Radiocarbon Dating

Radiocarbon dating is one of the most widely used scientific dating methods in archaeology and environmental science. It can be applied to most organic materials and spans dates from a few hundred years ago right back to about 50,000 years ago - about when modern humans were first entering Europe.

What is Radiocarbon dating?

Radiocarbon dating (14C dating or carbon dating) is the principal method used for dating archaeological material spanning ~50,000 years ago to the modern era. The technique was developed in the late 1940s by Willard F. Libby and two graduate students (James Arnold and Ernie Anderson), at the University of Chicago. Libby won the Nobel Prize in Chemistry for the discovery in 1960.

There are three principal isotopes of carbon (the stable isotopes 13C and 12C constituting 1.11% and 98.89% of global carbon respectively). Radioactive carbon (14C) is formed in the upper atmosphere, around 10,000 metres in altitude, through the influence of cosmic ray bombardment and its impact on 14N. The replacement of a proton with a neutron in the nucleus of 14N results in an atom of 14C. Following production, 14C is rapidly oxidised and enters the biosphere principally through plant photosynthesis as 14C-labeled 14CO2 and from there to animal lifeways and all living organisms.

As the level of 14C in living organisms decays, so it is replenished through the uptake of food and nutrients. When the death of an organism occurs, however, no further replenishment of 14C takes place and only decay continues. Libby’s great genius was to work out the so-called half-life of radiocarbon. This value of 5568 ± 30 years is still used today, even though we know it is slightly wrong (the Libby half-life was retained: to change it would have made comparisons with previous dates problematic. The calibration of radiocarbon dates makes this issue redundant, since both the tree rings that make up the calibration curve and radiocarbon dates on unknown materials are both calculated using Libby’s half-life). Every ~5568 years that passes the amount of radiocarbon remaining in the dated material decreases by 50%. After around 10 half-lives have passed, we reach the limit of the technique at around 50-55,000 years.

Radiocarbon dates are expressed in years BP (‘Before Present’, with 0 BP equivalent to 1950 AD). Dates are given as an age and a standard deviation (or ± value), which represents plus or minus one standard deviation (68% probability) from the mean. Radiocarbon dates are measured with reference to modern reference standards, usually the oxalic acid II standard, the ratio of whose 14C activity is tied to the primary radiocarbon standard which is 1890 wood. In our laboratory we measure around 20% of our throughput of measurements as standards, they range from bone from the Mary Rose wreck, which sank in 1545 AD, to tree rings of absolutely known age, to bones that date to far before the radiocarbon age limit of 50,000 BP. The latter are dated to quantify our exact background limit in the lab.

Complications

There are several issues that arise to complicate the radiocarbon process. The first is calibration. The radiocarbon content of living organisms was assumed from the beginning of the development of the technique to be in equilibrium with the level of atmospheric 14C, although it became apparent later that this was not quite true. We now calibrate our radiocarbon dates to convert them into calendar time. This is achieved using samples of known age that comprise the calibration curve. The latest is the INTCAL20 curve.

The second major influence is contamination. This occurs when material from a non-sample source becomes incorporated within the sample, resulting in an admixture of carbon. If this carbon is of a different age then erroneous dates will result. The job of the radiocarbon lab is to using physical and chemical means to remove the contamination. In addition, we have developed improved methods to isolate so-called compound specific parts of the dated samples we analyse. An example is to extract a single amino acid from bone collagen, thereby eliminating all contamination.

What can be dated?

The material to be dated must have once been a living organism. Since all living organisms on Earth are carbon-based, this means there is a wide variety of possible samples that can be dated. In Oxford the most common samples dated are bone (42% of all samples), charcoal (15%), wood, plant remains, charred seeds, teeth, shell, antler, textile, pottery, peat and cremated bone. Care must be taken to select good samples that avoid the possibility of inbuilt age. In the case of wood, for example, selecting the exterior bark or final tree rings of growth will result in accurate results. We usually advise the selection of single entities; samples which grew within a single calendar year. A grain of barley or wheat would be a good example.

Some of the most common materials for radiocarbon dating are: