Carbon has three isotopes, two stable and one cosmogenic. Natural variation of the two stable isotopes of carbon can be useful for understanding food webs and carbon cycling in ecosystems. Carbon-14, a cosmogenic isotope with a half life of 5715 years, is useful for age dating as well as for tracing hydrologic processes, such as groundwater flow and ocean circulation.

• Cost of Analysis
  
• Origin
  
• Measurement Techniques
  
• Hydrological Applications
  
• Other Applications
• References and Further Reading
• Internet Resources

Cost of Analysis (return to top)
d¹³C
Isotope Ration Mass Spectrometry (IRMS): $37.50/sample
Accelerator Mass Spectrometry (AMS): $125-$235/sample
(See ISO-Analytical for more information)

(See also the UA Laboratory of Isotope Geochemistry for more information)

¹⁴C
Radiometric Counting: $200-$300/sample
AMS: $400-$2400/sample


Origin (return to top)

¹²C & ¹³C:
These two stable isotopes of carbon are found naturally on Earth. Living matter (i.e., bacteria and plants) takes up carbon through CO₂ in the atmosphere. This matter is often isotopically selective, generally preferring to break the weaker, light-isotope bonds. Fractionation is constrained to definable ranges for relatively stable environmental conditions. Measuring this fraction, ¹³C/¹²C or d¹³C, enables the research scientist to determine a variety of factors regarding a particular sample.

(See Stable carbon isotopes in paleoceanography, by Ellen Thomas, for more information)

(See Evolution of carbon in groundwaters, in ch. 5 of Environmental Isotopes in Hydrogeology, by Ian Clark and Peter Fritz, for more information)

¹⁴C:
¹⁴C is formed in two different ways. Cosmogenically, ¹⁴C is created when cosmic rays in the Earth's atmosphere cause some of the atoms in the upper atmosphere to fly apart into pieces. Neutrons that come from these fragmented molecules run into other molecules, causing chemical reactions. When a neutron reacts with a ¹⁴N (nitrogen) atom, the result is ¹⁴C.

¹⁴N + neutron ® ¹⁴C + proton

¹⁴C is also formed anthropogenically, or through man-made reactions. Much ¹⁴C has been added to the atmosphere due to the nuclear bomb tests from the 1950s and the use of nuclear power.

Measurement Techniques (return to top)

Sample treatment:
Dissolved Inorganic Carbon (DIC)
Water is hydrolyzed and acid is added. The mixture is combined thoroughly and the resulting gas is collected. Sample must be dried well.

Dissolved Organic (DOC)
Water is evaporated and the residue is combusted in oxygen. Generally a low carbon value is expected.

Inorganic Solids - Carbonates
McCrea CO₂ Method of 1950
Amount in carbonate materials are measured by reacting 100% water-free phosphoric acid (H³PO⁴) at 25^oC with carbonate materials to convert calcite to CO₂. As long as all the CO₂ is recovered, there is no fractionation of ¹³C. The ¹³C/¹²C of the gas is the same as the calcite. The CO₂ gas is then run through the IRMS.

CaCO₃ + 2H⁺ ® CO₂ + H₂O + Ca²⁺

Organic Solids
Organic compounds are generally oxidized at high temperatures (850-1000^oC ) in a stream of oxygen or by an oxidizing agent such as CuO.

d¹³C:

Gas Source Isotope Ratio Mass Spectrometry (IRMS)
Gas source mass spectrometry is the most common method for analysis of stable carbon isotopes. Carbon isotopes are run through the mass spectrometer as CO₂ gas. Different sample treatment methods can be used to convert carbonates, HCO₃/CO₃ and DOC samples into CO₂ gas for analysis.
(See Carbonate, DOC, DIC and organic carbon sample treatments sections above for more information)

(See SAHRA's Gas Source page for more information)

(See Carbonate, organic carbon, and hydrocarbon, in ch. 5 of Environmental Isotopes in Hydrogeology, by Ian Clark and Peter Fritz, for more information)

In the last few years, a new methodology to measure d¹³C contents of individual compounds in complex organic mixtures has been developed. This GC-C-MS technique, which uses a capillary column gas chromatograph, a combustion interface to produce CO₂ , and a modified conventional gas mass-spectrometer (as described above), has the capability to measure individual carbon compounds in mixtures of subnanogram samples with a precision of better than ±0.5%.

Accelerator Mass Spectrometry
AMS can also be used for stable carbon isotope ratios. An AMS instrument provides the means to directly count the number of atoms in a sample, so that even extremely small (micrograms) samples can be used for quantitative determinations of very low-level isotopic concentrations to a precision of 0.5‰.
(See SAHRA's AMS page for more information)

(See SAHRA's AMS page for more information)

¹⁴C:

Radiometric Counting
This method determines the amount of ¹⁴C present in a sample by measuring its radioactivity. This is done by converting the carbon in the material being dated to a gas such as CO² and methane. The sample is then placed in a radiation detector. There are two types of counting systems in use:

• Gas Counting - A sample is converted to methane or CO², which is used to fill a proportional gas counter. The decay of a ¹⁴C atom triggers an electrical discharge in the gas, which is electronically detected. The rate at which the decay occurs depends on the number of ¹⁴C atoms present in the sample gas.
  
