Carbon Dating

Of all the elements, carbon is the one that seems most indispensable to life - the one without which life on any planet is hardest to envisage. This is because of carbon's remarkable capacity for forming chains and rings and other complex molecular architectures. It enters the food web via photosynthesis, which is the process whereby green plants take in carbon dioxide molecules from the atmosphere and use energy from sunlight to combine the carbon atoms with water to make sugars. All the carbon in ourselves and in all other living creatures comes ultimately, via plants, from carbon dioxide in the atmosphere. And it is continually being recycled back to the atmosphere: when we breathe out, when we excrete, and when we die.

Most of the carbon in the atmosphere's carbon dioxide is carbon-12, which is not radioactive. However, about one atom in a trillion is carbon-14, which is radioactive. It decays rather rapidly, with a half-life of 5,730 years, as we have seen, to nitrogen-14. Plant biochemistry is blind to the difference between these two carbons. To a plant, carbon is carbon is carbon. So plants take in carbon-14 alongside carbon-12, and incorporate the two kinds of carbon atom in sugars, in the same proportion as they exist in the atmosphere. The carbon that is incorporated from the atmosphere (complete with the same proportion of carbon-14 atoms) is rapidly (compared to the half-life of carbon-14) spread through the food chain, as plants are eaten by herbivores, herbivores by carnivores and so on. All living creatures, whether plants or animals, have approximately the same ratio of carbon-12 to carbon-14, which is the same ratio as you'll find in the atmosphere.

So, when is the clock zeroed? At the moment when a living creature, whether animal or plant, dies. At that moment, it is severed from the food chain, and detached from the inflow of fresh carbon-14, via plants, from the atmosphere. As the centuries go by, the carbon-14 in the corpse, or lump of wood, or piece of cloth, or whatever it is, steadily decays to nitrogen-14. The ratio of carbon-14 to carbon-12 in the specimen therefore gradually drops further and further below the standard ratio that living creatures share with the atmosphere. Eventually it will be all carbon-12 - or, more strictly, the carbon-14 content will become too small to measure. And the ratio of carbon-12 to carbon-14 can be used to calculate the time that has elapsed since the death of the creature cut it off from the food chain and its interchange with the atmosphere.

That's all very well, but it only works because there is a continuously replenished supply of carbon-14 in the atmosphere. Without that, the carbon-14 with its short half-life would long since have disappeared from the Earth, along with all other naturally occurring isotopes with short halflives. Carbon-14 is special because it is continually being made by cosmic rays bombarding nitrogen atoms in the upper atmosphere. Nitrogen is the commonest gas in the atmosphere and its mass number is 14, the same as carbon-14's. The difference is that carbon-14 has 6 protons and 8 neutrons, while nitrogen-14 has 7 protons and 7 neutrons (neutrons, remember, have near-enough the same mass as protons). Cosmic ray particles are capable of hitting a proton in a nitrogen nucleus and converting it to a neutron. When this happens, the atom becomes carbon-14, carbon being one lower than nitrogen in the periodic table. The rate at which this conversion occurs is approximately constant from century to century, which is why carbon dating works. Actually the rate is not exactly constant, and ideally we need to compensate for this. Fortunately we have an accurate calibration of the fluctuating supply of carbon-14 in the atmosphere and can take this into account to refine our dating calculations. Remember that, over roughly the same age range as is covered by carbon dating, we have an alternative method of dating wood - dendrochronology - which is completely accurate to the nearest year. By looking at the carbon-dated ages of wood samples whose age is independently known from tree-ring dating, we can calibrate the fluctuating errors in carbon-dating. Then we can use these calibration measurements when we go back to organic samples for which we don't have tree-ring data (the majority).

How it works.