Carbon dating 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.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.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.Using very old trees (such as the Bristlecone Pines in the western U. A.), it is possible to make measurements back to a few thousand years ago.To extend this method further we must use the fact that tree ring widths vary from year to year with changing weather patterns.Here is a laymen's explanation with a few slightly more technical details in the footnotes: There are different isotopes of carbon.
Since the calendar age of the tree rings is known, this then tells you the age of your sample.
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 1950 and that the half-life of radiocarbon is 5568 years.
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.
The wood in these rings once laid down remains unchanged during the life of the tree.