How does radiocarbon dating
Using the INTCAL98 curve it is possible to convert radiocarbon years to calendar years by projecting the radiocarbon age onto the curve and observing the intercept on the calendar year axis.This can be done manually, but there are now computer programs in the public domain which will provide a more consistent and accurate calibration.In Nyerup's time, archaeologists could date the past only by using recorded histories, which in Europe were based mainly on the Egyptian calendar.
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.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).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.This difference which is called the reservoir age is caused both by the delay in exchange rates between atmospheric CO and oceanic bicarbonate, and the dilution effect caused by mixing surface waters with upwelling deep waters which are very old.A reservoir correction factor must therefore be applied to conventional radiocarbon dates based on the remains of marine organisms.The INTCAL98 calibration curve thus constructed has been accepted by international consent (see Stuiver et al., 1998).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."Everything which has come down to us from heathendom is wrapped in a thick fog; it belongs to a space of time we cannot measure.We know that it is older than Christendom, but whether by a couple of years or a couple of centuries, or even by more than a millenium, we can do no more than guess." [Rasmus Nyerup, (Danish antiquarian), 1802 (in Trigger, 19)].