Radiocarbon calibration

Radiocarbon calibration is the process of converting raw carbon isotope percentages into estimates of the actual age of a sample. This is necessary mainly because the atmospheric 14
C/12
C ratio has not been historically constant.

The process begins with a fixed formula that converts the isotope ratio into a number of "radiocarbon years" on the assumptions that the environmental isotope ratio has always been the same as in 1950 and that the half life of 14
C is 5568 years. In order that the radiocarbon age would be the same no matter when it was measured, this uncalibrated age is usually given in "14C years BP", where BP (literally "before present") means "before 1950".

The second step is to adjust the uncalibrated age to obtain a more accurate estimate of the age in calendar years. This takes into account that the real half life of 14
C is 5730 years, and also takes into account the variation in environment isotope ratios over the years. The adjustment is done by applying a "calibration curve" that is periodically updated. The points on the curve have been determined experimentally by such means as measuring the isotope ratio in wood whose age is firmly established by counting tree rings. Since the curve is not monotonic, sometimes more than one calendar age is possible. In addition, uncertainties in both the isotope measurement and the calibration curve mean that calibrated ages are cited as confidence intervals consisting of one of more ranges and the associated probability. For example, a calibrated age might be "2120–2144 or 2150–2171 cal BP with 90% confidence".

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