Understanding Carbon Isotopes

Almost all elements are made up of more than one isotope, i.e. atoms of the same element but which have different masses. In fact this is why most quoted atomic weights are not whole numbers, because they are averages of a number of different atomic masses. Carbon is no exception and is made up of isotopes with masses 12, 13 and 14 (written 12C, 13C, 14C, but read carbon-12 etc.).

In geology, isotopes are used in two quite different ways. Some isotopes are radioactive and decay to produce isotopes of a different element over time. The study of radiogenic isotopes is the basis of many geological dating techniques and is also an important branch of igneous geochemistry. Many other elements are made up of isotopes which are stable — they do no experience radioactive decay. Stable isotopes can become preferentially concentrated because of differences in their mass. This makes them useful in geochemical fingerprinting, and allows us to identify reaction pathways and ultimately distinguish between different types of geological process. Where the mass difference is large, greater is the likelihood of fractionation. Thus in the case of hydrogen, 2H is double the mass of 1H and isotopic fractionation is extensive. Normally, the mass difference is not as great as this. In the case of carbon isotopes, 13C is 8.3% heavier than the isotope 12C.

Rather confusingly the isotopes of carbon include both radiogenic (14C) and stable isotopes (12C, 13C). It is the stable isotopes of carbon which are the focus here. Isotope mass fractionation may take place in two ways. It may be the result of an entirely physical process, such as evaporation. The conversion of water to water-vapour will tend to physically separate the heavy and light isotopes of oxygen in water. In this way water with light oxygen will tend to become water vapour, whereas water with heavy oxygen will tend to remain as liquid water.

Alternatively, isotopic fractionation takes place during a chemical reaction. In this case it is the speed of the reaction which is important. In other words there is a kinetic control on the fractionation. In detail the strength of a chemical bond is dependent upon atomic mass, such that bond strength increases with the substitution of heavier isotopes. In biological processes, when inorganic carbon is used to make organic compounds, 12C is more weakly bonded and reacts more readily than 13C, because of its lighter mass. This means that organic matter tends to become enriched in 12C relative to the reservoir of inorganic carbon from which it has been drawn.

Stable isotopic fractionations are measured relative to a standard. In the case of carbon the standard is a fossil belemnite (the PDB standard). Isotopic fractionations are normally small and so values are measured in parts per thousand (‰) and expressed as δ13C values as follows:

δ13C ‰ = [(13C/12Csample - 13C/12Cstandard) / (13C/12Cstandard)] * 1000

The crucial reaction for detecting biogenic activity in ancient graphite, is thought to be oxygenic photosynthesis involving the enzyme 'Rubisco'. For example, within a cyanobacterial cell, the conversion of bicarbonate (inorganic carbon) to carbon dioxide (en route to becoming organic carbon) is speeded up in the vicinity of Rubisco producing a carbon isotope fractionation of -22 ‰.