Dendrochronology and radiocarbon dating

Radiocarbon Dating, Tree Rings, Dendrochronology

dendrochronology and radiocarbon dating

Dates from dendrochronology can be used as a calibration and check of radiocarbon dating. The field of dendrochronology had a developmental "head start" of at least several decades relative to the inception of radiocarbon dating in the late s, but. The field of dendrochronology had a developmental “head start” of at least several decades relative to the inception of radiocarbon dating in the.

The rings are more visible in trees which have grown in temperate zoneswhere the seasons differ more markedly. The inner portion of a growth ring forms early in the growing season, when growth is comparatively rapid hence the wood is less dense and is known as "early wood" or "spring wood", or "late-spring wood" [17] ; the outer portion is the "late wood" sometimes termed "summer wood", often being produced in the summer, though sometimes in the autumn and is denser.

Many trees in temperate zones produce one growth-ring each year, with the newest adjacent to the bark. Hence, for the entire period of a tree's life, a year-by-year record or ring pattern builds up that reflects the age of the tree and the climatic conditions in which the tree grew. Adequate moisture and a long growing season result in a wide ring, while a drought year may result in a very narrow one.

Direct reading of tree ring chronologies is a complex science, for several reasons. First, contrary to the single-ring-per-year paradigm, alternating poor and favorable conditions, such as mid-summer droughts, can result in several rings forming in a given year. In addition, particular tree-species may present "missing rings", and this influences the selection of trees for study of long time-spans. For instance, missing rings are rare in oak and elm trees.

Apologetics Press - Dating in Archaeology: Radiocarbon & Tree-Ring Dating

Researchers can compare and match these patterns ring-for-ring with patterns from trees which have grown at the same time in the same geographical zone and therefore under similar climatic conditions.

When one can match these tree-ring patterns across successive trees in the same locale, in overlapping fashion, chronologies can be built up—both for entire geographical regions and for sub-regions. Moreover, wood from ancient structures with known chronologies can be matched to the tree-ring data a technique called cross-datingand the age of the wood can thereby be determined precisely. Dendrochronologists originally carried out cross-dating by visual inspection; more recently, they have harnessed computers to do the task, applying statistical techniques to assess the matching.

To eliminate individual variations in tree-ring growth, dendrochronologists take the smoothed average of the tree-ring widths of multiple tree-samples to build up a ring history, a process termed replication.

dendrochronology and radiocarbon dating

A tree-ring history whose beginning- and end-dates are not known is called a floating chronology. This is a relatively simple matter if the ruins are only a few hundred years old.

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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.

Radiocarbon Tree-Ring Calibration

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.

dendrochronology and radiocarbon dating

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.

dendrochronology and radiocarbon dating

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. We could even measure the rate at which the candle is burning down. But how can we know when the candle was lit?

Dendrochronology

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.

dendrochronology and radiocarbon dating

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.

Intro to Dendrochronology - Thomas C Windes

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.

dendrochronology and radiocarbon dating

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.

Roughly 1, years later, the Flood upset the entire carbon cycle. Further, we know from the radiocarbon dating of tree rings that as we go back in time, we find less and less carbon