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In later years, the use of accelerator mass spectrometers and the introduction of high-precision carbon dating have also generated calibration curves.
A high-precision radiocarbon calibration curve published by a laboratory in Belfast, Northern Ireland, used dendrochronology data based on the Irish oak.
The tree rings were dated through dendrochronology.
At present, tree rings are still used to calibrate radiocarbon determinations.
These changes were brought about by several factors including, but not limited to, fluctuations in the earth’s geomagnetic moment, fossil fuel burning, and nuclear testing.
The most popular and often used method for calibration is by dendrochronology.
Libraries of tree rings of different calendar ages are now available to provide records extending back over the last 11,000 years.
The trees often used as references are the bristlecone pine (Pinus aristata) found in the USA and waterlogged Oak (Quercus sp.) in Ireland and Germany.
This tree-ring sequence, established by Wesley Ferguson in the 1960s, aided Hans Suess to publish the first useful calibration curve.Radiocarbon dating laboratories have been known to use data from other species of trees.In principle, the age of a certain carbonaceous sample can be easily determined by comparing its radiocarbon content to that of a tree ring with a known calendar age.It is also worth noting that the half-life used in carbon dating calculations is 5568 years, the value worked out by chemist Willard Libby, and not the more accurate value of 5730 years, which is known as the Cambridge half-life.Although it is less accurate, the Libby half-life was retained to avoid inconsistencies or errors when comparing carbon-14 test results that were produced before and after the Cambridge half-life was derived.
Results of carbon-14 dating are reported in radiocarbon years, and calibration is needed to convert radiocarbon years into calendar years.