Mystery of the green icebergs solved

Posted on 9. March 2019 by Rolf Stange

Green icebergs, also called jade icebergs, are rare. Actually so rare that they have sometimes been taken for a cock-and-bull story. But they do exist. I have seen several ones myself. So long ago that I have not been able to take proper photos, with the stone-age equipment that I had back then. So I do not have any photos that show the colour really well. Ok, next time.

Green iceberg (jade iceberg), Bransfield Strait

Green iceberg (jade iceberg) in Bransfield Strait, March 2003. Photo taken with pre-historical equipment (diafilm, scanned), so not much remains of the beautiful original colour.

But if you have the rare luck to see such an iceberg, then the green colour is very prominent indeed. It is not just a hint of green within the usual blue of an iceberg. It is really a different colour.

There has been a lot of speculation regarding the reason for the green colour. According to the most common theory, the green ice is marine ice: seawater frozen to the bottom of an ice shelf. Not terrestrial glacier ice, but frozen seawater. Not frozen on the surface of the sea, so it is not what is commonly referred to as sea ice. Hence the special term marine ice. The colour, now, was said to be due to a high content of organic matter of the frozen seawater. Phytoplankton trapped in the ice.

A team of scientists (Stephen G. Warren, Collin S. Roesler, Richard E. Brandt and Mark Curran) have now come up with a new theory which they describe in an article published in the Journal of Geophysical Research: Oceans. The good news: not everything we previously believed is wrong now. Also this recent publication confirms the marine origin of the ice in question. It is frozen seawater, frozen to the bottom of an ice shelf, that is responsible for the green colour of the jade icebergs.

A number of conditions have to be met to enable seawater to freeze to the bottom of an ice shelf. This happens at considerable depths of several hundred to more than thousand metres. And the water has to be very cold. The Amery Ice Shelf in east Antarctica has a grounding line depth (where the ice shelf rests on the sea floor) of 2400 metres. There, the freezing point of sea water is -3.7°C. If this water happens to move to higher levels under the ice shelf, then it is supercooled.

Green iceberg (jade iceberg) near the South Orkney Islands, January 2009. The marine ice has formed within cracks at the bottom of the ice shelf, which gives the green ice the appearance of being worked into the blueish-white, terrestrial glacier ice.

The difference to older theories is the question of what in the seawater exactly brings the green colour. Warren and his colleagues claim that the substance in question is not organic, but iron compounds. Measurements of an ice core from the Amery Ice Shelf that includes basal sections of marine ice do not show high concentrations of organic matter. Instead, the iron content was higher than expected.

The iron is part of various chemical compounds, but altogether of anorganic origin. The main mineral is Goethite, an iron-bearing hydroxide which is a common mineral in the upper crust. The source is rock material eroded at the glacier base.

The optical properties (absorption spectra) of Goethite dust tend to give a yellowish colouration. But the interplay of this brownish-yellowish colour with the blue of dense (without air bubbles) sea ice can produce exactly the jade-colour in question.

Complex matter! Not surprisingly, the authors of the study conclude that further research is needed: regarding the exact composition of organic (and anorganic) substances mixed with marine ice and their optical characteristics. Finally, such knowledge might be used to gain information about the chemical composition of icebergs from light spectra that can be measured efficiently by remote sensing, using aircraft or drones or even satellites.

And the chemical composition of icebergs, especially the iron content, is actually very important for the marine ecology of the Southern Ocean: minerals transported by icebergs can fertilise seawater that is otherwise poor in such nutrients. Such fertilisation can have significant effects on the biological productivity of the ocean. Hence, the beautiful “jade icebergs” gain unexpected importance for the ecological system of the Southern Ocean.

The volume of green ice is probably larger than one might believe, considering the rarity of green icebergs. But only smaller icebergs are actually able to turn over, exposing the green bottom. Larger tabular icebergs may transport much greater quantities of green ice but this remains then hidden at great depth. And of course the light has to be right to see it and you have to bee around … and not every ice shelf produces jade icebergs in quantities. The Amery Ice Shelf is so remote that people hardly every get there, other than scientists who work on the stations Mawson and Davis (both belong to the Australian Antarctic Division).

So, if you have every seen a green iceberg in reality, then you are one of a lucky few!

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