Radiocarbon dating samples

It is calculated on the assumption that the atmospheric radiocarbon concentration has always been the same as it was in 1950 and that the half-life of radiocarbon is 5568 years.For this purpose `present' refers to 1950 so you do not have to know the year in which the measurement was made.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.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.If we have a tree that is 500 years old we can measure the radiocarbon in the 500 rings and see what radiocarbon concentration corresponds to each calendar year.

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.The first indicates the proportion of radiocarbon atoms in the sample as compared to samples modern in 1950.The second is directly derived from this on the assumption that the half-life of radiocarbon is 5568 years and the amount of radiocarbon in the atmosphere has been constant.Radiocarbon measurements are always reported in terms of years `before present' (BP).This figure is directly based on the proportion of radiocarbon found in the sample.

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