  
• Liquid Scintillation Counting - A sample of carbon is converted to benzene, mixed with special organic compounds and placed in a transparent container. The benzene is produced as follows: CO², obtained by burning the sample, reacts with metallic lithium to form lithium carbide. The lithium carbide is hydrolyzed to acetylene, which is subsequently converted to benzene by catalytic trimerisation. The solution emits a pulse of light whenever a ¹⁴C atom decays due to beta-decay, and the light pulse is detected by sensitive photomultiplier tubes placed close to the container.

Accelerator Mass Spectrometry
Accelerator Mass Spectrometry (AMS) is typical method of analysis for small samples or samples that have very low levels of ¹⁴C. Carbon from the sample is converted into graphite. This graphite is then loaded into the sample chamber of the AMS where it is ionized. These ions are then separated and analyzed for ¹⁴C.
(See our AMS page for more information)



Hydrological Applications (return to top)

d¹³C:

d¹³C values are used to understand the biogeochemical reactions controlling alkalinity in watersheds.
(See USGS' Periodic Table - Carbon for more information)

d¹³C values can be used to detect changes in the ocean biological pump activity and provide links to climate history. The ratio of d¹³C in calcareous nano-fossils (e.g., plankton) vary through geological time. This variation, although quite small, can provide information about ocean productivity and chemistry and can be linked to climate history. Shallow oceans have more positive d¹³C values than deep oceans. The differences in d¹³C values between shallow and deep water can be used to measure the efficiency of the biological pump.

Carbon isotopes are useful tracers of seasonal and discharge-related contributions of different hydrological flowpaths to streamflow (shallow vs. deep flow-paths). The values are also useful in studying the origin, transport, and fate of dissolved organic carbon in streams and shallow groundwater in forested catchments. This helps in paleoceanographic research by detecting changes in the biological pump activity and magnitude and by delineating nutrient contents of water masses and bottom water conditions.

¹⁴C:

• Radiocarbon dating of groundwater provides a mechanism to monitor, understand and control exploitation of an aquifer. ¹⁴C dating can help determine whether a community is mining their water resources.
  
  
• When the appropriate field measurements are collected and appropriate corrections are applied for dilution, ¹⁴C measurements can provide insight into groundwater flow paths, recharge areas and sources of recharge.
  
  The Tucson Basin study provides a classic example of application of isotope techniques to the determination of origins and ages of groundwater in a large semiarid basin. It is possible to identify mountain-front and mountain-block recharge, and to specify which parts of the basin aquifers receive water from the major drainages entering the basin.
  (See also Chris Eastoe, Locating recharge zones with isotopes: the Tucson Basin Example, Southwest Hydrology 2(1): 22-23, 2003. pdf version or html version)
  
• Time based monitoring of radiocarbon content of a well can predict possible contamination in a drinking supply by dating the incoming water. The radiocarbon content of a well can reveal both the stability of, and the changes in the source waters at the pump head. The younger ages of the water each year indicate younger waters are being drawn down from above. This could be caused, for example, by over-pumping of the well or by expanded well drillings in other areas. In either case, it indicates that eventually there is a risk that contaminated surface waters could enter the drinking supply.

Other Applications (return to top)

d¹³C:

• d¹³C values can be used as a biological tracer in carbon cycles studies.
  
  
• d¹³C values can be used to distinguish between C3 and C4 plants. C3 plants are mostly terrestrial plants, forest plants, temperate zone plants, native North American and European plants, and crop plants. C4 plants are often found in hot, arid climates and efficiently use water.
  (See Evolution of carbon in groundwaters, in ch. 5 of Environmental Isotopes in Hydrogeology, by Ian Clark and Peter Fritz, for more information)
  
  
• d¹³C values determine areas and times of high productivity.
  Organic processes incorporate more ¹²C than ¹³C, so areas of high productivity are marked by higher residual d¹³C values.
  
  
• d¹³C values of organic materials provide information about contaminants by comparing "chromatograms" of pollutants and possible contaminants.
  
  
• d¹³C values can be used as tracers in the medical field.
  Through a breath test, analysis of d¹³C ratios now permit diagnosis of Helcobacter Pylori, a bacterium responsible for gastric ulcers.
  (See The ¹³CO₂ Project Website for more information)
  
  
• d¹³C values are useful in archaeological investigations, especially for determining paleo-diets in terms of the percentage of C3 and C4 species (which fix atmospheric CO₂ by two different photosynthetic reaction pathways).
  (See Overview of Stable Isotope Research, by the University of Georgia Institute of Ecology, for more information)

¹⁴C:

Useful in determining the age of virtually any substance that contains the smallest amount of carbon.

Equations

d¹³C = {[( d¹³C)_sample - (d¹³C)_standard ]/( d¹³C)_standard } *1000

References and Further Reading (return to top)

• Clark, I., and P. Fritz, Environmental Isotopes in Hydrogeology, Lewis Publishers, Boca Raton, 1997.



• Faure, G., Principles of Isotope Geology, 2nd ed., John Wiley and Sons, New York, 1986.



• Kalin, R.M., Radiocarbon dating of groundwater systems, in Environmental Tracers in Subsurface Hydrology, ed. by P.G. Cook and A.L. Herczeg, pp. 111-144, Kluwer, Boston, 2000.

Internet resources (return to top)

Institut für Medizintechnik, The ¹³CO₂ Project Website

Thomas, E., Stable carbon isotopes in paleoceanography

University of Georgia Institute of Ecology, Overview of Stable Isotope Research

USGS Periodic Table - Carbon

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