¹⁴C (radiocarbon) Dating

Virtually every life form on earth takes in isotopes of carbon, including ¹⁴C and ¹³C, for growth and food. The relative amount of carbon isotopes in the cells differs with each plant and animal because of a process called fractionation. Isotopic fractionation occurs when the absorption of one isotope is favoured over another, often because of the energy differences between isotopes. For instance, during photosynthesis the isotope ¹²C is preferred over ¹⁴C. This leaves the plant cells with less ¹⁴C for each atom of ¹²C than in the atmosphere. When this occurs, we say the plant tissue is depleted in ¹⁴C and enriched in ¹²C. The table below shows some of the fractionation effects for various plants and animals. d¹³C indicates the difference between the sample's ¹³C/¹²C ratio and that of a standard. When the value is negative, it means that the isotope ¹³C is depleted compared to the standard. The more negative it is, the more fractionation has occurred. The fractionation values for d¹⁴C is twice that of d¹³C. Most radiocarbon labs assume d¹³C=-25.0 (d¹⁴C=-50.0). Marine samples, which have a d¹³C of about -1.0, must be corrected for the fractionation difference. This is generally done by the dating laboratory before they report the results.

Material	d13C (per mille)
Marine HCO3	-1±2
Marine CO3	0±2
PDB dC13 standard	0
Soil CO2 and secondary bone carbonate	-5±3
Speleothems	-9±3
Atmospheric CO2	-9±2
Bone apatite and original carbonate	-12±3
Grains, seeds, maize and millet (C-4 plants)	-10±2
Freshwater plants (submerged)	-16±4
Grasses arid zone, sedges	-13±3
Straw, flax	-14±3
Marine organisms (organic)	-15±3
Freshwater plants (submerged)	-16±4
Succulents (Cactus, pineapple, etc)	-17±2
Oxalic acid 2 C14 standard	-17±2
Bone collagen (C3 diet), wood cellulose	-20±2
C3 plants, Grains (wheat, etc). Graphite, coal	-23±3
Fossil wood, charcoal	-24±3
Recent wood, charcoal	-25±3
Tree leaves, Wheat, straw etc.	-27±2
Peat, humus	-27±3

Carbon intake stops with death. This means that the proportions of the carbon isotopes, ¹²C and ¹³C, no longer change in response to the environment. Immediately after death, the ¹⁴C concentration begins to decrease, and it continues to decrease for about 40,000-60,000 years, until all the ¹⁴C has decayed into ¹⁴N. We keep track of the decay rate of ¹⁴C by measuring changes in the ratio of ¹⁴C/¹²C in the sample.

Using d¹⁴C to calculate age:

In order to use d¹⁴C to calculate sample age, we compare it to a standard from 1890, before fossil fuel use altered the atmosphere's d¹⁴C values. We also assume that the atmosphere's d¹⁴C was the same in the past as it was in 1890. Actually, we know this isn't true. ¹⁴C is created in the upper levels of the atmosphere by cosmic ray alteration of ¹⁴N. When the frequency of cosmic rays changes, such as happens during sunspot cycles, ¹⁴C production changes. In order to adjust for this effect, radiocarbon ages of individual tree rings are compared to the age of the rings based on tree-ring counts. The differences are attributed to ¹⁴C production changes.

Other factors that affect d¹⁴C:

The oceans are a large depository of carbon, including ¹⁴C. When ocean circulation slows, the stockpile of C held by the ocean grows. And when ocean circulations speeds up, that stockpiled C re-enters the atmosphere, depleted in ¹⁴C because of the long time it takes water to move through the oceans. For example, the water that comes to the surface around Antarctica began it's journey in the Arctic Ocean, thousands of miles and hundreds of years earlier. Plants and animals that absorb the depleted ¹⁴C will appear much older than they are. Antarctic penguins, for example, consume and absorb carbon so depleted in ¹⁴C that 1 yr. old penguins have radiocarbon ages of 1000-1300 years old.

Radiocarbon dating is a relatively new science, in use only since the 1960's. As our knowledge ¹⁴C increases, we expect that both the accuracy and precision of radiocarbon dates will increase. Technological advances have made analyses of very small samples possible, using accelerator mass spectrometry (AMS). AMS dating of small amounts of pollen, ostracode shells, and single plant seeds is now commonplace.

Results from AMS radiocarbon dating of pollen, ostrocodes and bulk sediment indicate that fractionation and contamination is an important concern in comparing the dates. Despite this, the dates occur in-sequence and indicate that we have sediments covering the last 15,000 years.

Penguin photo courtesy of Guillaume Dargaud

Learn more about radiocarbon dating from these sites