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IGERT program in Archeological Science

Principles of AMS

Radiocarbon dating

Theory

Pretreatment

Measurement

Correction

Age Calculation

Calibration

Cosmogenic Radioisotopes

Publication

 
 
 

Recognizing the need for calibration

    If the amount of 14C produced in the atmosphere were always the same, then we could calculate a "radiocarbon age" using the equation we have discussed directly as an estimate of sample age. Unfortunately, things are not quite that simple. This was recognized after Libby published his first "Curve of Knowns" [Arnold and Libby 1949]. It was observed that samples of known age from ancient Egypt did not fall exactly on the expected decay curve(as shown in the figure 1. below).

figure 1. Libby's first "Curve of Knowns"

Figure 1. Libby's first "Curve of Knowns"

    The cosmic rays striking the upper atmosphere vary in intensity with time by a small amount due to changes in the magnetic fields of the sun and the earth. Fluctuation in the amount of carbon dioxide in the atmosphere can also affect the concentration of 14C in the CO2. Some of these effects are illustrated in Figure 2, which shows the difference between the expected and measured 14C content in tree-rings over the last 8,000 years. The long smooth curve is a result of changes in the earth's magnetic field, whereas solar variations cause the more rapid fluctuations. We also know that many longer and shorter term changes in the relative amount of 14C in the atmosphere can also be signals of climatic changes as well as changes in cosmic radiation.

figure 2. a portion of the radiocarbon calibration curve

Figure 2. A portion of the radiocarbon calibration curve.
The vertical axis shows the measured radiocarbon age
and thehorizontal axis the true age based on tree rings.
Adapted from Taylor [1987].



Next page: Making the calibration