Radiocarbon dating to
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.
This figure is directly based on the proportion of radiocarbon found in the sample.
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.
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.