Dating by measurements

For older periods we are able to use other records of with idependent age control to tell us about how radiocarbon changed in the past.The information from measurements on tree rings and other samples of known age (including speleothems, marine corals and samples from sedimentary records with independent dating) are all compiled into calibration curves by the Int Cal group.To give an example if a sample is found to have a radiocarbon concentration exactly half of that for material which was modern in 1950 the radiocarbon measurement would be reported as 5568 BP.For two important reasons, this does not mean that the sample comes from 3619 BC: Many types of tree reliably lay down one tree ring every year.

A slightly different method is now more often used which is called the `probability method'.Since the calendar age of the tree rings is known, this then tells you the age of your sample.In practice this is complicated by two factors: These effects are most clearly seen by looking at a specific example.By using dead trees of different but overlapping ages, you can build up a library of tree rings of different calendar ages.This has now been done for Bristlecone Pines in the U. A and waterlogged Oaks in Ireland and Germany, and Kauri in New Zealand to provide records extending back over the last 14,000 years.

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Radiocarbon measurements are always reported in terms of years `before present' (BP).

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