Dating annual tree growth rings of known historical age has demonstrated that in the past there have been short term variations in atmospheric C levels.
Tree ring dating has been used to construct a probabilistic calibration curve extending back to 11857 calendar years BP (see figure below).
This method can be used to radiocarbon date one milligram of carbon or less and has the added advantage that the small sample size permits more selective sampling.
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.Price depends on the nature of the material and turnaround time required; the basic price for analysis with a turnaround time of less than 3 weeks is £395 plus the cost of sample preparation. C ratio therefore remains in equilibrium with the atmosphere.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.The average difference between a radiocarbon date of a terrestrial organism such as a tree and a marine shell of the same age is about 400 radiocarbon years.Thus, it is not possible to accurately date organisms which have died since 1950.