Radiocarbon (or Carbon-14) dating methods are used for dating materials of biological origin. Radioactive carbon is formed from the interaction of nitrogen in the atmosphere with cosmic rays impinging on the earth from outer space. The cosmic rays interact with the nucleus of the nitrogen, converting a proton into a neutron. This results in an unstable form of carbon with two extra neutrons, Carbon-14.
The half-life (the time required for half of a sample of radioactive atoms to disintegrate) of C-14 is about 5700 years. Since the amount of radioactive carbon in a modern sample is very small, (ratio of C14 to C12 is 1.3x10-12) only about 5 or 6 half-lives can be detected by normal methods, so the dating is limited to about 40,000 radiocarbon years. More recently, using mass spectrometric techniques, the dating has been extended in theory to 100,000 radiocarbon years, but this methodology appears to have generated more problems than solutions in general.
The accuracy of the method depends on our ability to determine with considerable precision, the atmospheric concentrations of carbon dioxide, and the equilibrium value for radiocarbon at the time the organism being dated was alive. These values are both subject to changes. The assumption that atmospheric concentrations of carbon dioxide have been constant is reasonably safe for the immediate past, although it does appear to be changing significantly at present due to the burning of fossil fuels. It is also conceivable that the earth's atmosphere has contained significantly higher concentrations of carbon dioxide in the past.
On the other hand, the equilibrium value for radiocarbon depends upon not only the carbon dioxide concentration, but on the rate of formation of C14 in the upper atmosphere. This value is far from certain in the past, and is itself dependent upon the rate of cosmic ray bombardment, which varies with sunspot activity and other factors. Whether cosmic rays can enter the earth's atmosphere depends upon the strength of the earth's magnetic field, something we know is changing, and has changed in the past.
Balancing these assumptions is the data from dendrochronology, the study of tree rings. Making the assumption that trees lay down an average of one ring per year, we can use the wood from ancient trees, such as the Bristlecone Pine, to check the accuracy of the assumptions involved in C14 dating. Such comparisons have demonstrated that for the recent past, the assumptions of C14 dating appear to be justifiable. However, the farther back in time we go, the farther the radiocarbon clock deviates from the dendrochronological clock, suggesting that beyond the range of the dendrochronological checkpoints, we may be dealing with significant unknowns. There are also reasons to challenge the standard Bristlecone chronology, which becomes C14 dependent somewhere around 2700 years B.C., the age of the oldest living tree.
Other dependencies are the assumption of a constant decay rate (probably the most reliable assumption) and the assumption of a closed system after death (no modern carbon or extinct carbon contamination).
It is possible to explain all of the dates from C14 studies within the bounds of a recent biosphere and global flood, if one allows for certain differeences in the past from present conditions. It is conceivable that on the preflood earth, the concentration of CO2 in the atmosphere was considerably higher than at present (it is possible that concentrations may have been as high as a few percent...100 times present concentrations) and/or the level of cosmic ray bombardment was lower or shielding via the earth's magnetic field was stronger than at present. In these cases, the level of C14 in the earth's atmosphere would have been considerably lower, yielding effectively infinite ages (>40,000 radiocarbon years) for organisms living in the preflood world. If subsequently, the level was increased, organisms living in close proximity to the flood would date successively younger, until the curve for atmospheric equilibrium became asymptotic, perhaps within the past few thousand years.
2010 Arthur V. Chadwick, Ph.D.