A brief introduction to stable carbon isotope analysis

1. School of something
FACULTY OF OTHER
School of Civil Engineering
FACULTY OF ENGINEERING
Stable carbon isotopes
Rosemary Nmavulem Orlu
cn09ro@leeds.ac.uk

2. Content
 What are isotopes?
 Stable isotopes
 Stable carbon isotope analysis
 Stable isotope terminology
 δ13C of terrestrial and aquatic environments

3. What are isotopes?
 The major elements associated with
organic compounds include carbon,
hydrogen, nitrogen, oxygen and sulphur.
 Each of these elements have at least two
stable isotopes.
 Isotopes are atoms that have the same
number of protons but have different
numbers of neutrons .
 Having a different number of neutrons
gives the atom a different mass.
 Isotopes fall into two classes namely
stable and unstable isotopes
(radioisotopes).

4. Stable isotopes
 Stable isotopes do not undergo radioactive
decay.
 They may however be radiogenic (i.e.
produced by radioactive decay).
 The relative abundance of an isotope is the
percentage of that particular isotope that
occurs in nature.
% 𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒 =
𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑖𝑠𝑜𝑡𝑜𝑝𝑒
𝑡𝑜𝑡𝑎𝑙 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑎𝑙𝑙 𝑖𝑠𝑜𝑡𝑜𝑝𝑒𝑠
 Usually the number of protons and neutrons
is similar and the less abundant isotopes are
often ‘heavy’, i.e. they have an extra neutron
or two.

5. Stable carbon isotope analysis (SCIA)
 The major elements associated with organic
compounds include carbon, hydrogen, nitrogen,
oxygen and sulphur.
 Each of these elements have at least two stable
isotopes that can be distinguished with mass
spectrometry.
 Isotope ratios of carbon are expressed in terms
of δ13C values, which are reported in per mil and
calculated relative to the standard (e.g. Pee Dee
Belemnite, PDB), according to the relationship.
𝛿13
C (‰) =
Rsample− Rstandard
Rstandard
x 103
 Gas chromatography with isotope ratio mass
spectrometer (GC-IRMS) allows for the
measurement of small but significant variations
in stable isotope ratios with high precision

6. Stable isotope terminology
 In stable isotope convention, isotope ratios are
expressed in terms of δ13C values which are
reported in per mil or per thousand indicated by
the symbol ‰.
 The delta symbol 𝛿 refers to the ratio of heavy
to light isotopes compared to a standard, times
1000.
 The ratio of heavy to light isotopes increases
with the value of 𝛿.
 A negative δ in an experimental observation is
indicative of the sample being depleted in the
less common isotope relative to the standard.
(usually Vienna Pee Dee Belemnite i.e. VPDB).
 A positive δ is indicative of a sample in which
the less common isotope is in greater abundance
compared to the standards.
- 80 - 60 - 40 - 20 0
atmospheric CO2
marine HCO3
marine carbonate
groundwater HCO3
fresh-water carbonate
coal
oil
+20
13δ VPDB (‰)
biogenic methane
Overview of stable carbon isotopic composition
of common organic and inorganic material
showing relative positions on a δ13C scale

7. δ13C of terrestrial and aquatic
environments
 Generally, soil CO2 is approximately 5% more
enriched in 13C than the associated vegetation and
soil organic matter.
 Soil CO2 is derived from root respiration,
decomposition of soil organic matter, and at least in
the upper 30cm of the profile, diffusion of
atmospheric CO2 in the soil.
 Freshwaters have been shown to have a δ13C range
of -15 to 0‰ while rivers have a range of -15 to -
5‰.
 Groundwaters range from approximately -30 to 0‰
but are mostly found in the -25 to -10‰ range
Carbon cycling processes in aquatic and
terrestrial environments

8. Further Reading
 Boutton, T.W. 1991. 11 - Stable Carbon Isotope Ratios of Natural Materials: II. Atmospheric,
Terrestrial, Marine, and Freshwater Environments. In: Coleman, D.C., Fry, B. ed. Carbon
Isotope Techniques. Academic Press, pp.173-185.
 Mermut, A.R., Amundson, R. and Cerling, T.E. 2000. The use of stable isotopes in studying
carbonate dynamics in soils. In: Lal, R., et al. eds. Global Climate Change and Pedogenic
Carbonates. Washington, DC: Lewis Publishers, pp.65–85.
 Meier-Augenstein, W. 2010. Stable isotopes analysis: general principles and limitations.
 Newton, R. and Bottrell, S. 2007. Stable isotopes of carbon and sulphur as indictors of
environmental change: past and present. Journal of the Geological Society. 164, pp.691-708.
 Sessions, A.L. 2006. Isotope-ratio detection for gas chromatography. J.Sep.Sci. 29. 1946-1961.