14C:12C ratio

Suppose the 14C/12C ratio in plants a thousand years ago was 10% higher than it is today. What effect, if any, would this have on the calculated age of an artifact found by the C-14 method to be a thousand years old ... 

Procedures for pretreatment of soil samples and synthesis of sample benzene for 14C analysis had been described in Chen et al. (2002b). Sample benzene was often left for 3-4 weeks to allow any radon with half-life of 3.82 days that may be present to decay. 14C activity of the CgFL was then determined using a 1220-QUANTULUS ultralow-level liquid scintillation spectrometer manufactured by WALLAC Company, Sweden. The 14C analyses were conducted at the Guangzhou Institute of Geochemistry, CAS. Results are reported as A14C, in parts per thousand of the 14C/12C ratio from that of the standard (oxalic acid decay corrected to 1950) (Stuiver and Po-lach 1977), and corrected for bomb 14C (Chen et al. 2002b), where ... 

This technique is useful only when dating objects that are less than 50,000 years old (roughly 10 times the half-life of carbon-14). Older objects have too little activity to be accurately dated. This technique depends on cosmic-ray intensity being constant or at least predictable in order to keep the 14C/12C known throughout the time interval. Also, the sample must not be contaminated with organic matter having a different 14C/12C ratio. 

Finally, in Table 1 are shown the Rochester results for an Australian sucrose sample which is being considered as a new contemporary standard and which is being measured by all the major radiocarbon dating laboratories worldwide. The 14C/12C ratio compared to that for the ratio in A.D. 1950 is shown along with an indication of the range of values obtained at other values. The sample sizes employed range from about 1 to 10 milligrams. 

Since the work of de Vries [l,2]x, Willard Libby s [3] basic assumption of a constant atmospheric 14C/12C ratio has been known... 

The production of 14C by cosmic rays has remained constant long enough to have established a steady-state or equilibrium in the 14C/12C ratio in the atmosphere ... 

C-dating of travertines is possible in some instances the initial 14C/12C ratio of speleothem or travertine will generally be less than that of the atmosphere at the time of deposition, due to admixture of dead carbon from the limestone from which the Ca and part of carbonate have been obtained. But, where active... 

Broecker and Li (1970) and Broecker (1974) found that the 14C/12C ratio in the deep ocean was 84 percent of this ratio in the pre-bomb surface ocean. Assuming that surface carbon (dissolved and falling debris) is the only source of deep ocean carbon, calculate the residence time tc of this element in the deep-ocean. The 14C decay constant is 1.2 x 10 4a 1. 

AMS is used for the dating of archeological artifacts because the carbon composing living organisms has a 14C/12C ratio determined by the 14C levels in the... 

The 14C content of SOM decreases with depth in the soil profile (Martin et al., 1990). An estimate of the passive pool could be obtained by measurements of the 14C of SOM in deeper layers (Harrison and Broecker, 1993). Physical fractionation and sequential extraction have also been used, and they have shown progressively lower 14C/12C ratios in decomposing residues. 

Figure 6.8. Change in atmospheric 14C with time in the Northern Hemisphere (heavy solid line) since 1955. Radiocarbon values are expressed as the per mil variation in 14C/12C ratio relative to a standard (see Appendix for definition of units).The lighter lines show the evolution of 14C for homogeneous, steady-state reservoirs with turnover times of 5,15, 60, and 120 yr.

The concentration of 14C is expressed in relation to the 14C 12C ratio in an international standard (oxalic acid). The 14C concentration in the bulk carbon of the standard is defined as 100% modern carbon (pmc). Thus a tree that died will contain only 50 pmc after 5730 years (one half-life). The 14C concentration of a sample is measured in specialized counting laboratories. 

Since the industrial revolution of the early nineteenth century, large amounts of fossil fuels (oil, coal, gas) have been combusted, causing an increase of about 10% in the concentration of atmospheric C02. This added fossil C02 was devoid of 14C and, correspondingly, lowered the 14C 12C ratio in the air by about 10%. 

The last two hypothetical examples are correct only if the groundwater behaves as a closed system with regard to its 14C 12C ratio. Table 11.2 presents 14C data of basalt aquifers in Hawaii of mainly pre-bomb age, as reflected in the low tritium concentrations. The waters are seen to contain up to 100 mg HC03/1 and have 14C values exceeding 91 pmc in seven out of eight reported pre-bomb cases. It seems, thus, that little or no 14C was lost due to reaction with rocks, and little or no dead carbon (i.e., devoid of 14C) was added by interactions with aquifer rocks, a fact reflected also in the low HCO3 concentration. 

In another field, Litherland et al. [80] succeeded in determining a 14C/12C ratio of 1 1015 and hence in dating a 40000-year-old sample with a 1 % error. A quantity of 14C corresponding to only 10s atoms was able to be detected in less than 1 mg of carbon [81]. 

Carbon-14, a /S emitter with a half-life of 5720 years, is produced in the atmosphere by the reaction N(n, p) C, the neutrons coming from cosmic rays. The dating is based on the assumption that the amount of C in the atmosphere (the ratio of i C/ C) remains constant over thousands of years. A living species incorporates this same ratio in all its carbon-containing molecules. When it dies, the incorporation of C ceases, and the C decays. The 14C/12C ratio determined from a wood sample taken from a dugout canoe at the bottom of a lake was found to be one-tenth the ratio determined from a wood sample less than 1 year old. How old is the dugout canoe ... 

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