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.
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).
Radiocarbon dates should always be reported either as `percent modern' or years `before present' (BP).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.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).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.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.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.