The Carbon 14, or radiocarbon dating method is one of the best-known methods of dating human fossils, and has been around since the late 1940s.The Carbon 14 (C-14) dating method is a radiometric dating method. A radiometric dating uses the known rate of decay of radioactive isotopes to date an object. Each radioactive isotope has a known, fixed rate of decay, which we call a half-life. The half-life is the amount of time it takes for a quantity to fall to half of the value that it started with.
This means that if we know the isotope and its rate of decay, then we can calculate how old the substance is.So now we need a little science lesson about carbon.
The element carbon occurs in nature in three isotopic forms. Carbon 12 (12-C) is stable and represents 98.9% of the carbon in the atmosphere. The rest (1.1%) is mostly made up of Carbon 13 (13-C), which is also stable, and Carbon 14 (C-14) which is unstable – or radioactive. In our atmosphere, only about one in a trillion carbon atoms is C-14.
Most of the C-14 in our atmosphere is produced in the upper atmosphere by the action of cosmic rays on nitrogen (N-14) to produce C-14. Once C-14 is produced, it starts to decay back to nitrogen. The atmosphere has constant levels of C-14 – the production of new C-14 in the atmosphere and the decay of C-14 balance each other in a steady state equilibrium.
These three different forms of carbon are oxidised and dispersed through our atmosphere. The oxidised carbon is absorbed by plants through photosynthesis. Once it is in the plants, it enters the food chain. When the C-14 enters the plant or animal, it remains in equilibrium with the atmosphere. However – once the tissue dies (i.e. the animal dies or a tree’s sapwood converts into hardwood) then the tissue is no longer being replaced and the level of C-14 continuously decreases through radioactive decay.
If we know how much C-14 was in the living tissue, we can measure the amount of C-14 in the dead plant or animal and then compare these to assess how long it has been dead. We can do this because we know the decay rate of C-14 (it has a half-life of 5,730 years). The result is the radiocarbon age of the sample.
But how do we know?
The next question, of course, is – how do we know the amount of C-14 that was in the living tissue when it died? Well, this isn’t simple to determine. As it turns out, the production of C-14 in the atmosphere has not been constant over time, because of fluctuations in the amount of cosmic rays reaching the earth. We’ve been able to confirm this by measuring tree rings and comparing their known age with the radiocarbon result. This allowed us to create carbon calibration curves. However, our tree ring chronology only goes back 12,000 years (look up dendrochronology if you are especially interested in this aspect of dating). So, we’ve had to use other materials such as corals, speleothems (remember the “cave popcorn”?), lake sediments and ice-cores to create calibration curves for radiocarbon dating. We are working hard to extend these calibration curves as much as possible – currently, we are at around 55,000 years. This means that radiocarbon dates are given as a calibrated date – which is our best estimate based upon our current calibration curves but introduces a margin for error. As research continues to update our calibration curves, we need to go back to the original radiocarbon dates and recalibrate them. That’s one challenge. Remember that we said earlier that the naturally occurring amount of C-14 in the atmosphere is about one in one trillion carbon atoms? That’s another challenge. In such small amounts, C-14 is very difficult to measure and it is also very sensitive to contamination. When we first started using radiocarbon dating, huge samples were required. Today we use (very expensive) accelerator mass spectrometers (AMS) to count C-14 atoms in a sample. And we are also constantly researching new methods of sample preparation to help reduce the likelihood of contamination. All of this combines to produce dates that can be difficult to assess as correct. Certainly, a single radiocarbon dating of a human fossil should be treated with caution.