2.7 million-year-old ice shatters records, reveals ancient atmospheres – and ice ages.

(Plural intended.) Science magazine looks back at lots of tiny, tiny pockets of the distant past – with a deep ice core that contains samples of our changing atmosphere for 2.7 million years – about 10 times longer than Homo sapiens has been around:

Scientists announced today that a core drilled in Antarctica has yielded 2.7-million-year-old ice, an astonishing find 1.7 million years older than the previous record-holder. Bubbles in the ice contain greenhouse gases from Earth’s atmosphere at a time when the planet’s cycles of glacial advance and retreat were just beginning, potentially offering clues to what triggered the ice ages. That information alone makes the value of the sample “incredible,” says David Shuster, a geochemist at the University of California, Berkeley, who is unaffiliated with the research. “This is the only sample of ancient Earth’s atmosphere that we have access to.”

Described at the Goldschmidt Conference in Paris by Yuzhen Yan, a graduate student at Princeton University, the ice revealed atmospheric carbon dioxide (CO2) levels that did not exceed 300 parts per million, well below today’s levels. Some models of ancient climate predict that such relatively low levels would be needed to tip Earth into a series of ice ages.

The Princeton-led team went after ancient ice sitting far closer to the surface, in the Allan Hills, a wind-swept region of East Antarctica 200 kilometers from McMurdo Station that is famous for preserving ancient meteorites. In such blue ice areas—just 1% of the continent’s surface—the ice flows across rocky ridges, tipping the record on its side. Deep, old layers are driven up, while wind strips away snow and younger ice, revealing the lustrous blue of compressed ice below. But these contortions also confound the neat ordering of the annual layers—making it impossible to date the ice by counting them.

Michael Bender, a Princeton geochemist who spearheaded the project, solved the problem by finding a way to date chunks of ice directly from trace amounts of argon and potassium gases they contain. Although not as precise as other dating methods, Bender says, the technique can date ice to within 100,000 years or so.

It’s plausible, [Oregon State University in Corvallis geochemist Ed] Brook says, that the next attempt could come back with ice 5 million years old—a time when temperatures are thought to resemble what Earth is heading toward with human-driven warming.

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