Carbon dating

Carbon dating, or carbon-14 dating, is a method for comparing the ages of organic materials such as bones or things made from organic (containing carbon) material.

The technique is based on comparing the levels of C-14 and C-12 isotopes in the sample. When the sample was living, the ratio of C-14 to C-12 in the material is the same as in the atmosphere. But after death the C-14 isotope decays into N-14, while C-12 does not. The loss in C-14 is then used to estimate how long ago the sample died, which is typically the same as how long the sample existed.

The rate of decay of C-14 is such that, if this rate of decay does not change over time, then 50% of the C-14 in the sample will decay in 5730 years: 50% of the C-14 in the sample is left, after 11460 years 25% will be left, after 17190 years 12.5% will be left, after 50,000 years only about 1/500th of the C-14 remains - and since even initially it is only present as a minute proportion of the whole (0.0000000001% of all Carbon atoms), measuring the exact quantity present accurately enough to be of use for dating purposes is extremely difficult. For this reason, scientists do not generally attempt to carbon date material that is believed to be older than about 50,000-60,000 years old.

Limits of Carbon Dating

Currently the upper limit for carbon dating lies at 60,000 years under optimal conditions, until the development of the ABOX technique in 2005 , the upper limit lay around 40,000 years, again under optimal conditions. This means that material as old as the upper limit only has minute quantities of C-14 left, in even older material the quantities of C-14 become virtually undetectable: a 61,000 year old sample will give the same result as a 100 million year old sample!

Fluctuations in the concentration of CO2 in the atmosphere have to be accounted for to calibrate the procedure. Carbon dating has been calibrated by comparing the theory with historical Ancient Egyptian sources and tree rings, these sources reveal the CO2 concentrations in the atmosphere over the last 10,000 years . Ice samples from the Antarctic reveal CO2 concentrations over the last 650,000 years .

An other limitation arises when dating certain animals. Various plants have differing abilities to exclude significant proportions of the C-14 in their intake. This varies with environmental conditions as well. The varying rates at which C-14 is excluded in plants also means that the apparent age of a living animal may be affected by an animals diet. An animal that ingested plants with relatively low C-14 proportions would be dated older than their true age.

Known Errors in Carbon Dating

Material that is younger than 150 years or older than 50,000 years is known to often give errors. The errors for material younger than 150 years is ascribed to changes in the ratio of C-14 to C-12 in the atmosphere. The errors for material older than 50,000 years is ascribed to statistical uncertainties inherent in the loss of multiple half-lives. Contamination by, for example present-day bacteria, will also result in ancient material being dated to be younger than the carbon dating upper limit.

Additional anomalous results from carbon dating, which reinforce its limitations, include:

• coal, natural gas, and carbonate rocks have yielded dates based on carbon dating between 30,000 and 60,000 years old^{[Citation Needed]};

• some unfossilized dinosaur bones have yielded dates between 10,000 and 50,000 years old^{[Citation Needed]}; and

• CO2 from gas wells have yielded dates less than 30,000 years old^{[Citation Needed]}.