Icebergs are beautiful to watch with their blue and white tints. These colors make sense for glaciers as ice absorbs more red light than blue.
However, since the early 1900s explorers have reported the perplexing existence of green icebergs around Antarctica. These impressive glaciers have never been explained by science until now.
Researchers have revealed a new theory as to what may be causing the emerald green hue. The culprit, they suspect, is iron oxides in rock dust from Antarctica's mainland.
Iron is a key nutrient for phytoplankton, autotrophic components of the plankton community that form the basis of our oceans' food web. However, the nutrient is scarce in many oceanic areas.
Now, researchers suspect that green icebergs are bringing iron from Antarctica's mainland to the open sea when they break off.
"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," said Stephen Warren, a glaciologist at the University of Washington and lead author of the new study.
"We always thought green icebergs were just an exotic curiosity, but now we think they may actually be important."
Warren first began studying green icebergs in 1988 when he took a core sample near the Amery Ice Shelf on the coast of East Antarctica. He was impressed by the hue and texture of the green ice.
"When we climbed up on that iceberg, the most amazing thing was actually not the color but rather the clarity," Warren said. "This ice had no bubbles. It was obvious that it was not ordinary glacier ice."
Conventional glacier ice has air pockets that reflect light. However, in Antarctica, some icebergs have a layer of marine ice, ice that is clearer and darker because it lacks air pockets.
In 1988, Warren found that the green parts of icebergs were made of marine ice and not glacier ice. He then first suspected that an impurity in the ocean water underneath the Amery Ice Shelf was responsible for the green hue.
However, in 1996, Warren and his colleagues discovered that green marine ice had the same amount of organic material as blue marine ice. This perplexed Warren.
It wasn't till a few years ago that Warren finally got a potential answer to his green hue investigation when an oceanographer at the University of Tasmania tested the iron content of an ice core from the Amery Ice Shelf. What she found became the basis of Warren's theory.
The bottom of the core of green ice had nearly 500 times more iron than the glacial ice above. This is when Warren began to suspect iron oxides in the marine ice were turning the glaciers green.
The researchers are now proposing to study icebergs of different colors for their iron content and light-reflecting properties. If Warren's theory proves correct, it would mean that green icebergs are crucial to our oceans.
The study is published in the Journal of Geophysical Research: Oceans, a journal of the American Geophysical Union.