Dating sediment layers

09 Mar

The problem, says Bronk Ramsey, is that tree rings provide a direct record that only goes as far back as about 14,000 years.

Marine records, such as corals, have been used to push farther back in time, but these are less robust because levels of carbon-14 in the atmosphere and the ocean are not identical and tend shift with changes in ocean circulation.

Organisms capture a certain amount of carbon-14 from the atmosphere when they are alive.

By measuring the ratio of the radio isotope to non-radioactive carbon, the amount of carbon-14 decay can be worked out, thereby giving an age for the specimen in question.

This is what archaeologists use to determine the age of human-made artifacts. The half-life of carbon-14 is only 5,730 years, so carbon-14 dating is only effective on samples that are less than 50,000 years old.

Dinosaur bones, on the other hand, are millions of years old -- some fossils are billions of years old.

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For example, a volcanic eruption may create a layer made of hardened ash. Radiometric dating measures radioactive elements in the rocks.

But even he “realized that there probably would be variation”, says Christopher Bronk Ramsey, a geochronologist at the University of Oxford, UK, who led the latest work, published today in Science.

Various geologic, atmospheric and solar processes can influence atmospheric carbon-14 levels.

She will lead efforts to combine the Lake Suigetsu measurements with marine and cave records to come up with a new standard for carbon dating.

The most widely known form of radiometric dating is carbon-14 dating.