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Nowadays, the internationally agreed upon calendar calibration curves reach as far back as about 48000 BC (Reimer et.
al., INTCAL13 and Marine13 radiocarbon age calibration curves 0 – 50000 yrs cal BP, Radiocarbon 55(4), 2013).
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.
It is also called “radiocarbon” because it is unstable and radioactive relative to carbon-12 and carbon-13.
Carbon-14 is a naturally occurring isotope of the element carbon.And indeed, results of calibration are often given as an age range rather than an absolute value.Age ranges are calculated either by the intercept method or the probability method, both of which need a calibration curve.If a sample has the same proportion of radiocarbon as that of the tree ring, it is safe to conclude that they are of the same age.In practice, tree-ring calibration is not as straightforward due to many factors, the most significant of which is that individual measurements made on the tree rings and the sample have limited precision so a range of possible calendar years is obtained.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.Tree rings provided truly known-age material needed to check the accuracy of the carbon-14 dating method.During the late 1950s, several scientists (notably the Dutchman Hessel de Vries) were able to confirm the discrepancy between radiocarbon ages and calendar ages through results gathered from carbon dating rings of trees.The science of dendrochronology is based on the phenomenon that trees usually grow by the addition of rings, hence the name tree-ring dating.Dendrochronologists date events and variations in environments in the past by analyzing and comparing growth ring patterns of trees and aged wood.