arbon 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)}).

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 (ma/u)	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 page on the Isotopes of Carbon.
and the 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.