Green Icebergs?

When you think of an iceberg you might think of a large floating block of white ice which is detached from a glacier or ice shelf. Well, instead they can be a range of different colours from white, blue and even green! 

What is an Iceberg?

icebergs

Icebergs are large blocks of ice which break away (clave) from a glacier or ice shelf and float freely, following ocean and wind currents. Icebergs are typically made up of two different types of ice, marine ice, sea ice, and glacial ice. This is often capped off with snow. 

When icebergs rotate in the water, the difference in colour of these three types of ice can be seen from the surface. Learn more about Icebergs here.

Bering Glacier Icebergs (Sam Beebe)

Glacier Ice

Glacier ice is frozen terrestrially in glacial systems and flows towards the sea, where can extend out into the sea to form an ice shelf.

Glaceri ice is often blue-white and bubbly in appearance, and is composed of freshwater. The bubbles are formed as the ice freezes and traps pockets of air. These bubbles refract light as it passes through the ice, preventing it from penetrating deep, and resulting in the whitening of the blue ice.  Glacier ice makes good ice cubes for your drink, and it fizzes when you add it!

A glacier of the modern South Patagonian Icefield (top right) flowing into a deep valley filled with seawater, known as a fjord (bottom left). Image from NASA.

Sea Ice

Sea ice is formed at the sea surface and is typically less than 1 m thick. Like the glacial ice, sea ice contains a lot of bubbles as a result of brine inclusions. As the brine trapped within the ice freezes, the volume of liquid decreases, resulting in the supersaturation of air within the remaining brine and the formation of bubbles [5]

Sea ice is a little salty, and may not be so good to use to cool your drink!

img_7154-copy
Icebergs caught in sea ice, viewed from the Twin Otter in early Summer before the sea ice melts. Photo: Bethan Davies
Schematic of marine ice formation and iceberg calving

Marine Ice

Marine ice is formed underneath ice shelves at depths of up to 400 m below the sea surface. At these depths, air is more soluble than at the surface, so as the water freezes, the air remains dissolved, resulting in fewer bubbles being trapped in the ice. Marine ice can grow up to 100 m thick and consists of roughly one-third of the ice shelf thickness[7].  

Marine ice is clearer and darker than glacial ice, as it lacks the bubbles which reflect light. When icebergs are rotated or capsized, the marine ice is visible from the surface.

Green icebergs (jade bergs) have been observed on several occasions and are believed to be made up of marine ice[1,2,8,11]

Schematic of marine ice formation and iceberg calving. (AGU)

What Makes The Icebergs Green? 

A green iceberg in the Weddell Sea

The Role of Organic Carbon

During the 80s and 90s, the green colour of marine ice was believed to have been caused by coloured dissolved organic matter[10], which is also responsible for the green and yellow colour of coastal waters[6]. Dissolved organic material is yellow, so the addition of this to the blue ice would result in a green iceberg. However, when experts measured the dissolved organic matter in the green icebergs, they found that green ice had the same amount of Dissolved organic material as the blue ice[9]. Therefore, there must be another factor turning the ice green.  

A green iceberg sighted in the Weddell Sea, Antarctica on February 16th, 1985. 
(Kipfstuhl et al., Journal of Geophysical Research: Oceans, 1992) 

The Role of Iron

Herraiz-Borreguero et al. (2016) sampled both marine and glacial ice from a core taken from the Amery Ice Shelf in East Antarctica. They analysed the particulate and dissolved iron content and found that the marine ice at the bottom of the core had an iron concentration almost 500 times greater than the glacial ice above. Iron oxides often have a yellow hue, so it was suggested that the combination of the iron oxides and blue ice causes the green colouration of marine ice. But where does the iron come from? 

Iron minerals are commonplace in the bedrock geology of Antarctica. As glaciers travel over land, they abrade the bedrock into a fine powder, known as glacial flour, which is carried by the glacier and deposited into the sea. Glacial flour can remain in suspension below the ice shelf for a long period of time and is slowly incorporated into the marine ice as it forms. 

Find Out More About Green Icebergs Here

References

1. Amos, A.F., 1978. Green iceberg sampled in the Weddell Sea. Antarctic Journal of the United States13(4), pp.63-64. 

2. Dieckmann, G., Hemleben, C. and Spindler, M., 1987. Biogenic and mineral inclusions in a green iceberg from the Weddell Sea, Antarctica. Polar Biology7(1), pp.31-33. 

3. Herraiz‐Borreguero, L., Lannuzel, D., Van Der Merwe, P., Treverrow, A. and Pedro, J.B., 2016. Large flux of iron from the Amery Ice Shelf marine ice to Prydz Bay, East Antarctica. Journal of Geophysical Research: Oceans, 121(8), pp.6009-6020. 

4. Kipfstuhl, J., Dieckmann, G., Oerter, H., Hellmer, H. and Graf, W., 1992. The origin of green icebergs in Antarctica. Journal of Geophysical Research: Oceans97(C12), pp.20319-20324. 

5. Moreau, S., Vancoppenolle, M., Zhou, J., Tison, J.L., Delille, B. and Goosse, H., 2014. Modelling argon dynamics in first-year sea ice. Ocean Modelling, 73, pp.1-18. 

6. Morel, A. and Smith, R.C., 1974. Relation between total quanta and total energy for aquatic photosynthesis 1. Limnology and Oceanography, 19(4), pp.591-600. 

7. Morgan, V.I., 1972. Oxygen isotope evidence for bottom freezing on the Amery Ice Shelf. Nature, 238(5364), pp.393-394. 

8. Moulton, K.N. and RL, C., 1976. Bottle-green iceberg near the south shetland islands. 

9. Warren, S.G., Roesler, C.S., Brandt, R.E. and Curran, M., 2019. Green icebergs revisited. Journal of Geophysical Research: Oceans124(2), pp.925-938. 

10. Warren, S.G., Roesler, C.S., Morgan, V.I., Brandt, R.E., Goodwin, I.D. and Allison, I., 1993. Green icebergs formed by freezing of organic‐rich seawater to the base of Antarctic ice shelves. Journal of Geophysical Research: Oceans, 98(C4), pp.6921-6928. 

11. Wordie, J.M. and Kemp, S., 1933. Observations on certain Antarctic icebergs. The Geographical Journal81(5), pp.428-434. 

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.