Radiocarbon dating not accurate
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.Since the calendar age of the tree rings is known, this then tells you the age of your sample.
It gives the time range, from which you can be 95% sure the sample came.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.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.
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.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).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.