Radiocarbon carbon fractionation

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 first uses of radiocarbon in deep-sea core dating were based on few data points and depended on extrapolation assuming the constant rate of titanium deposition (Arrhenius et al., 1951) or interpolation (Suess, 1956) for determination of rates of accumulation and chronology. The first systematic study of radiocarbon incorporating possible changes in accumulation rates with depth in a core was performed by Broecker et al. (1958). They showed that accumulation rates of both the carbonate fraction and the detrital fraction varied with time in the equatorial Atlantic and those variations were linked to paleoclimatic indicators inferred from paleontologic data (Figure 3). 

Once the aggregate nature and the occurrence of other carbonate materials are estabhshed, the binder carbonates are separated by a combined mechanical and physical procedure (Ortega et al., 2008). This method removes the carbonate fraction, lime lumps and the charcoal particles. The extraction procedure allows to obtain binder reliable for dating without using partial acid digestion and several radiocarbon measurements of complex interpretation. In order to test the effectiveness of mechanical separation and to verify the purity of the binder. Scanning Electron Microscope (SEM), X-ray diffraction (XRD) analyses and thermogravimetric analysis (TGA) were performed. To test the developed procedure, historic lime mortars from the parish church of Santa Maria la Real (Zarautz, northern Spain) have been dated. 

The burial of the palaeopodzol took place after 1200 AD. This is still in line with the period of forest clear cutting. Time, available for the development of the micropodzol (2S), de stable period between the depositions of S2 and S3, was (based on OSL datings) maximal 130. The radiocarbon ages of the carbon fractions, extracted from the 2Ah (with the exception of FUL) look too old, but the OSL age of the burial of the micropodzol is fits with the heat degradation after the introduction of deep stable management. 

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]... 

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]. 

Castanha, C., Trumbore, S., and Amundson, R. (2008). Methods of separating soil carbon pools affect the chemistry and turnover time of isolated fractions. Radiocarbon. 50(1), 83-97. 

Megens, L., van der Plicht, de Leuw, J.W., and Smedes, F. (2002) Stable carbon and radiocarbon isotope composition of particle size fractions to determine origins of sedimentary organic matter in an estuary. Qrg. Geochem. 33, 945-952. 

Trumbore, S., Vogel, J.S., and Southon, J. (1989) AMS 14C measurements of fractionated soil organic matter an approach to deciphering the soil carbon cycle. Radiocarbon 31, 644-654. 

Fractionation and Contamination. The ratio 14C/12C in certain materials may be affected by isotopic fractionation. For example, the uptake of carbon dioxide and its incorporation into plant tissue may be accompanied by substantial fractionation that depends on the plant species. With marine organisms, fractionation may also be important, especially when inorganic carbonate and bicarbonate are involved. Corrections for fractionation must be made for precise radiocarbon dating. 

Table II. Radiocarbon Determinations on the Carbonate, Collagen, and Amino Acid Fractions of Two Human Bone Samples...

Because of the large number of conditions and factors which can influence variations in the carbon isotope ratios of certain types of samples, this aspect of radiocarbon studies especially in archaeological contexts has been perhaps the least understood but the most often cited as an explanation for anomalous results by archaeologists. The fact is that, with few exceptions, problems of contamination for most Holocene age samples can be solved usually by applying standard pretreatment approaches or, in the case of more severe problems, by applying fraction studies. 

A living plant contains about the same fraction of carbon-14 as atmospheric carbon dioxide. The observed rate of decay of carbon-14 from a living plant is 15.3 counts per minute per gram of carbon. How many counts per minute per gram of carbon will be measured from a 15,000-yr-old sample Will radiocarbon dating work well for small samples of 10 mg or less ... 

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