Calibration

Radiocarbon activity in nature is not constant, with variations caused by changes in cosmogenic production of ¹⁴C, necessitating the use of a calibration curve to compensate for these changes.

The first calibration curves converting conventional radiocarbon ages to calendar ages were compiled from data from pine tree rings obtained by Wesley Ferguson in the 1960s, spanning 8,000 years. Since then, the calibration dataset has been gradually expanded, initially with tree rings from other species, especially oak, and later with data from sources such as annual sediment layers in lakes. These curves continue to be refined and extended. In 1970, separate calibration curves for the northern and southern hemispheres began to be distinguished.

Calibration results in one or more age intervals, with their width determined by the precision of the analysis and the shape of the calibration curve. Some periods of history are difficult to date due to unfavorable trends on the calibration curve, while others can be dated with high precision.

On the steep parts of the calibration curve (in blue), the width of the dating result is mainly influenced by the precision of the radiocarbon activity determination. The more precise it is (in red), the narrower the resulting interval (in green). The steepest section of the curve occurs at the so-called "bomb peak" in the mid-20th century when there was a sharp increase in radiocarbon activity due to nuclear weapons testing. This part of the curve allows for very precise dating of samples from this period.

During periods when radiocarbon activity fluctuated or stagnated (referred to as plateaus), the dating result breaks down into several intervals with approximately comparable probabilities, covering long time spans. Even the most precise determinations do not significantly improve this situation.