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26-Jun-2020 10:39

Radiocarbon dates are presented in two ways because of this complication.

The uncalibrated date is given with the unit BP (radiocarbon years before 1950).

Radioactive decay can be used as a “clock” because it is unaffected by physical (e.g. For instance, the amount varies according to how many cosmic rays reach Earth.

This is affected by solar activity and the earth’s magnetic field.

Luckily, we can measure these fluctuations in samples that are dated by other methods.

Tree rings can be counted and their radiocarbon content measured.

The second difficulty arises from the extremely low abundance of C, making it incredibly difficult to measure and extremely sensitive to contamination.

In the early years of radiocarbon dating a product’s decay was measured, but this required huge samples (e.g. Many labs now use an Accelerator Mass Spectrometer (AMS), a machine that can detect and measure the presence of different isotopes, to count the individual C atoms in a sample.

A huge amount of work is currently underway to extend and improve the calibration curve.

The calibrated date is also presented, either in BC or AD or with the unit cal BP (calibrated before present - before 1950).

The calibrated date is our “best estimate” of the sample’s actual age, but we need to be able to return to old dates and recalibrate them because new research is continually used to update the calibration curve.

In 2008 we could only calibrate radiocarbon dates until 26,000 years.

Now the curve extends (tentatively) to 50,000 years.

Rachel Wood does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.