A RACE is on to retrieve the first million-year-old sample from deep within Antarctica's ice. It's a prize that could help us understand what drives major changes in Earth's climate.
Every 100,000 years or so, the Earth swings into an ice age - but it wasn't always this way. Until around 1 million years ago, our planet danced to a faster beat, with the ice age pulses occurring every 40,000 years. No one knows why the tempo slowed.
Currently, the shifts between ice ages and warm interglacial phases are thought to be influenced by three cyclical changes to Earth's motion. The Earth's axis wobbles or "precesses" on a 26,000-year cycle; it changes its average tilt on a 41,000-year cycle; and it shifts its orbit from being roughly circular to more elliptical on a 100,000-year cycle.
These changes alter the intensity of sunlight hitting the Earth at high latitudes, and so affect the extent of glaciation. The puzzling thing about the shift that happened a million years ago is that there was no obvious change to any of these cycles to make it happen.
"It's a real head spinner," says Tas van Ommen at the Australian Antarctic Division in Hobart, Tasmania. But climatologists are keen to find an explanation. "If we don't understand the switch, then we cannot claim to understand why we have the climate we have today," saysEric Wolff of the British Antarctic Survey in Cambridge, UK.
One possible explanation is that there was a slow decline in the concentration of carbon dioxide in the atmosphere, starting around 3 million years ago. This could have weakened the greenhouse effect and cooled the Earth so much that the tilt towards the sun every 41,000 years no longer provided enough heat to melt the glaciers that formed in between. Confirmation of this idea requires a direct record of the ancient atmosphere - and this can be recovered by analysing the air that became trapped in tiny bubbles within ice as the snow it formed from fell to Earth.
In 2005, the European Consortium for Ice Coring in Antarctica (EPICA) drilled an ice core in Dome C on east Antarctica's plateau that stretches our record of the ancient atmosphere back 800,000 years (Quaternary Science Reviews, DOI: 10.1016/j.quascirev.2010.10.002). That's frustratingly short of the crucial transition period, so to extract an older core, the EPICA consortium must now go back to its drill site.
It has been joined in the chase for the million-year-old core by three other teams: one from the Australian Antarctic Division; a US contingent; and one from the Chinese Arctic and Antarctic Administration. Although the groups collaborate, there is no doubt each wants to win the prize.
"China is there already, at Dome A," in east Antarctica, says van Ommen. The Australians are also close to committing to a drill site: van Ommen has just returned from a survey of the Aurora basin in east Antarctica, which is believed to hold the thickest ice in Antarctica.
Despite their head start, however, the Chinese may have run into trouble. Last month, Robin Bell of the Lamont-Doherty Earth Observatory in Palisades, New York, and her colleagues found that ice sheets in Dome A are growing from the bottom up. This could mean that any ancient ice that was once there may have melted and been replaced (Science, DOI: 10.1126/science.1200109).
Similar problems may stymie work at other potential drill sites, van Ommen says, but Wolff remains optimistic that the million-year-old ice core will be found. It is only recently that very deep cores have been drilled - and three of them contain ice more than 160,000 years old. "It would be surprising if we happened to have already collected the oldest ice available," Wolff says.