Seasonal changes in cellular growth near the bark of a tree leave rings buried in its wood. The size of those records is tied to the growth of the. An independently developed tree ring chronology for bristlecone pine in the White Mountains, California, provides a basis for testing the. Tree rings provided truly known-age material needed to check the accuracy of the carbon dating method. A high-precision radiocarbon calibration curve published by a laboratory in Belfast, Northern Ireland, used.
The internal agreement of these American dendrochronologies confirmed that dendrochronologists are able to accurately match ring patterns. But another independent check came along which was even better than the Douglas fir chronology. European Tree-ring Chronology While American scientists were building bristlecone pine and Douglas fir chronologies, European scientists were actively building a very long tree-ring chronology using oak trees.
The more recent part of the chronology was constructed from oak logs used in various historic buildings. The more ancient part of the chronology was constructed from oak logs preserved in peat beds, for example.
The European oak chronology provided an excellent check of the American dendrochronologies. The two were obviously independent. Ring-width patterns are determined by local environmental factors, such as temperature and rainfall. The patterns in America could not bias the work on patterns in Europe, because the specimens came from two different local climates, separated by an ocean.
The scientists worked independently of one another. Also, oak trees and bristlecone pine or Douglas fir trees are very different. Bristlecones, for example, are evergreens which grow very slowly, at high altitude, in a cold, arid environment, and live for thousands of years. None of these things are true of the oaks used in the European chronology. They are deciduous, grow relatively rapidly, at low altitudes, in relatively warm, moist environments, and live for only hundreds of years.
If the science of dendrochronology was characterized by significant random error, the American and European tree-ring chronologies would certainly disagree with each other.
Are tree-ring chronologies reliable?
In fact, a comparison of the European and American chronologies showed very close correlation. The pattern of radiocarbon in the rings showed a maximum divergence, even at very old ages, of only around 40 years. This objective, quantitative test of dendrochronology showed it to be reliable and accurate.
Multiple Rings Per Year? These checks show that tree-ring chronologies are not subject to significant random error. However, some critics of dendrochronology go on to suggest that trees in ancient history grew multiple rings per year, perhaps due to Noah's Flood, for example.
A number of evidences argue strongly against such a claim. First, the agreement of independent chronologies from separate continents discussed above must be taken into account. If Noah's Flood, or some other phenomenon caused trees to grow multiple rings per year, it must have affected different species in widely separated locations in exactly the same way. This does not seem likely.
Second, radiocarbon dates on objects of known age have confirmed the reliability of radiocarbon dating, and hence dendrochronology, when applied to the last 2, years, at least. The radiocarbon dates on the Dead Sea Scrolls are a good example.
Thus we know that trees growing in the last 2, years or more haven't been growing multiple rings per year. Third is an argument which is perhaps the most definitive falsification of the idea that trees grew more than one ring per year in ancient history. Here is a greatly condensed version of this argument.
Our sun occasionally goes through periods of quiescence. During these periods few sunspots are seen on the sun's surface and the solar wind is reduced. This lets more cosmic radiation into the upper atmosphere of the earth, which allows more radiocarbon to be produced in the atmosphere.
These periods of quiescence occur in two varieties, one lasting an average of 51 years, and the other lasting an average of 96 years. How does this relate to tree-rings? During these periods of quiescence, atmospheric radiocarbon concentrations are higher. This difference in radiocarbon concentration is recorded in tree rings which are growing during the period of quiescence. If trees were growing two or three rings per year at the time one of these episodes occurred, two or three times as many rings would be affected than if trees were only growing one ring per year.
In other words, if trees were growing one ring per year, a year period of solar quiescence would affect 51 tree rings. If trees were growing three rings per year, a year period of solar quiescence would affect about rings. Thus, a record of ring growth per year is preserved in the number of rings affected by these periods of solar quiescence.
Radiocarbon Dating, Tree Rings, Dendrochronology
In fact, at least 16 of these episodes have occurred in the last 10, years. Dendrochronology and Carbon Dating 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.
- Are tree-ring chronologies reliable?
They can determine the exact calendar year each tree ring was formed. Dendrochronological findings played an important role in the early days of radiocarbon dating. Tree rings provided truly known-age material needed to check the accuracy of the carbon dating method. During the late s, 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, years.
The trees often used as references are the bristlecone pine Pinus aristata found in the USA and waterlogged Oak Quercus sp. Radiocarbon dating laboratories have been known to use data from other species of trees.
Radiocarbon Tree-Ring Calibration
Radiocarbon Tree-Ring Calibration 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, 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. 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.
Calibration Curves The first calibration curve for radiocarbon dating was based on a continuous tree-ring sequence stretching back to 8, years. This tree-ring sequence, established by Wesley Ferguson in the s, aided Hans Suess to publish the first useful calibration curve. 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. Nowadays, the internationally agreed upon calendar calibration curves reach as far back as about BC Reimer et.