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Tree ring dating has been used to construct a probabilistic calibration curve extending back to 11857 calendar years BP (see figure below).
This method is not without its own inaccuracies and has limited precision, but it does provide dates in terms of calendar years.
In addition, material from the last 300 years gives unreliable ages, mainly due to the widespread burning of fossil fuels and more recently the explosion of nuclear bombs, both of which have artificially increased the amount of C in the atmosphere.
Thus, it is not possible to accurately date organisms which have died since 1950.
This can be done manually, but there are now computer programs in the public domain which will provide a more consistent and accurate calibration.
Because the calibration curve is not a straight line it is possible to have many intercepts on the calendar year axis, each with its own probability range.
Two other U-Th dates provide a tentative extension to the INTCAL98 curve which has been used in this report to provide a calibration curve back to about 40k years BP.
Suitable specimens are selected by picking through the residue.Measurement of the amount of C remaining in a dead organism will therefore give the date of its death.