Antarctic sediments muddy climate debate

Ocean-floor sediments drilled from Antarctic regions recently covered by ice shelves suggest that those shelves were only 2,000 years old. This finding could compel scientists to reassess whether the current destruction of polar ice is due primarily to human-caused global warming.

In the early 1990s, part of the ice shelf atop the Prince Gustav Channel, which separates the Antarctic peninsula and James Ross Island, broke apart. In the area formerly covered by the shelf, the channel’s water depth is between 600 and 800 meters. Scientists collected sediment cores 5 to 6 m in length from the ocean floor in February and March 2000.

Scattered throughout the seafloor ooze were telling grains of rock, says Carol J. Pudsey, a geologist with the British Antarctic Survey in Cambridge, England. She and her colleague Jeffrey Evans separated the grains larger than about 1 millimeter from the smaller particles, which may have been washed into the area by ocean currents.

They found that some of the flecks and pebbles had been scraped from James Ross Island and the Antarctic mainland by the glaciers that fed the ice shelf. However, other grains and pebbles didn’t match the types of rock in nearby sources and could only have been carried into Prince Gustav Channel by far-traveling icebergs. Those bergs, in turn, would have dropped their sedimentary burden in the channel only if the ice shelf was absent during at least part of the year.

By carbon-dating the organic material found in sediment layers rich in large grains, the researchers could determine when icebergs had wafted into the channel. Their analysis suggests that from about 2,000 to 5,000 years ago, much of the channel was seasonally open water. In areas where sediments had accumulated slowly, cores included berg-delivered material from as much as 30,000 years ago, Pudsey notes. She and Evans report their findings in the September Geology.

A surge in temperatures along the Antarctic peninsula in the past few decades has been linked to the demise of several ice shelves there. Larsen A, the southern neighbor of the Prince Gustav ice shelf, disintegrated during a strong summer storm in January 1995 (SN: 5/12/01, p. 298: Big Bergs Ahoy!). The Larsen B ice shelf, which used to sit adjacent to Larsen A, continues to shed icebergs and may disappear within the next decade.

Some scientists suspect the dramatic rise in carbon dioxide and other industrial greenhouse gases is to blame for the recent regional warming. Several climate models predict that global warming would be accentuated in polar regions. However, the news that the Antarctic peninsula’s ice shelves may have come and gone at least once since the end of the last ice age, about 11,000 years ago, suggests that people may not be fully to blame for the disappearance now under way.

“The current rate of warming may be unusual, but our research shows definitively that this area has been warm and icefree at times in the past few thousand years,” says Pudsey. Because she and Evans analyzed core segments between 5 and 10 centimeters in length–each corresponding to at least a century–they weren’t able to calculate the precise rate of past warming episodes.

The new data from the Prince Gustav Channel should help indicate whether the instability of the Antarctic peninsula’s ice shelves is part of a natural cycle, says Ólafur Ingólfsson, a geologist at Göteborg University in Sweden.

The new evidence for past warming backs up data garnered from land-based studies of lake sediments and ancient, abandoned penguin rookeries, he notes. According to that research, conditions along the peninsula were warmer and more humid between 2,500 and 4,000 years ago.

The collective indication is that the Prince Gustav ice shelf, and others in the area, are short-lived, Ingólfsson adds.

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