There are two main methods used for calculating age ranges from the calibration curve: The first method to be employed was called the `intercept method' because it can be done by drawing intercepts on a graph.
This method will tell you the years in which the radiocarbon concentration of tree rings is within two standard deviations of your measurement (e.g.
This plot shows how the radiocarbon measurement 3000 -30BP would be calibrated.
The left-hand axis shows radiocarbon concentration expressed in years `before present' and the bottom axis shows calendar years (derived from the tree ring data).
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.
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.
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.These values should correspond exactly to normal historical years BC and AD.The term cal BP means the number of years before 1950 and can be directly compared to calendar years.The results of calibration are often given as an age range.In this case, we might say that we could be 95% sure that the sample comes from between 1375 cal BC and 1129 cal BC.These are the basis for the calibrations performed by the programs like CALIB and Ox Cal. 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.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).