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How atmospheric carbon-14 has changed

How do we know how much carbon-14 there was 10 000 years ago?

Carbon dating relies on looking at the ratio of carbon-14 to carbon-12 of an ancient sample to the same ratio in the atmosphere today.

One of the assumptions of this method is that the proportion of carbon-14 to carbon-12 in the atmosphere has always stayed the same.

There are lots of ways of telling the age of things: what things are buried above and below them, how they are made, where they were found and so on.  If the ratio of carbon-14 to carbon-12 had not been fairly constant then other techniques would show up discrepancies.  In general this doesn't happen.

But in the last few hundred years human activities have had the effect of changing the proportion enough for scientists to need to make compensations.

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New carbon-12 makes a sample seem younger

Coal and oil were formed from organisms that died millions of years ago.  All the carbon-14 in them has already decayed.  When coal and oil are burned the carbon dioxide released contains no carbon-14, only carbon-12 (and about 1% stable carbon-13).

This tends to dilute what carbon-14 is already there.

Because there's less carbon-14 than there was, a sample that's spent a long time decaying to get down to modern levels may appear to be pretty modern.

So adding carbon-12 tends to make samples look younger than they are.

New carbon-14 makes a sample seem older

In the 1950s there were many nuclear weapons tests.  Nuclear explosions produce huge fluxes of neutrons.  It is neutrons that cause nitrogen-14 to turn into carbon-14.

Nuclear weapons testing may have doubled the amount of carbon-14 in the atmosphere for a few years.

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