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Biogeochemistry

Why Are Some Icebergs Green?

Source: Warren, S. G., C. S. Roesler, R. E. Brandt, and M. Curran (2019), Green icebergs revisited. Journal of Geophysical Research – Oceans, doi:10.1029/2018JC014479

Dozens of times a day around the Antarctic coastline, large chunks of ice sheet break off and crash into the ocean. Once floating in the open water, these pieces are known as icebergs. Thousands of icebergs form each year from Antarctica’s glaciers, which account for a significant portion of the total ice sheet mass loss.

When you picture an iceberg, chances are you’re imagining something that’s big and white. In reality though, icebergs come in different shades of white, blue, and even green. An iceberg’s color results from how it absorbs light, which in turn can be related to the processes that formed the ice itself. Pure ice, for example, appears blue since it absorbs longer wavelengths of visible light (reds) more effectively than shorter ones (blues). Because glacier ice is created from compressed snow, most icebergs appear bluish-white (in between the blue of pure ice and white of snow).

But what about green icebergs? A recent study led by Stephen Warren at the University of Washington uses ice samples from the Amery Ice Shelf in Antarctica to investigate what gives some icebergs this unique color. The explanation for green icebergs starts with a phenomenon called “marine ice.” Marine ice is seawater that freezes to the base of an ice shelf, creating a layer as thick as 100 m in some cases. However, the ice crystals that make up marine ice actually form well below the ice shelf base in the surrounding seawater and then rise up through the water column (since ice is less dense than water). As they rise, the ice crystals incorporate organic and inorganic particles from the seawater before attaching to the ice shelf base.

Example of green iceberg (photo from Gerhard Dieckmann)

The unique formation process of marine ice means that it contains much less air bubbles than glacial ice, resulting in a gemstone-like clarity. Furthermore, its color ranges from blue to green depending on the exact particles that get incorporated into the ice as the crystals rise. Past studies hypothesized that dissolved organic carbon is responsible for the green color in marine ice. Testing this is difficult however, since marine ice exists on the bottom of ice shelves and is therefore difficult to access.

 

 

When an iceberg breaks off from an ice shelf, it is difficult to tell whether or not there is marine ice since it sits at the bottom of the iceberg. You may have heard the phrase “tip of the iceberg,” referring to the small, perceptible part of a larger issue. This saying comes from the fact that up to 90% of an iceberg’s mass is below the sea surface. Therefore, the clear, dark marine ice is only exposed if the iceberg capsizes, such as in the image below.

Marine ice visible on the underside of a capsized iceberg (photo from Alex Cornell)

Scientists in this study took measurements of marine ice samples from the Amery Ice Shelf. They found very low concentrations of dissolved organic carbon in green ice samples, suggesting that it is not responsible for the unique color (as was suggested by past studies). Instead, they found high concentrations of iron. Iron-oxide minerals are likely responsible for the green color since they absorb shorter wavelength light, which could shift the color of the ice from blue to green.

Why does it matter that iron is responsible for the green coloration on some icebergs? While it may seem like a small and unimportant detail, this result has implications for biological productivity in the Antarctic. This is because iron is the limiting nutrient for phytoplankton in the Southern Ocean. Icebergs with green marine ice may be able to fertilize phytoplankton growth in regions far away from the ice shelf. Furthermore, changing the distribution of phytoplankton blooms would in turn impact ecosystem structure and oceanic carbon uptake. As global temperatures rise and ice melt increases, accounting for iron from icebergs, especially green icebergs, will become increasingly important to improving global climate models.

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