No. 87: May-Jun 1993
Antarctic icebergs frequently incorporate sediments that turn them dark, even zebra-striped on occasion. But it is the bottle-green icebergs that provoke the most interest, for it is not certain why they are green. Is it due to intrinsic optical properties of the ice or impurities? A recent paper by J. Kipfstuhl et al suggests the latter and tells us where these translucent, strikingly green bergs originate.
"A comparison of samples from a translucent green iceberg with a core from the Ronne Ice Shelf revealed an excellent agreement in isotopic composition, crystal structure, and incorporated sediment particles. Marine shelf ice which constitutes the basal portion of some ice shelves is considered to be the source of green icebergs. It [the green ice] most likely results from 'ice pump' processes which produce large amounts of ice platelets in the water column beneath ice shelves. These subsequently accumulate and become compacted into bubble-free, desalinated ice."
The inclusions in the green ice are probably trapped by the accumulating platelets. Since the green ice is confined to the bottoms of the bergs, it becomes apparent only after the bergs capsize. The green icebergs in the Weddle Sea originate mainly from the Amery Ice Shelf.
(Kipfstuhl, J. et al; "The Origin of Green Icebergs in Antarctica," Journal of Geophysical Research, 97:20,319, 1992.)
Similar phenomenon. The ice calving from Alaskan glaciers is often a beautiful blue color -- the consequence of the ice's high density rather than inclusions. Alaskans export this ice to Japan where it is popular in drinks at tony parties!.
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