Apologetics Press - Dating in Archaeology: Radiocarbon & Tree-Ring Dating
The growth rings of an unknown tree species, at Bristol Zoo, England is the method of scientific dating based on the analysis of tree-ring growth patterns. The tree was a member of a population of bristlecone pine trees growing at treeline . These tools have been important in archaeological dating of timbers of the cliff. Recently, the technique of tree-ring dating (dendro chronology) as applied to bristlecone pines has figured prominently in discussions of the validity and. tree-rings in the bristlecone pines of the White Mountains of California from the can be corrected by the system of cross dating between trees worked out by . care of the laboratory measurements without any delays because of equipment.
Both radioactive and nonradioactive 12C,13C forms of carbon can react with oxygen to form carbon dioxide, which becomes part of the atmosphere. From here it can enter plants by respiration, animals by feeding, and the oceans by exchange with the atmosphere Figure 1. The part of radiocarbon in the carbon cycle Early in these studies, Willard F.
Libby and his coworkers realized that they could use this process as a tool for dating objects containing carbon. Take, for instance, a piece of charcoal from an ancient campsite. While the wood was alive and growing, it was taking in carbon dioxide. Its ratio of common carbon to radioactive carbon closely matched the ratio in the surrounding air. But after that ancient camper cut it for firewood, it no longer took in carbon dioxide. The carbon slowly decayed, while the amount of carbon stayed the same.
Theoretically, if we know the ratio of these two isotopes, and the decay rate, we can calculate the radiocarbon age of the charcoal. The decay rate for carbon, expressed as a half-life, is years e. Measurement Limits Until the last few years, laboratories measured carbon content indirectly by extracting all the carbon from a sample and then counting its radioactive emissions. Unfortunately, many of these systems required relatively large samples to obtain accurate results.
Archaeologists faced the dilemma of either preserving or dating their precious finds. The application of accelerator mass spectrometry AMS to carbon isotope analysis has changed this picture dramatically.
An AMS system has the advantage of counting individual carbon atoms. However, being able to measure tiny amounts of carbon is not the same as proving that objects are thousands-of-years old. Radiocarbon Assumptions and Problems Like other radiometric methods, radiocarbon dating faces technical problems and operates under some questionable assumptions.
Dendrochronology - Wikibooks, open books for an open world
Perhaps the most critical assumption of radiocarbon dating is that the rates of carbon production and decay are in a state of balance or equilibrium, and have been so for millions of years. However, we have reason to think that this is not true, as we will see in a later section. Radiocarbon dating assumes a constant decay rate for the breakdown of carbon At present, we have no firm evidence for any systematic change in this rate.
Contamination by groundwater, soil, or foreign matter is always a potential problem. However, people working with radiocarbon dating feel confident that good sample collection can overcome this problem. Some organisms may exclude the heavier carbon isotopes preferentially, making them look too old e. Comparison of carbon and carbon with the stable isotope carbon is supposed to correct this problem see Aitken,pp. This venerable science began in the early part of the twentieth century when A. Douglass was looking for a way to investigate the historical relationship between solar activity and climate.
He noticed variations in the width of annual growth rings in yellow pine trees growing around Flagstaff, Arizona. The year-to-year variations were the result of changes in rainfall, while the larger patterns were perhaps the result of some longer-term trend. Douglass used a cross-identification system to match patterns in trees of the same age. He later extended his work to the giant redwoods of California. Eventually he had a chronology going back more than three thousand years.
In the mids, Douglass began to apply tree rings to dating in archaeology. 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.
Removal of the bark of the tree in a particular area may cause deformation of the rings as the plant overgrows the scar. 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"  ; 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.
Geochronology/Dendrochronology - Wikiversity
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.
The alteration of light and dark wood is what makes the ring easily recognized and counted to determine the age of the tree. The width of the ring reflects the growth rate of the tree in a particular year. The growth rate depends greatly on the amount of rain during the growing season. The width of a ring can be used to reconstruct rainfall patterns in the past.
Understanding the climate in the past helps us to understand the current and predict future climate trends.
Click here for more information on tree rings Why is crossdating important? Occasionally, trees will produce more than one ring in a year. The extra ring is called a false ring and it can be the result of drought stress in the middle of a growing season.
Other times a tree can go a year without producing a ring. This can happen when the tree is suffering from severe stress like being burned in a fire, attacked by insects, or adverse weather conditions. When you crossdate you use a tree that doesn't have a missing ring to find where other trees do have missing rings. Click here for more information on crossdating back to list Analyze and Interpret the Data After collecting the increment cores and counting the tree rings, the scientists needed a way to organize all the information.
They chose to display it in a table. Using a table they could keep track of all the information that they collected and calculated with mathematics on each tree. When they calculated the final results, they were happy to find that their hypothesis was correct.
There are Rocky Mountain bristlecone pines older than 1, year old, much older! They found 12 trees older than 1, years. They discovered that four trees were more than 2, years old.
This is a very common way that scientists communicate to one another the process and results of their work. In the paper they described each aspect of their investigation including: The question that they wanted to answer: How they selected their study sites and trees The methods that they used to determine tree age The result of their investigation Discussion and explanation of the results Critical Thinking Questions What tools and techniques did the scientists use to gather the data?
How did the scientist use mathematics in their investigation? How did the scientist critical thinking skills help them to avoid making errors when counting the rings?