BioEd Online

Scientists have drilled right to the bottom of an Antarctic glacier, creating a hole that is three kilometres deep. The ice core that they extracted is shorter than that from a similar hole made in 2004, but oddly it seems to be just as old.

Preliminary tests show that the bottom of the core might be as much as 900,000 years old. "That was a big surprise to us," says Heinrich Miller, a glaciologist from the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, who leads the effort.

The age will have to be confirmed in their laboratory back in Germany, "but we're pretty confident," says Miller.

The drilling effort is part of the European Project for Ice Coring in Antarctica (EPICA), which dug out and analysed a similar core from an area called 'Dome C', on the side of the continent close to Australia (see ' Frozen Time')¹,²,³.

Drilling began at Kohnen Station in Antarctica's Dronning Maud Land in 2001, and the first 2,500 metres or so of ice has already been returned to laboratories in the Northern Hemisphere for analysis. The team broke through the bottom of the glacier on 17 January after removing a total of 2,774 metres of ice.

North and south

Both cores were taken from masses of ice that have built up slowly over time through precipitation. But whereas Dome C got its ice from water originating in the Indian and Pacific Oceans, the Kohnen core is essentially built from the Atlantic Ocean.

This means that researchers will be able to look at ancient climate data from the northern and southern regions of a single ocean. "This gives us the best opportunity to compare climate signals in Antarctica with those in Greenland," says Miller.

This comparison should clarify the Atlantic's role in past climate change, by showing whether the world warmed or cooled first in the north or the south, or both at the same time. The core should also reveal changes in the extent of Antarctic sea ice that are "invaluable" for fine-tuning climate models, Miller told news@nature.com.

Data traps

The relative ratios of different oxygen isotopes in the ice reveal how warm the atmosphere was at the time the snow fell. Tiny bubbles of air trapped in the ice also hold a record of the atmosphere's composition, such as how much of the greenhouse gas carbon dioxide was around. Other climate clues come from particles of dust trapped in the core.

The Kohnen core should also provide more detailed climate evidence than the one from Dome C, at least for more recent times. In the upper parts of the core, ice accumulated more than twice as fast as at Dome C, so it will be easier to distinguish between ice layers laid down in consecutive years. The ice at the bottom of the core, however, seems to be at least as old as that at the bottom of Dome C, so it must be very compressed.

Jean-Robert Petit, an EPICA team member from the glaciology lab LGGE in Grenoble, France, hopes that the core will yield evidence of a reversal of Earth's magnetic field that happened about 780,000 years ago. This would show up as traces of beryllium-10, an isotope formed in the atmosphere from the bombardment of cosmic rays that would have leaked through while the Earth's magnetic field was temporarily weakened.

For some unknown reason researchers haven't been able to find this trace in the Dome C core. But if they can find it in the Kohnen core, this would help to date that part of the ice, improving the accuracy of climate models based on its data.

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