Radiocarbon fractions

The half-life (t1 ) of a radioisotope is the amount of time it takes for that isotope to undergo radioactive decay and be converted into another. It is also a measure of the stability of the isotope the shorter its half-life, the less stable the isotope. The half-life of radioisotopes ranges from fractions of a second for the most unstable to billions of years for isotopes that are only weakly radioactive. In the case of radiocarbon (carbon-14), for example, the half-life is 5730 years (see Fig. 61). 

The relative concentration of radiocarbon in the samples dated is corrected for isotope fractionation, based on the ratio C-13/C-12 (see text below). 

In this way, it is possible to reach an extremely high selective sensitivity down to 1 part in 1015, which in 14C dating corresponds to being able to date samples about 50 000 years old. Moreover, modern systems can measure isotopic ratios in modern carbon, both C/ C and C/ C, with an ultimate precision as good as 2%o and l%o, respectively. The former value corresponds to determining the conventional radiocarbon age with an absolute error, smaller than in the past, better than 20 years, while the l%o precision for the 13C/12C allows an adequate correction for isotopic fractionation effects. Even in routine measurements, at least in the case of historical samples, a precision of 5%o in the 14C/12C measured value is standard, corresponding to an uncertainty in the radiocarbon age of 40 years.[27]... 

The measurement of both 14C and 12C is necessary to estimate the radiocarbon age on the basis of the isotopic ratio 14R measuring 13C allows us to evaluate the effects of isotopic fractionation, according to which the measured radiocarbon concentration can be corrected. For details, see Aitken.1... 

Agren GI, Bosatta E, Balesdent J (1996) Isotope discrimination during decomposition of organic matter a theoretical analysis. Soil Sci Soc Am J 60 1121-1126 Balesdent J (1987) The turnover of soil organic fractions estimated by radiocarbon dating. Sci Total Environ 62 405-408... 

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. 

The values of E(t) so computed are listed in Table 4. The correction for fractionation of carbon dioxide at the sea surface is a serious one. It makes the interpretation of 13C/12C variations in wood difficult and militates against the use of the isotope ratio of carbon as a thermometer. This correction, when applied to variations of carbon-14 in wood, is able to explain the Suess radiocarbon "wiggles" of about 100 years duration each, without the need to invoke changes in the neutron flux from the sun [54]. 

Taylor, R. E., Slota, P., Fraction Studies on Marine Shell and Bone Samples for Radiocarbon Analyses, In Radiocarbon Dating, pp. 422-432, Berkeley University of California Press, 1979. 

The analyses involved the exposure of developed chromatograms to x-ray films followed by scraping of radioactive spots and quantifying them. Relative amounts of various compounds in the fractions are indicated on the righthand side against numbered spots. Spots A-6, B-4, and C-5 represent unchanged cis-chlordane A-7 was a mixture of dichloro-chlordene and oxychlordane. A-3, B-3, C-4, and D-4 represent chlordene chlorohydrin. A-2, B-2, C-2, and D-2 were complex spots with the heptachlor diol as the major compound. A-l, B-l, C-l, and D-l were polyhydroxy derivatives or conjugates. Identities of other spots are not known. Recoveries in fractions A, B, C, and D were, respectively, 60.1%, 0.6%, 2.2%, and 2.9% of the applied radiocarbon. 

Unfortunately, most of the DOM in seawater is LMW (75 to 80%) and its chemical composition has not been as well studied as that of the HMW fraction. LMW DOM is thought to be composed primarily of biopolymers containing 10 or fewer monomers. Radiocarbon measurements indicate LMW is older than HMW DOM, suggesting that LMW is fer less reactive than HMW DOM. 

Balesdent, J. (1987). The turnover of soil organic fractions estimated by radiocarbon dating. Sci. Total Environ. 62, 405-408. 

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