A Staff Report from the Straight Dope Science Advisory Board

How does carbon-14 dating work?

October 2, 2000

Dear Straight Dope:

Hey, what I can't figure out is carbon dating. They can carbon date the Shroud of Turin, and realize it wasn't made 2,000 years ago, right? Well, before it was shroud, it was, like, wool or something, and before it was wool or somethnig, it was hairs on a sheep's back, right? Blah, blah, blah (you can see where I'm going with this). So at what point does something's carbon b-day start? Am I missing the point entirely here? I need your help, man.

SDStaff Ken replies:

The broad principles behind carbon dating are fairly simple to explain.

Look at your standard periodic table. Carbon is element 6, with an atomic mass of 12. Sunlight (cosmic radiation) strikes the upper atmosphere and converts a small amount of nitrogen (element 7, atomic mass of 14) into the unstable radioactive iostope carbon-14. The C-14 then gets bound into carbon dioxide gas molecules in the atmosphere, and thus absorbed into plant life. The plants are consumed by animals, and thus C-14 appears in all living creatures on Earth.

When an organism dies, the C-14 in its cells is no longer replenished and so begins to decay, to revert back to nitrogen, at a constant, measurable rate. The half-life is about 5730 years. Therefore, we need only count the amounts of carbon-12 and carbon14 in the organic material, and compare the results, and voila! we have an initial estimate of the age — that is, the point at which the organism stopped absorbing C-14.

There are some assumptions underlying this process, and some limits to accuracy.

First hurdle: carbon dating is only effective on organic material. For example, we cannot carbon-date oil paintings, because some of the carbon in the paint comes from within the Earth's crust, which may never have had any C-14 in it at all. We know that the "blood" on the Shroud of Turin is not blood at all, but paint pigment; however, we know this from polarized light microscopy, not from carbon-dating. We could not accurately use carbon-dating to date the paint pigment.

Second (and more serious) hurdle: calculating the radioactive decay based on a fixed half-life assumes that the amount of C-14 in the atmosphere has been reasonably constant, and that the rate of absorption by plants and animals has also been reasonably constant. This is not so; in some periods organisms have absorbed more or less C-14 than in others. Fortunately, we are mostly able to correct for this.

There are trees that live for thousands of years, and their age can be exactly determined by counting the rings. Each ring is examined for the amount of C-14 absorbed that year. Thus, carbon-daters know the precise adjustments, year by year, to make to an initial radiocarbon dating to account for fluctuations in absorption rates over time.

Consequently, carbon-dating within the past (say) thousand years or so can be adjusted fairly accurately. (There could still be local variation in the amounts of C-14, etc.) Carbon dating for much older samples has a greater margin of error, since we have have no way of knowing the variation in rate of C-14 absorption prior to the tree-ring era.

Third hurdle, which gets directly to your question. Carbon dating tells us how long ago an organism died (that is, stopped absorbing C-14.) For dating the remains of a short-lived plant or animal, such as flax or sheep, this is not a problem. But, for example, imagine a tree that was cut down, used to build a house that stood for a hundred years, then was demolished and the old wood burnt for fuel. We find the hearth, and want to use the ashes to date the fireplace. Carbon dating would give the point at which the tree was cut down, not the point at which the wood was burnt, possibly over a hundred years later.

The final hurdle is contamination. There are several sources of contamination. Samples can be contaminated with older or younger carbon. In the fireplace example, for instance, several logs might have been burnt at the same time, but all cut down at very different periods. Groundwater or carbonated rocks such as limestone can also contaminate a sample. Generally, contamination is removed by chemical cleaning prior to dating.

Given all these constraints, a sample can only be dated to a range of time. Usually, radiocarbon dates are expressed with a plus or minus margin of error. If someone tells you something has been carbon-dated to a specific point, without giving you a margin of error, you should be suspicious.

So, with that all in the back of your head, carbon-dating the Shroud of Turin tells us that the linen (it's linen, not wool, and we refrain from any comments about woolly thinking) was made from flax that was cut around the year 1325 AD, plus or minus 65 years.

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