Tree, radiocarbon dating

The next problem was which trees to measure. Many tree ring sequences can be counted with an accuracy of about one year. Those which are not yet tied to modern sequences by overlapping ring patterns (said to be "floating"), can be dated in favorable cases with an accuracy of about 30 years by radiocarbon dating, depending on the age and the number of radiocarbon measurements which are made. 

Obviously this wide distribution of the 14C formed in the atmosphere lakes time it is believed to require a period of 500-1000 years. This time is not. however, a deterrent to radiocarbon dating because of two factors die long half-life of I4C and the relatively constant rate of cosmic-ray formation of l4C in the earth s atmosphere over the most recent several thousands of years. These considerations lead to the conclusion that the proportion of 14C in the carbon reservoir of the earth is constant, and that the addition by cosmic ray production is in balance with the loss by radioactive decay. If this conclusion is warranted, then the carbon dioxide on earth many centuries ago had the same content of radioactive carbon as the carbon dioxide on earth today, Thus, radioactive carbon in the wood of a tree growing centuries ago had the same content as that in carbon oil earth today. Therefore, if we wish to determine how long ago a tree was cut down to build an ancient fire, all we need to do is to determine the relative 14C content of the carbon in the charcoal remaining, using the value we have determined for llie half life of 14C. If the carbon from Ihe charcoal in an ancient cave has only as much 14C radioactivity as does carbon on earth today, then we can conclude that the tree which furnished llie firewood grew 5730 30 years ago. 

Clark, R. M., Renfrew, C., "Tree-Ring Calibration of Radiocarbon Dates and the Chronology of Ancient Egypt, Nature (1973) 243, 266. 

Floating Tree-Ring Chronologies Using Radiocarbon Dates, Archaeometry (1972) 14,519. 

The isotopic isolation of in each tree ring had crucial implications for future radiocarbon studies and continues to be very much a part of current research. Specifically, the examination of the radiocarbon contents in tree-ring-dated samples of wood provided the principal data that permits an in-depth examination of the assumption that the production of radiocarbon by cosmic rays had been maintained at a constant level. For archaeologists especially, it is important to know if there have been... 

Hiller A., Boettger T., and Kremenetski C. (2001) Medieval chmatic warming recorded by radiocarbon dated alpine tree-hne shift on the Kola Peninsula, Russia. LLolocene 11, 491 97. 

Study of very old trees, such as the bristlecone pines in California, have allowed researchers to develop calibration curves that adjust the results of radiocarbon dating experiments for the variation in the ratio that... 

The size of tree rings can be related to CO2 content, where the number of rings indicates the age of the tree. The amount of CO2 in ice can be directly measured from portions of polar ice in different layers obtained by drilling. The age of CO2 can be determined by radiocarbon dating and other methods. 

In radiocarbon dating, a reasonable assumption is that the ratio of carbon-14 to carbon-12 in the atmosphere has been relatively constant for the past 50,000 yr. However, because variations in solar activity control the amount of carbon-14 produced in the atmosphere, that ratio can fluctuate. We can correct for this effect by using other kinds of data. Recently scientists have compared carbon-14 data with data from tree rings, corals, lake sediments, ice cores, and other natural sources to correct variations in the carbon-14 clock back to 26,000 yr. 

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