Antarctic researchers have a new hypothesis that could solve the century-old mystery of why some icebergs are green.

Icebergs are normally white or blue, but sailors and explorers to Antarctica since the early 1900s have reported sightings of lustrous jade and emerald green icebergs in parts of the Southern Ocean.

Tests on ice samples collected from underneath the Amery Ice Shelf suggest that iron oxides in seawater are the likely the explanation for these rare, frozen curiosities.

The research, led by Professor Steve Warren from the University of Washington, has been published in the journal JGR Oceans, and includes Australian Antarctic Division Glaciologist, Dr Mark Curran, as a co-author.

Dr Curran said the study suggests that the unique colour of the icebergs is the result of yellow-tinted iron oxide in seawater combining with the crystalline blue of the ice, to produce the distinctive jade green.

“I first saw one of these intriguing icebergs from the deck of the Aurora Australis in Prydz Bay in 1992, and no one could tell me what caused the colour,” Dr Curran said.

“It’s a question that has intrigued many seagoing Antarctic scientists and explorers over the years, and so it was a real honour to play a part in this fascinating piece of research.”

The most commonly sighted Antarctic icebergs are made from glacial ice, which is fresh water from the snow that falls on the Antarctic plateau and becomes compacted over thousands of years.

Jade icebergs are formed under very different conditions, when mineral-rich seawater freezes to the underside of an ice shelf in layers, then eventually breaks off and floats away.

These jade bergs contain layers of the pure blue-white ice from the glacier and greener ice below, formed from frozen seawater. Some icebergs of blue glacial ice contain green stripes of marine ice, formed by seawater freezing up into basal crevasses.

A number of theories have previously been put forward about why frozen seawater turns such a remarkable shade of green, including the presence of dead phytoplankton or dissolved organic carbon. The researchers found that these materials were not present in large enough amounts to explain the colour.

The scientists also believe that these mineral-rich ice blocks could play a role in promoting biological activity in the Southern Ocean, by transporting nutrients to areas where iron is in short supply.

Iron is an essential trace nutrient for the growth of marine phytoplankton, which are the tiny plants at the base of the Antarctic food web.

The research is based on iron measurements published in 2016 by Laura Herraiz-Borreguero of CSIRO, Professor Delphine Lannuzel of the University of Tasmania’s Institute for Marine and Antarctic Studies, and their colleagues.