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 have reported seeing emerald green icebergs around certain parts of Antarctica. A new study published in the Journal of Geophysical Research: Oceans suggests that the iron content found in marine ice is sufficient to explain the unusual color of these icebergs.
Close-up view of an iceberg near Mawson Station, November 20, 1988, much of it covered by snow. At the far left is bubbly blue-white glacier ice; the center of the picture shows the clear, dark green, bubble-free marine ice. The boundary between glacier ice and marine ice is nearly vertical. Image credit: Warren et al, doi: 10.1029/2018JC014479.
Glacier ice, originating from snowfall, flows off the Antarctic Ice Sheet to float on the ocean as ice shelves.
At the front of the ice-shelf, icebergs break off. They appear bluish-white, intermediate between the blue of pure ice, and the white of snow, because glacier ice contains numerous bubbles that scatter light.
Seawater freezes to the base of some ice shelves, forming marine ice, which incorporates organic and inorganic particles from the water.
On the resulting composite icebergs, the marine ice can be exposed to view if the iceberg capsizes. The marine-ice part of such icebergs is clear, dark, and often green in color, because red or yellow particles from the seawater, in combination with the blue of ice, can shift the color to green.
Now University of Washington’s Professor Stephen Warren and co-authors report in their paper that they suspect iron-oxide minerals in rock dust from Antarctica’s mainland are turning some icebergs green.
“Iron is a key nutrient for phytoplankton, microscopic plants that form the base of the marine food web. But iron is scarce in many areas of the ocean,” they explained.
“If experiments prove our 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.”
Professor Warren 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.
When he and his colleagues analyzed 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,” Professor Warren said.
But when the scientists 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 color.
The problem nagged at Professor Warren until 2016, when another research team tested an ice core from the Amery Ice Shelf for its iron content. The researchers found marine ice near the bottom of the core had nearly 500 times more iron than the glacial ice above.
Iron oxides found in soil, rocks, and common rust tend to have warm, earthy hues — yellows, oranges, reds and browns.
So Professor Warren began to suspect iron oxides in the marine ice could turning blue ice green. But where was the iron coming from?
As glaciers flow over bedrock, they grind rocks to a fine powder known as glacial flour. When the ice meets the sea, this glacial flour flows into the ocean. If the rock dust becomes trapped under an ice shelf, the particles could be incorporated in marine ice as it forms.
“We suspect iron oxides in glacial flour from rocks on Antarctica’s mainland are responsible for creating the stunning emerald icebergs,” Professor Warren and colleagues said.
“We now propose to sample icebergs of different colors for their iron content and light-reflecting properties. If our theory proves correct, green icebergs could be more important than scientists thought.”
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Stephen G. Warren et al. Green Icebergs Revisited. Journal of Geophysical Research: Oceans, published online January 10, 2019; doi: 10.1029/2018JC014479
