Muddy lake bed holds radiocarbon 'Rosetta stone'

Mud is not renowned for its clarity, but the murky gloop at the bottom of a Japanese lake could provide the clearest, most accurate way yet of calibrating radiocarbon dates. The finding promises to help us pin down exactly when important events in prehistory occurred, including periods of climate change and the date of the Neanderthals' extinction.

We gauge the age of artefacts containing organic matter ? like bones, plant remains and wooden tools ? by comparing the amounts of the radioactive isotope carbon-14 and the stable carbon-12 that they contain. While it is alive, an organism's tissues contain a ratio of the two isotopes that matches the ratio in the atmosphere. Since carbon-14 decays at a steady rate, you can work backwards to figure out when the carbon isotope ratios would have matched that of the atmosphere, and so figure out how long ago the organism died.

The trouble is that the atmospheric ratio of carbon-14 to carbon-12 changes over time, making it difficult to establish exactly how much was in the organism when it was alive, and making the time-of-death calculation less accurate. That's where Lake Suigetsu comes in.

Every year for at least the last 60,000 years, a layer of algae has formed on the lake bed. Each layer includes the organic remains of leaves and twigs, which Christopher Bronk Ramsey at the University of Oxford and his colleagues have analysed to work out their carbon-14 content ? a project that began more than a decade ago.

Layer count

Because counting the layers provides an independent way to establish exactly how old the organic remains are, the team used the information ? together with the carbon-14 content of the remains ? to work out exactly how much carbon-14 was in the atmosphere when the organic remains in each layer were trapped.

"This is massively important," says Chris Turney, a climate scientist at the University of New South Wales in Sydney, Australia, who was not involved in the research. He says it is a bit like a Rosetta stone that will allow researchers from a range of disciplines to work out the precise age of organic remains.

"You've got all these different records all scattered around the planet and nobody's known how to link them all in," says Turney. The Lake Suigetsu record offers a way.

We already have an independent carbon-14 record that came from analysing tree rings, but it stretches back only 12,655 years. The Lake Suigestu data will extend that accurate record to 60,000 years before the present.

Radiocarbon dating expert Paula Reimer, an archaeologist from Queen's University Belfast, UK, says the accuracy with which we can determine an artefact's age could be improved by up to 1000 years as a result of this work. "This is significant if you're trying to measure rates of change such as glacier advances and retreats of species," she says. "You might be able to see if climate change was a factor in the extinction of Neanderthals, for instance."

Unpicking cause and effect

One of the joint leaders of the research, Takeshi Nakagawa at Newcastle University, UK, says the Lake Suigetsu data could help date artefacts that are 25,000 years old to within a century. Such improvements could establish the chronological order of extinctions, human movements and major environmental events, helping to distinguish cause from effect. For example, says Joe Dortch, an archaeologist at the University of Western Australia in Crawley, the work will help us figure out whether humans or climate changes led to the extinction of megafauna, beginning in Australia around 45,000 years ago.

"[The new study] is something that a lot of us have been working towards, but this is a big step up. This makes a massive change," says Turney.

Journal reference: Science, DOI: 10.1126/science.1226660

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