Big berg tells important story

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A supergiant iceberg floating in Antarctica’s Ross Sea is providing scientists with valuable new data on how icebergs form and the direction of currents in Antarctic waters.

The tabular berg, known as B-9, broke away from the eastern part of Ross Ice Shelf in October 1987 and has now drifted more than 1000km around eastern Ross Sea (see map page 14).

When B-9 formed it was approximately 154km long and 36km wide — one of the largest bergs on record. The shape of the berg was largely determined by weaknesses in the ice shelf. As early as 1971 huge rifts (one of them 100km long, 5km wide and extending almost the full 300m depth of the ice shelf) had been observed in the area. Such zones of weakness are natural “fault lines” along which big bergs form.

The actual calving process is probably trig­gered by flexing of the ice sheet due to ocean swells.

Ross Ice Shelf is basi­cally a glacier, or more accurately, the confluence of a number of glacial flows extending from the Antarc­tic continent. Ice forming on the land spreads out­ward under its own weight into the sea to create a wedge-shaped shelf. The pressure of new ice flowing over and under the old results in a massive thick­ness of ice which, in the case of Ross Ice Shelf, is estimated to extend 50m above sea level.

After 17 months B-9 is still wandering around the cold waters of the Antarctic continental shelf.The berg hasn’t changed much and is still about 140km long. Antarctic icebergs finally melt when they drift into warmer waters between latitudes 68 and 45 South, but B-9 is still several hundred kilometres from such waters and it may take several years to reach them.

Since it formed, B-9 has drifted in a slow but complex manner. It has been pushed at an average speed of about 2.5km per day by a succession of ocean currents. Between late June and mid Decem­ber 1988 it followed an almost circular course ­perhaps suggesting a huge current gyre in the Ross Sea.

By studying such a large, easily identifiable iceberg we can learn a lot about how and where icebergs drift, and their behaviour. This information will help us predict how possible offshore mineral activities in the region would be threatened by icebergs and, on the other side of the equation, how the Ross Sea ecosystem could be af­fected by environmental changes.

More immediately, the formation of B-9 throws light on how large bergs are created, and the mecha­nisms by which ice shelves, and on a larger scale the entire Antarctic ice sheet, lose mass. This information is important because the ice sheet exerts a major influence over processes affecting the world’s climate and sea levels.

The information B-9 is providing on currents in Ross Sea is important for several reasons. These currents influence the annual freezing of the sea around Antarctica, and sea ice drift and melt; they also affect the marine ecosystem (including krill, penguins and whales) by influencing availability of food and habitat, and hence breeding success.

Ross Sea currents may also play a part in deter­mining how the greenhouse effect could influence Antarctica.

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