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Tree ring cross dating
The most recent determination of the spontaneous C-14 conversion rate indicates that, within an uncertainty of about 30 years, in 5715 years half of an initial amount of C-14 will have converted into N-14.
The initial C-14/C-12 ratio is not accessible to experimental determination, and must be assumed.
Accordingly, any C-14 age is based on an assumption.
The density difference between early and late growth produces visible features known as tree rings.
Variation in the width of these rings results from year-by-year variation in the conditions favorable to growth of a particular portion of a tree.
The less C-14 present in a sample, the older it will date.
To determine a date, one must have data concerning: At the best laboratories the C-14/C-12 ratio can be determined to about one-thousandth of the value that characterizes contemporary plants and animals.
Radiocarbon corrections beyond that are often based on attempts to match the thickness variations of tree rings in old wood samples.
If a similar pattern of variation in tree-ring thickness is found in two pieces of wood, the two are assumed to have grown at the same time.
But because of the uncertainty in matching a wood specimen against a master sequence only on the basis of growth-ring patterns, there is uncertainty regarding the validity of a master tree-ring sequence in a range that has been extrapolated beyond an unquestioned historical reference point. The flow that contained this log has been dated by stratigraphy (dating of rock layers) to have occurred within the range AD 1482-1668. The age of the growth-ring immediately adjacent to the bark is designated as the "bark date." Segments of 20 or more tree-rings beginning from either edge of this 290-ring sequence were compared for possible match against the Douglas fir master tree-ring sequence.
The magnitude of these uncertainties is indicated by tree-ring study of a Douglas fir log from a Mt. Computer-calculated coefficients of cross-correlation statistically significant at or beyond the p=0.001 level (99.9% confidence) indicated 113 possible bark dates within the range AD 1410-2240 (projected bark dates that are beyond the present are italicized).