Carbon-14 is a radioactive isotope of carbon, and is as a result often used in to date archaeological finds. The presence of Carbon-14 in all organic material (including wood, textiles and bones) comes about because of the bombardment of the atmosphere by cosmic rays, which is subsequently absorbed by the living object.
Carbon dating, using C14, works because there is a known half-life for the isotope of 5,730±40 years (originally thought to be 5,568±40 years), and once the living object dies no new Carbon-14 will be absorbed, and beta decay occurs. This rate of decay is what is measured to provide a date. The ±40 years in the half-life of the carbon isotope, immediately means though that date provided will not be exact.
Some people will say that radio carbon dating is inaccurate, and will quote dinosaur bones being dated to 16,000 years or live sea molluscs being dated as dying thousands of years ago. Most inaccuracies though come about because the carbon dating is being applied outside of its accepted perimeters.
Carbon dating is generally only used to date items up to 50000 years, and more commonly up to 20000 years. Older items will have so little carbon-14 left that other sources of carbon will have contaminated the object to such a degree that dating is impossible. Other forms of dating, including uranium-thorium dating, can provide more accurate dating figures for older items.
Organic material associated with marine life should also not be dated with carbon dating. Marine life doesn’t absorbed carbon-14 from the atmosphere and so levels are normally highly skewed.
In most cases recently deceased organic material is also not dated with carbon dating. The burning of fossil fuels, and the used of atomic weapons, have both greatly changed the amount of carbon-14 found in the atmosphere, and living items during this period will also have greatly altered levels of the isotope.
Figures taken from carbon dating are normally calibrated against other known dating methods, including dendrochronology, ice-core samples, and speleothem dating. These methods can provide guidance on atmospheric changes and other influences which might impact on the decay rate of the C14. Calibration curves are created for locations around the world, and then by taking the carbon dating result and plotting it on the curve a date can be estimated.
Carbon-14 dating remains a highly accurate method for dating many finds within its accepted perimeters, although no scientist is going to claim that a date given is 100% accurate.