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.
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.
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.
