Radiocarbon dating samples
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.
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.
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.
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.
This figure is directly based on the proportion of radiocarbon found in the sample.
between 2940BP and 3060BP for the measurement 3000 -30BP).
A slightly different method is now more often used which is called the `probability method'.
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.