Apologetics Press - Dating in Archaeology: Radiocarbon & Tree-Ring Dating
Dendrochronology and Radiocarbon Dating: The Laboratory of Tree-Ring Research Connection. Steven W Leavitt, Bryant Bannister. inception of radiocarbon dating in the late s, but that evolution was sufficiently advanced so tree-ring science could assure success of the 14C enterprise. rings from the beginning of this century until and (2) carbon 14 This means that a typical single radiocarbon date for wood or charcoal.
Tree rings are used to calibrate radiocarbon measurements. Calibration is necessary to account for changes in the global radiocarbon concentration over time. Results of calibration are reported as age ranges calculated by the intercept method or the probability method, which use calibration curves.
The internationally agreed calibration curves for the period reaching as far back as BC are those produced by PJ Reimer et al. Calibration curves have a dendro timescale on the x-axis and radiocarbon years on the y-axis.
Calibration is not only done before an analysis but also on analytical results as in the case of radiocarbon dating —an analytical method that identifies the age of a material that once formed part of the biosphere by determining its carbon content and tracing its age by its radioactive decay.
Carbon is a naturally occurring isotope of the element carbon. Results of carbon dating are reported in radiocarbon years, and calibration is needed to convert radiocarbon years into calendar years.
It should be noted that a BP notation is also used in other dating techniques but is defined differently, as in the case of thermoluminescence dating wherein BP is defined as AD It is also worth noting that the half-life used in carbon dating calculations is years, the value worked out by chemist Willard Libby, and not the more accurate value of 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 test results that were produced before and after the Cambridge half-life was derived. Radiocarbon measurements are based on the assumption that atmospheric carbon concentration has remained constant as it was in and that the half-life of carbon is years.
Calibration of radiocarbon results is needed to account for changes in the atmospheric concentration of carbon over time. The most popular and often used method for calibration is by dendrochronology. 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.
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.
In the mids, Douglass began to apply tree rings to dating in archaeology.
Radiocarbon Tree-Ring Calibration
His idea was to match ring patterns in the timbers of Native American structures, with the ring patterns in yellow pines. This is a relatively simple matter if the ruins are only a few hundred years old. But if they predate the living trees, then it is necessary to use indirect methods. Douglass bridged the gap by overlapping patterns of successively older timbers. This classic technique is called cross dating. From this longest-living of all trees, they have constructed a chronology going back almost ten thousand years.
For example, say we wanted to date a piece of German oak furniture. We could try to match a pattern of rings on the furniture, with a pattern of rings in living oaks from a forest near to where it was made. Using our tree-ring chronology for German oaks, we might get a date of A.
In contrast, if we applied radiocarbon dating, all we could say is that the piece dates to sometime in the seventeenth century. Problems with Tree-Ring Dating The most questionable assumption in dendrochronology is the rate of ring formation.
General principles of biology and climate suggest that trees add only one ring each year. Individual bristlecone pines, which grow very slowly in arid, high altitude areas of western North America, will sometimes skip a year of growth.
This might make a tree appear younger than it really is, but dendrochronologists fill in the missing information by comparing rings from other trees. However, trees would appear too old if they grew more than one ring per year. Most dendrochronologists, drawing on an influential study by LaMarche and Harlanbelieve that bristlecone pines do indeed add only one ring per year.
Yet not all scientists accept this study. According to Harold Gladwinthe growth patterns of the bristlecone trees are too erratic for dating. Lammerts found extra rings after studying the development of bristlecone saplings. He suggested that the existing chronology should be compressed from 7, to 5, years.
Other problems relate to the analysis of growth-ring patterns. As with conventional jig-saws, some people are better at pattern recognition than others and, if the analogy is not too brutal, there are those who recognise the problems, and those who might try to force the pieces together.
It has to be remembered that there is only one correct pattern: Simply because two pieces look alike does not necessarily mean that they fit togetherp. Computers can provide an important tool for some of this analysis.
But researchers must still judge the statistical significance of an apparent match. Also, they must consider variables like local climate and aging, which affect the width of the rings. However, we do not know the ratio at the time of death, which means we have to make an assumption. In other words, the system of carbon production and decay is said to be in a state of balance or equilibrium.
Yet this assumption is questionable, even for an old Earth.
The problem is akin to a burning candle cf. Without stretching the analogy too far, let us imagine that the wax represents carbon We could take a ruler and measure the length of the remaining candle.
Radiocarbon Dating, Tree Rings, Dendrochronology
We could even measure the rate at which the candle is burning down. But how can we know when the candle was lit? We simply cannot answer this question without knowing the original length of candle. Perhaps we could make a guess from a nearby unlit candle, but it would only ever be a guess. In the old-Earth model, the process of making carbon began billions of years ago.
The evolving atmosphere filled rapidly with carbon, but this rate slowed as carbon found its way into the oceans and the biosphere. Eventually, the carbon would break down into nitrogen, thus completing the cycle. Geologists freely admit that this process has not always been in equilibrium, but they maintain that this will not affect the radiocarbon method in any practical way.
He settled on a specific decay rate SDR of Libby never seriously questioned the discrepancy between these two numbers. He felt that his method was accurate, and that the numbers were close enough. These problems encouraged a systematic study in which researchers used the radiocarbon method to date tree rings.
Two levels of error emerged. One was a small-scale, short-term variation that can make a given radiocarbon date appear up to four hundred years older or younger than expected Taylor,Figure 2. Much of this error may be the result of sunspot activity, which in turn affects solar radiation and the production of carbon A second error comes from an S-shaped, long-term trend Figure 2. One bend of the curve peaks in the middle of the first millennium A.
Radiocarbon ages during this period overestimate dendrochronological ages by up to a hundred years. The curve switches direction around B. The discrepancy grows as we go back in time, so that by the fifth millennium B. Major trend in the plot of dendrochronology vs. Dates above dashed zero line overestimate tree-ring ages; dates below underestimate tree-ring ages after Taylor,Figure 2.
No one can explain this major trend adequately on the assumptions of an old Earth or an equilibrium system. Not only are these the most significant events to have ever affected the physical world, but they occurred over a relatively short time span of only a few thousand years.
In a world with such a history we would expect nonequilibrium conditions. Production of carbon began only 6, years ago—the approximate time of Creation.