by Catherine Brahic, New Scientist, September 18, 2008

Seen emerging from a mud cliff, the smooth, near-black surfaces look like a long-lost objects from some high-tech civilisation. In fact, they are huge chunks of ice – the oldest ice ever found in North America.

More than 750,000 years old, the wedges have survived through times when the planet was even warmer than it is today. Duane Froese of the University of Alberta in Canada and colleagues say their discovery could us predict the fate of the deep Arctic permafrost and its frozen methane stores.

Ice wedges form when spring melt-water runs into fissures and freezes. Thermal expansion widens the cracks and allows the wedge to grow, up to around 3 metres wide and 6 metres deep.

The team discovered the ancient ice several years ago at Dominion Creek – a site in the Canadian Yukon, just east of the Alaskan border. It had been exposed by mining activities in the area. They were able to date it thanks to a layer of ancient volcanic ash that had been deposited a few dozen centimetres above its top margin. The ash was about 740,000 years old, making the ice the oldest known in North America.

"Much of the relic ice in the region dates to the last 100,000 years and most to the last 25,000 years," says Froese.

Carbon store

Remarkably, this means the Dominion Creek ice wedges must have survived two interglacial ages: 120,000 and 400,000 years ago. Computer models estimate that temperatures at the time rose to between 2 °C and 3 °C higher than the average today, which to Froese "illustrates how stubborn permafrost can be in the face of climate warming."

Permafrost is ground that remains frozen all year round. This arrests the decay of vegetation, preventing the carbon which it contains from being released into the atmosphere as greenhouse gases.

But Froese's findings do not mean that the Arctic permafrost, including the huge stores that cover much of Siberia, will survive the few degrees of warming predicted for the end of this century.

For starters, Andrew Slater of the University of Colorado points out that although they are surrounded by permafrost, the ice wedges themselves are very different. "A block of solid ice like this wedge takes considerably more energy to thaw than a mixture of soil, ice and organic matter," he says.

Melted away

Moreover, the top of the ice wedges are several metres beneath the soil surface and reach down a further 6 metres.

"The near-term worries about northern hemisphere permafrost concern the shallow layer. There is abundant evidence that this permafrost is warming," and is releasing carbon dioxide and methane, says Froese. How fast the upper few metres will degrade remains uncertain.

Even less certain is how long the deeper permafrost will hold out. "The survival of this ice for the last 750,000 years or so indicates that it is quite resilient to climate change," says Froese. Slater agrees that it may survive for hundreds, thousands or nearly a million years.

For the ancient ice wedges at Dominion Creek, however, the game is up. Since their discovery they have melted away entirely. Others, though, may remained buried just a few metres beneath the nearby forest floor.

Journal reference: Science (DOI: 10.1126/science.1157525)

Link to article: http://environment.newscientist.com/channel/earth/dn14756-americas-oldest-ice-discovered-then-it-melts.html