport these vital nutrients out to sea.

"This could be an important source of iron," explained Warren. 

"These things would be like big shopping carts for the microbes that live there," added Scambos.