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Antarctic Fossil Mystery Points to Warming Future

Marine microorganisms in Antarctic mountains hint at potential ice sheet collapse.

A 30-year-old Antarctic mystery may have just been solved -- and the answer could have troubling implications for a warming world.

The story begins in 1984, when a team of researchers from Ohio State University discovered fossil diatoms in East Antarctica. There was just one problem: diatoms are microscopic marine organisms, and the fossils were a mile above sea level, in exposed sedimentary rocks in the Transantarctic Mountains.

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The question of how they got there divided scientists into two groups: one that argued the diatoms had been blown into the mountains by high winds, and another that suggested the fossils' presence pointed to a major upheaval. According to this second scenario, the massive East Antarctic Ice Sheet retreated dramatically during warm periods of the Pliocene Epoch, between 3 and 4.5 million years ago; its retreat allowed the ocean to encroach more deeply into Antarctica, and the diatoms in that ocean were transported to the mountains as glaciers returned and expanded.

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The new study, published in the journal Nature Communications and led by Northern Illinois University geologist Reed Scherer, argues that both camps were right. Yes, diatoms were blown into the mountains by the wind, but only after melting glaciers and rising sea levels had carried them inland.

Using sophisticated ice sheet and climate models, Scherer and colleagues found the ice sheet experienced a series of retreats and re-advances during the Pliocene warm periods; the retreats were not as dramatic as some scientists had earlier suggested, but were significant enough to uncover bays of open seawater, with conditions ripe for production of copious amounts of plankton diatoms.

When diatoms die, their tiny shells coat the seafloor, and this, Scherer and colleagues argue, is key to understanding what happened next. As the ice retreated, its weight was removed. That allowed the land beneath it to rise up over the course of several thousand years, forming islands amid the encroaching ocean. Those islands were covered with diatoms, and cyclonic winds then carried these lighter, dead diatoms up into the mountains.

As well as being of interest in clearing up a long-simmering scientific dispute, the study's findings are important because they show not only that the East Antarctic ice sheet can retreat significantly, but that it did so -- by up to 300 miles, according to the findings of Scherer and colleagues -- in conditions that were not dramatically different from those we are creating today RELATED: Antarctic Library 'Would Protect Ice Memory'

At the time of the mid-Pliocene Warm Period, average global temperatures were about 1 to 2 degrees Celsius higher than today, but concentrations of atmospheric carbon dioxide, at around 400 parts per million, were pretty much where they are now. The prospect of a significant collapse of the East Antarctic ice sheet -- as well as that covering West Antarctica, which is modeled to collapse first -- portends a dramatic rise in sea levels worldwide.

"During certain intervals of Pliocene warmth, the sea level could have been as much as 75 feet higher than it is now," Scherer said. "The rise in atmospheric carbon dioxide from burning fossil fuel has now elevated the concentration to 400 parts per million, matching for the first time the levels of the warm Pliocene. This makes the old debate about whether the ice sheet was notably smaller than it is now more relevant than ever.

"The question is always how quickly could sea levels rise, and we're probably looking at several hundred years into the future before reaching a peak high that matches the Pliocene, but the problem of progressive sea-level rise is already upon us."

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Added study co-author Richard Alley of Penn State University:

"This is another piece of a jigsaw puzzle that the community is rapidly putting together, and which appears to show that the ice sheets are more sensitive to warming than we had hoped. If humans continue to warm the climate, we are likely to commit to large and perhaps rapid sea-level rise that could be very costly. No one piece of the puzzle shows this, but as they fit together, the picture is becoming clearer."

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