Dating tree rings

Radiocarbon years are calibrated from determinations of the 14C activity and stable isotopic carbon ratios of dendrochrono-logically dated tree rings [4]. The stable isotope data are required to normalize the dates to average wood with 613C value of -25 per mil (13C/12C fractionation relative to PDB reference standard). Photosynthetic and other plant physiological processes may produce differential isotopic fractionation between species, within the same species in different localities and even within the same tree under changing environmental conditions. 

Significant interpretative problems in the use of radiocarbon values are apparent immediately when the C-dated tree ring data are examined (Figures 3 and 4). If we ignore for the moment the short-term perturbations, the radiocarbon and dendrochronological values approximately agree ( 1-3%) for the last 2000 years. However, as one moves back in time from about the beginning of the Christian era, there is an increas-... 

Moreover, there are fluctuations in the production rate so that the initial value of ratio in the atmosphere and surface ocean also fluctuates proportionately. This variation has been observed in independently dated tree rings on land and in coral accumulations in the sea (Bard et al., 1990). All these factors plus the intrinsic properties of accumulation, bioturbation, and preservation must be considered to translate a ratio to an age expressed in calendar years. 

A fundamental assumption made for most dating with atmospheric radionuclides is that the cosmic radiation flux and hence, the natural production of the radionuclides has been constant with time. Various studies of this problem using 14C and tree-ring calibration have been made. Isotopic studies of meteorites have also been useful [17]. Considering the probable lack of basic accuracy of dating water, the problem of changes in cosmic ray flux is not serious. 

Stokes, M. A., Smiley, T., An Introduction to Tree-Ring Dating, The University of Chicago Press, Chicago, 1968. 

It is necessary to calibrate the 14C time scale for greater dating accuracy. However, the second-order variations are at least as important as the first-order constancy of atmospheric 14C. For example, they provide a record of prehistoric solar variations, changes in the Earth s dipole moment and an insight into the fate of C02 from fossil fuel combustion. Improved techniques are needed that will enable the precise measurement of small cellulose samples from single tree rings. The tandem accelerator mass spectrometer (TAMS) may fill this need. 

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. 

As pointed out by Muller (1978), there are well-documented differences between the ages of materials determined by dating with radioisotopes and the ages determined by other means, such as tree-ring counting. In addition to systematic effects, there are statistical errors due to the limited number of atoms observed. Both types of errors can be considered to be fluctuations in n, the number of atoms observed. A relationship can be derived between the magnitude of these fluctuations and the resulting error in the estimation of age of the sample ... 

Tree-ring counting as a measure of age is described under Pine Trees. However, it is of note to mention an August 1990 report on how tree rings were used to reveal the age of the oldest road." which was found m a peat bog in 1970. The road is located in southwestern England. It dates back 4000 years. 

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