Carbonate fractions, radiocarbon determinations

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. 

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

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). 

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. 

To this purpose, isotopic data presented in this paper were obtained from several selected Gorleben groundwaters as part of the colloid characterisation programme. The contents of major and minor ions, light isotopes ( H, H, and and the U/Th isotopes were measured. Radiocarbon and were measured in dissolved inorganic carbon (DIG), ion the humic acid (HA-colloids) and fulvic acid (FA-solution) fractions of dissolved organic caibon (DOC). The and were also determined in dissolved sulphate phase. The U/Th isotope measurements were carried out on total and surface solid phases, colloid fraction (1-1000 nm particle size, HA) and solution (<1.5 nm, FA). 

As the international standard of A is the applied standard of the US National Bureau of Standards (NBS), namely the oxalic acid with radioactivity of 14.3 decays per minute per 1 g of carbon (1.176-10 °% C). However, most labs use in their calculations activity of the internationally accepted modem radiocarbon standard. This standard is defined as specific activity of in wood tissue, which grew in 1950. During assimilation, carbon is subjected to isotopic fractionating (see below), which results in its enrichment with light isotope. In order to eliminate the effect of isotopic fractionating, beside the radiocarbon activity is also determined isotopic ratio and a correction is introduced for average value 6 C... 

Pathways of carbon flow in natural environments have also been reconstracted using bulk stable carbon and nitrogen isotopes as well as compound-specific isotopic analysis of individual biomarker lipid component (Hayes et al, 1990) based on the fractionations involved during primary (photosynthetic) and secondary (heterotrophic) processes. Recently, compound-specific radiocarbon analysis of individual biomarker hpid has been shown to be a valuable technique to determine the source of marine organic matter (Eglinton et al., 1997). 

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