When icebergs break off from Antarctica, they typically glisten in a deep blue to white colour. But tales of green icebergs floating around parts of the icy continent have been told by sailors since at least the 1900s. Researchers from the University of Washington (UW), US, now claim to have cracked the mystery of the emerald-like icebergs. In a study published in the Journal of Geophysical Research: Oceans, researchers proposed the culprit behind the mysterious green tinge is iron oxide.
Glaciologists studying the phenomenon suspect significant amounts of iron oxide in rock dust from Antarctica’s mainland are trapped in the ice.
Iron oxide is a key element in the diet of phytoplankton, a microscopic plant-based organism, which forms various forms of marine bacteria and green algae.
Now, according to the UW study, the icebergs act like a delivery service of sorts, carrying iron oxide into those parts of the ocean where the element is scarce.
When the surface ice melts away from the iceberg, it releases the iron oxide which phytoplankton can then feed on.
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Stephen Warren, a glaciologist at UW, said: “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.
“We always thought green icebergs were just an exotic curiosity, but now we think they may actually be important.”
Pure icebergs typically appear blue because the ice reflects more red light than blue light on the visible spectrum.
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But the occasional oddity of green icebergs appearing on the horizon has prompted Professor Warren and his colleagues to investigate the mystery.
The researchers first obtained a sample of green ice during an Australian expedition in 1988, when they encountered a green iceberg near the Amery Ice Shelf on Antarctica’s east coast.
The green iceberg they found had a deep emerald-like hue to it, unlike anything the scientist has ever seen before.
Professor Warren said: “When we climbed up on that iceberg, the most amazing thing was actually not the colour, but rather the clarity.
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“This ice had no bubbles. It was obvious that it was not ordinary glacier ice.”
Icebergs typically form when consecutive layers of snow build up and solidify over long periods of time, creating pockets of our in-between.
In Antarctica, however, some icebergs form so-called marine ice – ocean water frozen to the bottom of an under-hanging ice shelf.
These particulate ice shelves do not have air pockets to reflect light and these are the parts which turn green.
The study notes: “The marine‐ice part of such icebergs is clear, dark, and often green in colour, because red or yellow particles from the seawater, in combination with the blue of ice, can shift the colour to green.
“Previously, dissolved organic carbon had been proposed to be responsible for the green colour.
“The recent finding of large concentrations of iron in marine ice from the Amery Ice Shelf suggests that the colour of green icebergs is caused more by iron‐oxide minerals.
“These icebergs travel great distances from their origin; when they melt they can deliver iron as a nutrient to the Southern Ocean.”
