The mystery of the rare emerald icebergs in Antarctica

Several articles have been published about this strange phenomenon. Most recently, scientists have proposed a new idea about why the mysterious icebergs are emerald in color.

The search for the mystery of the green icebergs began during an expedition by Australian scientists in 1988.

“Normal icebergs come from snow, and as the snow is compressed under its own weight into ice, the air in the snow is trapped like bubbles. So glaciers contain a lot of bubbles, and icebergs are very shiny. But these icebergs are not like that,” said researcher Warren.

However, the emerald ice had no bubbles, suggesting it was not ordinary glacial ice. Warren took a core sample from a glacier near the Amery Ice Shelf in East Antarctica and compared it to other green ice samples from Australian expeditions in the 1980s. He found that the emerald colour was clearly due to sea ice, not glacial ice.

Most icebergs seen by sailors in Antarctica are white or blue, some even have stripes. True green is very rare.

At first, Warren’s team suspected impurities in the seawater below were turning the ice blue, possibly from trapped microscopic particles of dead marine plants and animals. But a sample of the ice proved their theory wrong: The blue sea ice had similar amounts of organic matter.

It wasn’t until a few years ago that Warren was inspired to try something different. His inspiration came from research by University of Tasmania oceanographer Laura Herraiz-Borreguero, who found that the Amery Ice Core had nearly 500 times more iron than the ice above it.

Warren wondered if it was possible that iron oxides were turning the common blue color of ice a deep blue. If so, where did the iron come from? These compounds are rare in many parts of the ocean. Warren believed the answer might lie in glacial limestone powder. These iron-rich particles then flowed into the ocean and became trapped under the ice shelf, where they mixed with marine ice as it formed.

This discovery could play a role in sustaining life in the oceans. Iron is an important nutrient for many other organisms. If blue icebergs are transporting iron from the Antarctic continent to the Southern Ocean, it could be a crucial process for marine life.

“Iron is a limiting nutrient for phytoplankton in the Southern Ocean, so biological oceanographers are keen to quantify different sources of iron,” Warren writes in the Journal of Geophysical Research Letters.

Phytoplankton are the base of the food chain in the Southern Ocean. Photosynthesis by these phytoplankton also removes CO₂from the atmosphere, so they are important in the global carbon cycle. With global warming, if the ocean water flowing under the ice shelves becomes warmer, then perhaps less sea ice will form and less iron will be transferred to phytoplankton.

To confirm this hypothesis, the team recommends further analysis of short cores from the iceberg to measure dissolved organic carbon and particulate organic carbon relative to depth, as well as iron mineralogy.

Minh Long