Carbon Dating

The short summary
The theory
Are there things that can't be carbon-dated?
Can we prove that carbon dates are accurate?
Are there inaccurate carbon dates?
For more information

The Short Summary

Carbon dating can be used on material which was living in the last few tens of thousands of years, and which got its carbon from the air. The method has become more accurate in the last few decades.

The Theory

Carbon 14 is a radioactive isotope of carbon. It is produced in the upper atmosphere by radiation from the sun. (Specifically, neutrons hit nitrogen-14 atoms and transmute them to carbon.)

Land plants, such as trees, get their carbon from carbon dioxide in the air. So, some fraction of their carbon is C14. The same is true of any creature that gets its carbon by eating such plants. We can measure this in living things today.

Suppose such a creature dies, and the body is preserved. The C14 will undergo radioactive decay, and after 5730 years, half of it will be gone. Eventually, all of it will be gone. So, if we find such a body, the amount of C14 in it will tell us how long ago it was alive.

Are There Things That Can't Be Carbon-Dated?

Yes. The method doesn't work on things which didn't get their carbon from the air. This leaves out aquatic creatures, since their carbon might (for example) come from dissolved carbonate rock. That causes a dating problem with any animal that eats seafood.

We can't date things that are too old. After about ten half-lives, there's very little C14 left. So, anything more than about 50,000 years old probably can't be dated at all. If you hear of a carbon dating up in the millions of years, you're hearing a confused report.

We can't date oil paints, because their oil is "old" carbon from petroleum.

We can't date fossils, for three reasons. First, they are almost always too old. Second, they rarely contain any of the original carbon. And third, it is common to soak new-found fossils in a preservative, such as shellac. It is also standard to coat fossils during their extraction and transport. Acetone is sometimes used while extracting fossils, because it dissolves dirt. In short, unless you have evidence to the contrary, you should assume that most of the carbon in a fossil is from contamination, and is not originally part of the fossil.

We also can't date things that are too young. The nuclear tests of the 1950's created a lot of C14. Also, humans are now burning large amounts of "fossil fuel". As the name suggests, fossil fuel is old, and no longer contains C14. Both of these man-made changes are a nuisance to carbon dating.

If you hear of a living tree being dated as a thousand years old, that is not necessarily an example of an incorrect dating. Trees only grow on the outside. Wood taken from the innermost ring really is as old as the tree.

Can We Prove That Carbon Dates Are Accurate?

There are two ways to do this. We can date things for which historians know a "right answer". And, we can date things that have been dated by some other method.

Historians don't have "right answers" for really old things. However, carbon dating has done well on young material like the Dead Sea Scrolls, the Minoan ruins, and acacia wood from the tomb of the pharoah Zoser.

Some corals can be carbon dated, and also dated by another radioactive material, Thorium-230. Pollen found in the Greenland icecap has been carbon dated, and also dated by counting ice layers. The three methods confirm each other.

Trees grow a thick ring in a good year, and grow a thin ring in a bad year. It is sometimes possible to match up tree-ring patterns between different trees. When enough suitable trees are found, living or dead, the matching is completely accurate. Then, we have wood for which we know the right answer.

So, carbon dating has been calibrated against the rings of California bristlecone pines, and Irish bog oaks, and the like. When this was first done, it turned out that carbon dating had been giving too-young dates for early civilizations. Apparently, the production of C14 by the Sun has changed by several percent across the last 10,000 years. We know (from other measurements) that the Sun hasn't fluctuated by more than 10 percent in the last million years. However, even this small an adjustment was a bit of a shock. For example, Stonehenge suddenly became older than the Pyramids, instead of younger.

Since then, several other calibrations have been done, which confirm and extend the tree-ring one. Some were done by finding lakes with atmospherically derived carbon in their annual layers of silt (called varves). In those particular lakes, the varves can be counted, and the varves can also be carbon dated. See below for details about the 45,000 annual varves in Lake Suigetsu.

Are There Inaccurate Carbon Dates?

Yes. There are three kinds.

The first kind are datings of things that should't be carbon dated. For example, polar bears that eat seals aren't getting their carbon from an atmospheric source.

The second kind are datings on contaminated samples, or on samples which are a mixture. Old samples contain much less C14, so the measured date of older samples is strongly affected by even small amounts of contamination.

The third kind are dates which were measured before the 1970's. In the 70's:

In short, all carbon datings published in the 1950's and 1960's are suspect.

For More Information

If this sort of thing interests you, you should find the journal Radiocarbon and read one of the issues devoted to calibration. For example, see Radiocarbon 46,1029 (2005), which has a calibration curve that goes back 26,000 years.

On the Web, you could visit a dating laboratory, visit a dating service, read an encyclopedia entry or read a critique.

The Lake Suigetsu varve calibration was reported by ABC News and was published:

Atmospheric Radiocarbon Calibration to 45,000 yr B.P.: Late Glacial Fluctuations and Cosmogenic Isotope Production, Kitagawa and van der Plicht, Science 1998 February 20; 279: 1187-1190
NOTE: free registration is required by Science Online.

Last modified: 26 June 2005

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