Understanding Carbon 14 dating – a beginner’s guide!
The basics
Carbon 14 (C14) dating is a widely used way of dating organic material (anything once living and containing carbon) up to 60,000 years or so old. It’s particularly useful for dating relatively recent objects, a few hundred to a few thousand years old.
C14 dating is based on the fact that carbon atoms come in different forms, one of which is radioactive. The carbon in the carbon dioxide (CO
2) in the air contains a mixture of carbon
(C) atoms with 6 neutrons and 6 protons making up the nucleus of each atom – normal C12
- plus 1% C13 with 7 neutrons, and importantly for dating a very small proportion of atoms with 6 protons and 8 neutrons – C14.
Unlike the other forms of carbon atoms, C14 is radioactive and unstable and decays over time – half of the C14 atoms in any sample will decay into C12 in 5730 years.
The C14 in the air is constantly replenished by the action of cosmic rays on nitrogen N14, which makes new C14 in the atmosphere about as fast as it is lost through radioactive decay. The proportion of C14 in the carbon dioxide in the air is therefore stable.
During their lifetimes plants take in carbon dioxide from the air during photosynthesis, using the carbon to make their stems, leaves, fruits, etc (and releasing the oxygen we breathe).
This carbon-based vegetable matter is in turn is eaten by animals, who may in their turn be eaten by other animals themselves. Once the plant or animal dies however, it stops photosynthesising or eating and so stops absorbing new carbon. The C14 store it accumulated in life slowly decays, half of it decaying every 5730 years.
By comparing the ratio of C14 to C12 in a sample with the ratio in the air, and knowing how quickly C14 decays, we can tell how long it is since the specimen was alive.
If there’s half as much C14 as we’d expect in a freshly dead sample we would know that the specimen was
5730 years old, if a quarter as much it would be 11460 years old, and so on.
The technique gets harder and less accurate as specimens g et older and there’s less and less C14 left to be measured. 60,000 years old is about the limit.
Complications
As with any measurement there’s a potential for error, so dates are expressed as something like 1650 + or – 35. That means there’s a 95% chance that the exact true age is between
1615 and 1685 years old.
These error ranges assume that there’s no gross contamination with either modern or ancient carbon, but if apparent dates seem completely inexplicable then contamination is always a possibility.
Dates are expressed as “BP” – before present – but time marches on so “The Present” is defined as 1950 so that reports don’t soon go out of date. To convert to dates AD / BC (also known as BCE / CE, Before Common Era / Common Era, as more culturally neutral terms) simply subtract the date given from 1950. 1350 Before Present = 600 AD, not (in 2014) 664 years old.

Dates are also expressed as “calibrated” C14 dates. Put very simply, the ratio of C14 to ordinary C12 in the atmosphere is stable day to day but varies over much longer periods of time, due to factors such as variations in the amount of solar radiation that makes new C14, volcanic eruptions which release large amounts of ancient carbon whose C14 has long since decayed, and more recently the burning of huge amounts of fossil fuels which also contain ancient carbon and hence also no C14.
Using samples from specimens dated by other means, eg tree rings, we can correct for this variation, a process known as “calibration”. Corrections are also made for other known influences – for instance in areas with hard water, some of the carbon in a skeleton will come from the water not the food. The hard water contains dissolved carbon (as carbonate) that might be millions of years old, making the skeleton appear older than it is.
There’s a huge amount more to know about C14 dating – if you’re interested search on line
– but hopefully this guide will help you understand the basic concept and the terms used.
Bill Jenman 2014