Carbon dating
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What is it?
Carbon dating, also known as 'radiocarbon dating' or 'C-14 dating', is a technique for determining when a piece of formerly-living matter (i.e. wood, leather, etc) died. It is one of several radiometric dating methods.
How does it work?
The theory
The element carbon is found in all known life-forms. Like many other elements, carbon has a number of isotopes; the three most common are Carbon-12, the most common of all, which is stable; carbon-13, also stable; and the radioactive carbon-14, which produces nitrogen-14 when it decays. All three carbon isotopes are found in nature, in a ratio of about 1012:1010:1. While the nuclear/radioactive properties of the three carbon isotopes are quite different, their chemical properties are indistinguishable.
The C-14 content of the environment changes by various processes. There are two processes which most strongly affect the C-14 content of the atmosphere:
- C-14 is generated by cosmic radiation hitting nitrogen-14 atoms in the upper atmosphere. This increases the C-14 content of the carbon dioxide in the air. By photosynthesis the C-14 goes into plants, then animals.
- C-14 is radioactive and decays into nitrogen-14. This happens in every circumstance.
Since cosmic radiation is not constant, the C-14 content of the atmosphere varies. When a living thing ingests carbon atoms from the environment, it will acquire the three isotopes of carbon in whatever the standard ratio happens to be at the time -- and this is the basis for carbon dating. Since C-14 decays to nitrogen the C-14 content of any living creature would continually decline if it was not, at the same time, continually ingesting more C-14 from their environment. Since living creatures stop ingesting carbon (or anything else!) when they die, it follows that the C-14 content of a dead carcass must continually decline. Thus, knowing the C-14 content of a piece of once-living matter can tell you how long it's been since that object was alive.
Practical details
You need to know the half-life of C-14; this has been measured at 5730 years. In other words, after 5730 years have passed, half of the original supply of C-14 atoms will have decayed, so only half of the original C-14 atoms will still be there; after 11460 years (i.e., two half-lives), one-quarter (= .25) of the original supply of C-14 atoms will be left; after 17190 years (i.e., three half-lives), one-eighth (= .125) of the original supply of C-14 atoms will be around; and so on. By the time ten half-lives (57,300 years) have passed, only about .001 of the original C-14 atoms will still be there. Given that C-14 makes up only one-trillionth of all carbon atoms in the first place, measuring C-14 content at all is a bit tricky, and by the time ten half-lives have rolled around, the C-14 content is too small to allow any meaningful statement about the true age of the object being dated.
To verify and add confidence to radiocarbon dates the C-14 content of the environment at the time of death of an organism must be known. Fortunately, one can find the true age of a piece of wood by comparing its tree rings with those of other trees (dendrochronology) and cross-referencing that absolute age with the present-day C-14 content of that wood. Currently calibration with bristle-cone pines is good for about 6000 years into the past. Correlation by use of annual growth layers in coral, sediment layers, and speleothems extend the calibration back 26,000 years. In addition recent studies of correlation of dated ocean sediment cores with Greenland ice cores, varves (layering in lake sediments) from Japan and a detailed study of speleothems from a submerged cave in the Bahamas will add confidence and perhaps extend the calibration back to about 45,000 years. Research and fine-tuning continue.
Creationist claims about carbon dating
- Carbon dating gives inaccurate results
- Carbon-14 in Coal Deposits indicates a young earth
- Variable C14/C12 ratio invalidates C14 dating
- A freshly killed seal was C14 dated at 1300 years old
- Ancient oil is C14 dated as only 50,000 years old
- Living snails were C14 dated at 2,300 and 27,000 years old
- Triassic wood from Australia was dated at 33K years old
- Vollosovitch and Dima mammoths yielded inconsistent C14 dates
- Biologists think that Carbon 14 proves evolution
