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. The Cal prefix indicates that the dates are the result of radiocarbon calibration using tree ring data. The term CalBP means the number of years before 1950 and can be directly compared to calendar years. This requires a computer since the calculations are more complicated.
Calibration curves The main calibration curves that have been used are given here.
In the Southern Hemisphere an offset should be applied. A very useful source of data on this is the Marine Reservoir Correction Database at Queens University Belfast. For the period after 1950, there is no inernationally agreed calibration curve. However, there is a great deal of data on the atmospheric radiocarbon concentration. A good source for this is the WWW site, Atmospheric Carbon Dioxide and Carbon Isotope Records .