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Frozen Wonders
Forget the white mass of a typical iceberg. The real spectacles are rare jade and striped icebergs.


Photo © Steve Nicol

While all icebergs fertilize the ocean with nutrients, boosting plankton production and starting the biological engine of the Southern Ocean (see “Life on Ice,” January-February 2009), a special kind of iceberg may be even more potent than others: emerald green or jade bergs.

When seawater at depths of more than 1,200 feet freezes to the underside of massive ice shelves like East Antarctica's Amery Ice Shelf, it forms "marine ice." Enormous hunks of ice calve—or break off—from the ice shelf, creating icebergs. When one of these icebergs overturns, its jade underside is revealed. The wondrous color of this "marine ice" results from organic matter dissolved in the seawater at those great depths, says Collin Roesler, associate research professor at the University of Maine's Darling Marine Center, who has studied green icebergs.

The organic matter in marine ice is degraded phytoplankton—not phytoplankton itself, which cannot survive at those dark depths. It includes complex organic molecules like proteins, lipids, amino acids, and sugars, as well as carbon, iron, nitrogen, silica, and sulphur. Jade icebergs not only store these dissolved organic compounds, they also transport them from great depths back to the surface, where they can be taken up by phytoplankton.

Photo © Steve Nicol

Green icebergs are infrequently seen because their verdant bellies are underwater; it’s only when they flip over, a rare event, that their richly colored regions can be seen before they melt. "No one has studied the waters around these green icebergs because they're relatively rare," Roesler says, "but it sure would be interesting."

Striped icebergs, perhaps even more scarce than jade bergs, are thought to form in one of two ways: either meltwater refreezes in crevasses formed atop glaciers before they calve icebergs (creating blue stripes), or seawater freezes inside cracks beneath ice shelves (creating green stripes).

Steve Nicol has seen more green and striped icebergs than just about anyone. Nicol is the leader of the Australian Antarctic Division's Southern Ocean Ecosystems Program. In the course of his work over the past 20 years, he has made eight lengthy research voyages to the seas off East Antarctica, with some cruises lasting as long as three months.

Photo © Steve Nicol

In an area off East Antarctica, at 58°49'S 77°0'E, the sea floor rises to relatively shallow depths, creating a feature called BANZARE Bank, which commemorates Australian explorer Douglas Mawson's British Australian and New Zealand Antarctic Research Expedition of 1929-31.

Nicol and his shipmates have seen many vividly colored icebergs at BANZARE Bank, probably due to ocean circulation bringing water northwards from the Amery Ice Shelf, a major source of green bergs.

Whenever a jade or striped berg is spotted, Nicol says, "the ship swarms with camera-wielding scientists duplicating each others’ efforts to capture forever sights that few people are privileged to observe."

Thanks to Nicol's generosity, now we, too, can catch a glimpse of these rare, ephemeral beauties.

 

Read related story: “Life on Ice

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