# Radioactive carbon dating fossils

By looking at the ratio of carbon-12 to carbon-14 in the sample and comparing it to the ratio in a living organism, it is possible to determine the age of a formerly living thing fairly precisely. So, if you had a fossil that had 10 percent carbon-14 compared to a living sample, then that fossil would be: t = [ ln (0.10) / (-0.693) ] x 5,700 years t = [ (-2.303) / (-0.693) ] x 5,700 years t = [ 3.323 ] x 5,700 years Because the half-life of carbon-14 is 5,700 years, it is only reliable for dating objects up to about 60,000 years old.

If the fossil has 35% of its carbon 14 still, then we can substitute values into our equation.Green plants absorb the carbon dioxide, so the population of carbon-14 molecules is continually replenished until the plant dies.Carbon-14 is also passed onto the animals that eat those plants.After death the amount of carbon-14 in the organic specimen decreases very regularly as the molecules decay.Carbon-14 has a half-life of 5,730 ± 40 years, meaning that every 5,700 years or so the object loses half its carbon-14.The ratio of carbon-12 to carbon-14 at the moment of death is the same as every other living thing, but the carbon-14 decays and is not replaced.

The carbon-14 decays with its half-life of 5,700 years, while the amount of carbon-12 remains constant in the sample.

By examining the object's relation to layers of deposits in the area, and by comparing the object to others found at the site, archaeologists can estimate when the object arrived at the site.

Though still heavily used, relative dating is now augmented by several modern dating techniques.

Radiocarbon dating can be used on samples of bone, cloth, wood and plant fibers.

The half-life of a radioactive isotope describes the amount of time that it takes half of the isotope in a sample to decay.

Archaeologists use the exponential, radioactive decay of carbon 14 to estimate the death dates of organic material.