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) 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!
Those who have the privilege to have seen South Georgia with their own eyes will remember the island as a wildlife paradise. Seals, penguins, flying seabirds – they are all there. The colonies are hugely impressive.
And there have been good news about the conservation status of South Georgia in recent years. The reindeer that Norwegian whalers had introduced from 1911 were culled until 2014. In South Georgia, the reindeer population had a density of up to 85 animals per square kilometre, in contrast to about 5 per sq km in Spitsbergen. It does not surprise that the vegetation in South Georgia suffered severely from the constant trampling and grazing from thousands for reindeer in those areas where they existed. This was evident when you compared those parts of South Georgia where reindeer roamed the tussock grass with other areas where they never existed. The eradication project gained priority when the ongoing retreat of glaciers threatened to enable reindeer to migrate to other parts of the island which had been naturally closed of until now. Wherever possible, reindeer were herded and slaughtered; elsewhere, they were shot by hunters. The last ones were culled in 2014. Since then, the vegetation and accordingly many bird species associated with the tussock gras communities can return to their original habitat in those areas formerly inhabited by reindeer.
Getting rid of rats was another and much larger challenge. These came also with the whalers to the island. They thrived soon in many areas – a disaster for many ground-breeding seabirds, meaning pretty much all of them (there are no trees on South Georgia). Rats take eggs and chicks from nests and burrows and they don’t even hesitate to have a go at chicks from large albatrosses.
In a herculean effort that lastet over several years, the South Georgia Heritage Trust has eradicated rats on South Georgia. The evaluation phase is still going on, but years of monitoring have not shown any traces of rats still being present on the island. Birds such as the South Georgia pipit which had been restricted to small off-lying islands for many decades soon started to re-establish themselves on the main island.
The recovery of whale and seal populations after centuries of intense catching and hunting is also good news. Today, thousands of Fur seals crowd South Georgia’s beaches again. Some whale species will need centuries to return to pre-industrial levels if they ever do, while others, noticeably the Humpback whale, make a swifter return. The overall development is positive.
But all these good news and successes don’t mean that everything is as it should be. Bycatch in fisheries has been an issue for decades, being the reason for dramatic declines of many seabird species not only in South Georgia, but in many areas of the world. This includes iconic species such as the Wandering albatross and many of its close relatives. Also here, there have been regional improvements: in South Georgia’s waters, strict regulations have brought major improvements. Here, bycatch does not have a significant effect on populations anymore, birds drowning on longlines are rare exceptions today. That is the good news. The bad news is that fishing vessels elsewhere in the Southern Ocean (or elsewhere, for that sake) operate under much less strict regulations and often illegally. And many seabirds travel thousands of miles on their foraging trips. Regional protection will not be able solve the problem.
As a result, in spite of regional successes bycatch is still considered the one big main threat for many seabird populations. Also on South Georgia, several albatross species are still declining strongly – in a state which is already considered anything but stable and healthy. Species including Wandering, Black-browed and Grey-headed albatross are all listed on the IUCN list at various levels of risk. And all of them are still losing individuals at an alarming rate, as is confirmed by recent surveys..
Wandering albatross on nest on Bird Island, South Georgia.
The number of Wandering albatrosses breeding on South Georgia has declined by 18 % in just 11 years, from 2003/04 to 2014/15. Speaking in absolute figures, only 1278 were left out of 1553 in the beginning of the above-mentioned period. Black-browed albatrosses went through a very similar development with a loss of 19 %. For Grey-headed albatrosses, the situation was even much more dramatic with a loss of a stunning 43 %.
And it is not that this development started at a healthy level in 2003/04. Populations have declined at least since the 1970, when scientists started to monitor them more regularly. Today’s colonies are just a shadow of what they were when South Georgia and the Southern Ocean were still untouched by man, before whaling and sealing started. Sally Poncet and her co-authors consider bycatch in fisheries still the main threat for South Georgia’s albatrosses.