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In addition, samples need to be thoroughly cleaned to remove carbon contamination from glues and soil before dating.

This is particularly important for very old samples.

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.

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

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

The total mass of the isotope is indicated by the numerical superscript.

While the lighter isotopes C has decayed that what remains can no longer be measured. In 5,730 years half of the C in the atmosphere, and therefore in plants and animals, has not always been constant.

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Tree rings can be counted and their radiocarbon content measured.

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.

Australian National University provides funding as a member of The Conversation AU.

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