This figure is directly based on the proportion of radiocarbon found in the sample. It is calculated on the assumption that the atmospheric radiocarbon concentration has always been the same as it was in and that the half-life of radiocarbon is years. To give an example if a sample is found to have a radiocarbon concentration exactly half of that for material which was modern in the radiocarbon measurement would be reported as BP.
For two important reasons, this does not mean that the sample comes from BC: How tree rings are used as a radiocarbon record Many types of tree reliably lay down one tree ring every year. The wood in these rings once laid down remains unchanged during the life of the tree. This is very useful as a record of the radiocarbon concentration in the past.
If we have a tree that is years old we can measure the radiocarbon in the rings and see what radiocarbon concentration corresponds to each calendar year. Using very old trees such as the Bristlecone Pines in the western U.
To extend this method further we must use the fact that tree ring widths vary from year to year with changing weather patterns. By using these widths, it is possible to compare the tree rings in a dead tree to those in a tree that is still growing in the same region. 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, years. 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.
Calibration curves 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 IntCal group.
For more detail see the OxCal manual. How radiocarbon calibration works Calibration of radiocarbon determinations is in principle very simple. If you have a radiocarbon measurement on a sample, you can try to find a tree ring with the same proportion of radiocarbon.
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: The pair of blue curves show the radiocarbon measurements on the tree rings plus and minus one standard deviation and the red curve on the left indicates the radiocarbon concentration in the sample. The grey histogram shows possible ages for the sample the higher the histogram the more likely that age is.
The results of calibration are often given as an age range. Some Conventions This is not intended to be an exhaustive summary of radiocarbon calibration conventions but a brief guide.