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Tree ring cross dating
Using radioactive carbon (carbon-14 C-14) to determine age is a complex process.
By comparing many pieces of wood and combining matches, tree-ring chronologies of over 11,000 years extent have been proposed for use in correcting carbon-14 dates.
The reliability of the system is dependent on the correctness of the tree-ring matches, and here there is considerable uncertainty.
The Bristlecone Pine master dendrochronological sequence that has been foundational for C-14 calibration has been based on 81 living-wood and 118 dead-wood specimens from the White Mountains of California.
A calibration that falls within a time span that has been established by wood specimens that have been dated by unquestioned historical records (usually by cross-referencing C-14 ages) can be relied on to give a high precision estimate of real time.
However, these tests were not used when the original dendrochronological correction scheme for carbon-14 dates was established.
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It appears that this original scheme is subject to reevaluation.
In order to determine what real-time age should be associated with a radiocarbon age, the radiocarbon data are often compared to historical and tree-ring data that are considered to be more reliable indicators of time.
Tree-ring data are especially important in the correction process for dates older than 1000 BC.
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regarding the validity of a master tree-ring sequence in a range that has been extrapolated beyond an unquestioned historical reference point.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).