Scientists have explained why some Antarctic icebergs are emerald green in colour rather than the normal blue, potentially solving a decades-long scientific mystery.
The researchers at the University of Washington in the US noted that pure ice is blue because ice absorbs more red light than blue light.
Most icebergs appear white or blue when floating in seawater, but since the early 1900s explorers and sailors have reported seeing peculiar green icebergs around certain parts of Antarctica.
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The new study, published in the Journal of Geophysical Research: Oceans, suggests that iron oxides in rock dust from Antarctica's mainland are turning some icebergs green.
The team formulated the new theory after Australian researchers discovered large amounts of iron in East Antarctica's Amery Ice Shelf.
Iron is a key nutrient for phytoplankton, microscopic plants that form the base of the marine food web. However, iron is scarce in many areas of the ocean.
If experiments prove the new theory right, it would mean green icebergs are ferrying precious iron from Antarctica's mainland to the open sea when they break off, providing this key nutrient to the organisms that support nearly all marine life.
"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.
"We always thought green icebergs were just an exotic curiosity, but now we think they may actually be important," Warren said.
He started studying the phenomenon on an Australian expedition in 1988, when he took a core sample from a green iceberg near the Amery Ice Shelf on the coast of East Antarctica.
The green ice he saw was a deep emerald hue much darker and clearer than that of normal icebergs -- a signal to scientists that green ice might be different from regular iceberg ice.
"When we climbed up on that iceberg, the most amazing thing was actually not the colour but rather the clarity. This ice had no bubbles. It was obvious that it was not ordinary glacier ice," Warren said.
When the researchers analysed that iceberg and other green icebergs sampled by Australian expeditions in the 1980s, they found the green parts were made of marine ice and not glacier ice.
They suspected an impurity in the ocean water underneath the Amery Ice Shelf was turning some marine ice green.
Their first thought was that dissolved organic carbon, microscopic particles of long-dead marine plants and animals, was getting trapped in the ice as the water froze to the underside of the ice shelf.
Dissolved organic carbon is yellow, so if pure ice is blue, the addition of yellow particles could turn the ice green, according to Warren.
However, when researchers sampled icebergs on a subsequent expedition in 1996, they found green marine ice had the same amount of organic material as blue marine ice, so something else had to be responsible for the green colour.