1. Tree rings are used to calibrate radiocarbon measurements;
2. Results calibration is necessary to account for changes in the global radiocarbon concentration over time;
3. Radiocarbon measurements are usually reported in years BP (Before Present), with zero BP defined as AD 1950;
4. Results of calibration are reported as age ranges calculated by the intercept method or the probability method, which use calibration curves;
5. The internationally agreed calibration curves for the period reaching as far back as 2500 BC are those produced by Gordon Pearson and Minze Stuiver;
6. Calibration curves have a dendro timescale on the x-axis and radiocarbon years on the y-axis;
7. In the case of radiocarbon dating, calibration is needed to convert radiocarbon years into calendar years.
There are two techniques in measuring radiocarbon in specimen samples—through radiometric dating and by Accelerator Mass Spectrometry (AMS). These mass spectrometers detect atoms of specific elements according to their atomic weights; however, they do not have the sensitivity to distinguish atomic isobars (atoms of different elements that have the same atomic weight, such as in the case of carbon 14 and nitrogen 14—the most common isotope of nitrogen).
Thanks to nuclear physics, mass spectrometers have been fine-tuned to separate a rare isotope from an abundant neighboring mass, and accelerator mass spectrometry was born. A method has finally been developed to detect carbon-14 in a given sample and ignore the more abundant isotopes that swamp the carbon-14 signal.
These two radiocarbon dating methods use modern standards such as oxalic acid and other reference materials. Although both radiocarbon dating methods produce high-quality results, they are fundamentally different in principle. Radiometric dating methods detect beta particles from the decay of carbon 14 atoms while accelerator mass spectrometers count the number of carbon 14 atoms present in the sample. Both carbon dating methods have advantages and disadvantages.
Two things to keep in mind about accelerator mass spectrometer dating, and that is it is very costly—establishing and maintaining an AMS costs millions of dollars. The other is, that because it uses such a small sample size, control of contaminants is difficult and rigorous pretreatment is needed to make sure contaminants have been eliminated and will not lead to substantial errors during the carbon dating process.
It is also important to keep in mind that radiocarbon measurements are reported in years BP, meaning “Before Present,” where “Present” or “0” is 1950 (not the current date, such as 2015). It is also important to know that half-life used in carbon dating calculations is 5568 years, the inaccurate value worked out by chemist Willard Libby, not the accurate value of known today of 5730 years, with Libby's known as the Cambridge half-life. Thus, the concept is flawed from the beginning, using less accurate measurement, but it was decided to do so to avoid inconsistencies or errors when comparing Carbon-14 test results that were produced before and after the correct half-life was determined. In addition, the BP dates are used because the system is based on the assumption that atmospheric Carbon-14 concentration has remained constant as it was in 1950.
Now we come to dendrochronology, or tree-ring dating, a system used to supplement radiocarbon dating figures when trying to extend dating beyond the time frame that the Carbon-14 is measurable within the specimen. In fact, dendrochronology played an important role in the early days of radiocarbon dating. Tree rings provided 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 tree rings were dated through dendrochronology.
At present, tree rings are still used to calibrate radiocarbon determinations. 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) and waterlogged Oak (Quercus sp.) in Ireland and German. In addition, radiocarbon dating laboratories have been known to use data from other species of trees as well.
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. 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, however, 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.
Calibration dates 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—the first calibration curve for radiocarbon dating was based on a continuous tree-ring sequence stretching back to 8,000 years, established by Wesley Ferguson in the 1960s, which aided Hans Suess to publish the first useful calibration curve that was based on the bristlecone pine and used tree rings for its calendar axis.
There have been many calibration curves published since Suess’s curve, but this proliferation brought more problems than solutions. 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.
Today, the internationally agreed calibration curves for the period reaching as far back as 2500 BC are those produced by Gordon Pearson and Minze Stuiver. There is no internationally agreed calibration curve for the period after 1950, but a great deal of data on atmospheric radiocarbon concentration is available. The problem is trying to find the details and methodology behind the published information. In fact, when digging deeper into dendrochronology we find that it is an opaque world because dendrochronologists tend to be a rather secretive bunch of Chronology Oracles, according to the British Heritage organization which several years ago reported the problem with the “mystery of the missing 4th century oak trees in England,” specifically those years from 434 B.C. to 315 A.D., in which it has resulted in conjuring up an England chronology for 404 A.D. through 1981 A.D. of overlapping tree-ring dates that result in patching and random floating of chronologies.
(See the next post, “How Far Back Can We Measure Dates? Part VI,” to see how this patching and floating of tree-ring dates has uncovered a huge gap in the dating sequence of tree-ring in the Middle Ages)
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