The Isotopes of Carbon

Carbon has three isotopes in nature 12C—common and stable 13C—rare and stable and 14C—very rare and radioactive. The heavy carbon isotope, 14C, is unstable and decays radioactively into 14N, emitting a beta ([ ) particle that can be measured in specialized laboratories. The half-life of 14C is 5730 years. The above information can be summarized in the following way  

The radioactive decay curve of 14C is given in Fig. 11.1. What fraction of an initial concentration of 14C is left after 10,000 years The answer, read from Fig. 11.1, is 27%. 

C) of which two are stable ( C and The rest are radioactive with half-lives between 

74s ( C) and 5726 years ( C). Only the stable isotopes and C (often referred to as radiocarbon ) are of interest in the carbon cycle. 

The most abundant isotope is C, which constitutes almost 99% of the carbon in nature. About 1% of the carbon atoms are C. There are, however, small but significant differences in the relative abun- 

The content of the material in a carbon reservoir is a measure of that reservoir s direct or indirect exchange rate with the atmosphere, although variations in solar activity (Stuiver and Quay, 1980,1981) also create variations in atmospheric content. Geologically important reservoirs (i.e. carbonate rocks) contain no radiocarbon because the turnover times of these reservoirs are much longer than the isotope s half life. The distribution of is used in studies of ocean circulation and studies of the terrestrial biosphere. 

Fractionation is another major process responsible for creating inhomogeneities in the isotope distribution. Physical, chemical, and biological processes may be sensitive to the molecular weights of the molecules involved. The definition of d used to describe variations in isotope composition was 

The most abundant isotope is which constitutes almost 99% of the carbon in nature. About 1% of the carbon atoms are There are, however, small but significant differences in the relative abundance of the carbon isotopes in different carbon reservoirs. The differences in isotopic composition have proven to be an important tool when estimating exchange rates between the reservoirs. Isotopic variations are caused by fractionation processes (discussed below) and, for C, radioactive decay. Formation of takes place only in the upper atmosphere where neutrons generated by cosmic radiation react with nitrogen  

The C content of a sample is described in a similar manner. The basis for Rq is an oxalic acid standard of the US National Bureau of Standards normalized for C fractionation and corrected for radioactive decay since a reference date January 1, 1950 (Stuiver and Polach, 1977). The absolute value of Rq is 1.176-10 (Stuiver et al, 1981). 

A C is used frequently in modeling, since no corrections for fractionation are necessary when modeling fluxes between reservoirs. 

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When plants are consumed as food by herbivorous animals, the isotopic signatures in the plants are passed on to the consumers. Therefore, provided the isotopic signatures of C3 and C4 plants are known, determining the isotopie signatures in the tissues of herbivorous animals enables one to determine the relative amounts of C3 and C4 plants that the animals consumed as food, and to reconstruct their diets. Moreover, since carnivorous and omnivorous animals, including humans, feed on herbivorous animals as well as on plants, determining the isotopic signatures of the isotopes of carbon in tissues of ancient animals and humans makes it possible to elucidate the components of their diets. 

The basic assumption of constant atmospheric X4C activity in radiocarbon dating is not strictly valid. We now have a record of the fluctuation of atmospheric 14C variations for the last 8,400 years B.P. obtained by measurement of the isotopes of carbon in dendrochron-ologically dated wood. Prior to contamination of atmospheric 14C activity by fossil fuel combustion and nuclear technology in the 20th century, the first-order secular variation can be closely approximated by a sine curve with a period of 10,600 years and an amplitude of ... 

This information is normally shown as a superscript (atomic mass) and a subscript (atomic number) to the symbol for the element. Hence 1%C represents the isotope of carbon (atomic number 6) with the atomic mass of 14. In practice the subscript is often omitted because the atomic number is unique to the element that is represented by the appropriate letter (e.g. C for carbon). For simplicity in the spoken form and often in the written form, isotopes are often referred to as carbon-14, phosphorus-32, etc. 

Hayes, J.M. 2001. Fractionation of the isotopes of carbon and hydrogen in biosynthetic processes. Pp. 225-278, in Stable Isotope Geochemistry Reviews in Mineralogy and Geochemistry, J.W. Valley and D.R. Cole, eds., Washington, D.C. Mineralogical Society of America. 

Atomic mass unit a unit based on a value of exactly 12 atomic mass units for the mass of the isotope of carbon that has exactly six protons and six neutrons in the nucleus ... 

Two important conclusions can be drawn from the simunary of the symmetry analysis of Ar/CO collisions in Table 6. First, no SIKIE is predicted for C substitution because the symmetry of the system is independent of the isotope of carbon involved. Second, because the predicted a based symmetry restrictions for Ar COj cluster formation are identical to those predicted for (002)2, dependence of the magnitude of observed 0 SIKIE on the conditions of CO2 formation is expected. However, the e/f parity label state propensities for El-produced COJ, inferred from 0 SIKIE in (COj) formation, are not sufficient to predict the magnitude of 0 SIKIE in Ar-COj formation because, for above the threshold for Ar formation, COj ions are also produced by the charge-transfer reaction,... 

It is convenient to denote isotopes in the form Jf E, where the superscript m denotes the mass number (i.e., sum of the number of protons and neutrons in the nucleus) and the subscript n denotes the atomic number of an element, E. For example, is the isotope of carbon which has six protons and six neutrons in its nucleus. The atomic weight of each naturally occurring element is the average of the weights contributed by its various isotopes. 

