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Stable Isotopes

Carbon Isotopes

Carbon has two naturally occurring stable isotopes (12C and 13C), and one naturally occurring radioactive isotope (14C). For studies of stable isotope biogeochemistry we measure the ratio of 13C to 12C in samples of interest (e.g., carbonate minerals), expressed relative to the ratio in a standard of known isotopic composition, such as the "Pee Dee Belemnite" (PDB; 13C/12C = 0.00112372 (1) (2)).

Formula for Delta 13 C

The d13C values of terrestrial carbon samples typically range from -40 to +10 per mil (parts per thousand) relative to PDB (3).

The KSC Center for Environmental BioGeoChemistry is not equipped to determine 14C ages.

Isotope Atomic mass
Natural abundance
(atom %)
12C 12.000000000* 98.93
13C 13.003354826 1.07
14C 14.003241982 half-life of 5715 years
decays by b- to 14N

See the WebElements page on the Isotopes of Carbon.
and the Chart of the Nuclides centered on C.

(1)From Table 9-1, p. 154, of David P. Mattey, 1997, Gas source mass spectrometry: isotopic composition of lighter elements, pages 154-170 in Robin Gill (editor), Modern Analytical Geochemistry, Longman (UK).

(2)The uncertainty in this value is much larger than the precision with which we can measure relative ratios, which is one reason for the use the "delta" notation; page 32 in Robert E. Criss, 1999, Principles of Stable Isotope Distribution, Oxford University Press.

(3)From Figure 2-27, p. 111, of Arthur H. Brownlow, 1996, Geochemistry, Prentice-Hall.

KSC Center for Environmental BioGeoChemistry

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January 2009