Science Questions

How does carbon dating work?

Sun, 8th Jun 2008

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Question

Paul, New Zealand asked:

If a carbon atom is in existence at the beginning of the Earth, spends some of its time in bacteria, then in fish and animals and so on. It gets into a tree and into furniture and we try and carbon date the furniture arenít we just carbon dating bacteria? How does it really work?

Answer

They key to carbon dating is that the carbon isnít the carbon thatís been on Earth ever since the Earth was formed.  The carbon thatís in carbon dating is carbon thatís been newly made.  Where that comes from is when cosmic rays - high energy particles from the sun -  hit the Earthís atmosphere they interact with atoms and send neutrons flying around.  when one of these neutrons hit a Nitrogen-14 (14N) atom, it knocks out a proton, and the 14N becomes Carbon-14 (14C).  This then circulates in the atmosphere but because this process is happening roughly at the same rate continuously the amount of carbon thatís in the atmosphere is roughly continuous.  Most of it ends up in the atmosphere as carbon dioxide so you have 14C carbon dioxide. Plants then pick that up in their process of photosynthesis and they turn it into sugar. You then eat the plant and all the time that you are alive youíre gaining radioactive carbon in your body which you incorporate into your body. The level in your body will be roughly constant because youíre taking it in at a roughly constant rate from the environment. The ratio of radioactive to non-radioactive carbon should be the same all the time you or a plant are alive.

But when you die you stop adding new carbon-14 to your body and the 14C youíve already got starts to break down to 14N because itís radioactive.  The half-life is about 5500 years or so.  When you find an ancient specimen all you have to do is to compare how many 14C atoms are in it to the number of 12C atoms.  The ratio tells you how long it was since it was last alive and this gives you a ballpark figure for its age.

This does make the assumption that the production of 14C and incorporation into the food chain is the same now as it was thousands and thousands of years ago.  This assumption but itís assumed to be a fairly reasonable and accurate way to do it.

The guy we have to credit is Willard Libby who discovered carbon dating in the 1940s, got the Nobel Prize for it actually.

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