Nielsen SG, Rehkamper M, Brandon AD, Norman MD, Turner S, O Reilly SY (2007) Thallium isotopes in Iceland and Azores lavas - Implications for the role of altered crust and mantle geochemistry. Earth Planet Sd Lett 264 332-345 Nier AO (1950) A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon and potassium. Phys Rev 77 789... 

Nuclides with spin 7=0 therefore have no nuclear magnetic moment. Two very important nuclei, the isotope of carbon and the isotope of oxygen, belong to this class of nuclides—this means that the main building blocks of organic compounds cannot be observed by NMR spectroscopy. 

A relative scale of atomic weights (as the weighted average of all forms, or isotopes, of a particular element found in nature) has been developed. The base of this scale is the assignment of a mass of 12.0000 to the isotope of carbon containing 5 protons, 5 neutrons, and 6 electrons. An atomic weight table can be found in Table 2.2. 

There can be varying numbers of neutrons in the atoms of an element. Most carbon atoms have six neutrons, but there are also carbon atoms with seven and eight neutrons. The differing numbers of neutrons does not affect the chemical properties of the atom. The different forms of an element that vary only in the number of neutrons in the atoms are called isotopes. Isotopes are named according to the total number of protons and neutrons they contain. Thus the isotopes of carbon with six, seven, and eight neutrons are called carbon-12, carbon-13, and carbon-14, respectively. Hydrogen has isotopes with one, two, and three neutrons. The most common form of hydrogen has only one neutron. 

Nier, A. O. (1950) A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon, and potassium. Physical Review, 77, 793-798. 

Two dominant themes run throughout the evolution of late type star compositions the abundances of the isotopes of carbon, nitrogen, and oxygen, and the abundances of the metals heavier than the iron peak - the neutron capture elements usually associated with the s-process. In addition to these elements, the abundance of lithium can also be a distinguishing characteristic of some groups, and can be used to interpret possible origins for some of these peculiar stars. 

In a study conducted by the Armour Research Foundation (Ref 41) a-Pb azide crystals wrapped in a thin A1 foil were subjected to fast and thermal neutrons in the heavy water pile at Argonne National Laboratory. With a thermal flux rate of about 10l4n/cm2/sec the crystals were irradiated for 8, 17 and 170 hours. The crystals decompd to a brown powder which was identified as Pb carbonate by X-ray techniques and infrared absorption spectra. From a mass spectrographic analysis of the isotopes of carbon and oxygen in the decompn products, it was determined that the mechanism of carbonate formation is a reaction with the atmosphere by broken surface bands produced by the neutrons. Subsequently, Raney (Ref 60) reported... 

We speculate that process changes could cause changes in sequence distribution. C NMR spectroscopy has been used to study similar copolymers (10,11,12) and hence should be of value in correlating process with sequence distribution, nmr is a means of looking at changes in the electronic environment of nucleii of the isotope of carbon, 13C. There are a number of texts on % NMR spectroscopy and its application to organic molecules, and polymers (13,14,15). The chemical shifts which one observes as characteristic of different carbons can be caused by... 

The subatomic particles differ in mass and charge. Their masses are expressed by the atomic mass unit, u (also called the dalton), which is also used to express the masses of individual atoms, and molecules (aggregates of atoms). The atomic mass unit is defined as a mass equal to exactly 1/12 that of an atom of carbon-12, the isotope of carbon that contains six protons and six neutrons in its nucleus. 

The isotope of carbon with seven neutrons, l3C, composes about 1.1% of carbon atoms. It is similar to hydrogen in that it has two nuclear spin states of different energy when it is in an external magnetic field. The spectroscopy that is done using this nucleus, l3C-NMR, provides direct information about the carbon chains in the compound, information that is often complementary to that obtained from H-NMR spectroscopy. 

The isotopes, though, are not present in equal amounts. Carbon-12 comprises 98.9% of all carbon, while carbon-13 accounts for 1.1%. Carbon-14 is present in a very small amount—about 1 x 10-10%. It makes sense that the average mass of all the isotopes of carbon is 12.01 u—very close to 12—since carbon-12 is by far the predominant isotope. 

While all the isotopes of carbon follow the same chemical or physical pathway, the rate at which this occurs varies as a function of their difference in mass. The pioneering studies of Harmon Craig in the early 1950s first pointed to the need to consider variations in the stable isotope ratios (13C/12C or of samples if one wished to obtain precise and comparable (or values (78). He determined that the effect of any... 

Solute isotope biogeochemistry focuses on isotopes of constituents that are dissolved in the water or are carried in the gas phase. The most commonly studied solute isotopes are the isotopes of carbon, nitrogen, and sulfur. Less commonly investigated stable, nonradiogenic isotopes include lithium, chloride, boron, and iron. 

Another widely used radiochronometric method involves study of the isotopes of carbon. Studies on these isotopes demonstrated that carbon-14 is taken in consistently by living organisms but that when the organism dies, carbon-14 starts to decay. Thus, a ratio of carbon-14 to the stable carbon isotopes, one of which is carbon-12, can provide an age in years since death of the organism. The ages of organic materials from the present as far back in time to 5 X 10" yr of age can be determined, with appropriate corrections for materials somewhat greater than 10" yr old. 

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