This process is constantly occurring, and has been for a very long time, so there is a fairly constant ratio of carbon 14 atoms to carbon 12 atoms in the atmosphere. Now living plants 'breathe' CO 2 indiscriminately they don't care about isotopes one way or the other , and so while they are living they have the same ratio of carbon 14 in them as the atmosphere. Animals, including humans, consume plants a lot and animals that consume plants , and thus they also tend to have the same ratio of carbon 14 to carbon 12 atoms.

How Carbon Dating Works | HowStuffWorks

This equilibrium persists in living organisms as long as they continue living, but when they die, they no longer 'breathe' or eat new 14 carbon isotopes Now it's fairly simple to determine how many total carbon atoms should be in a sample given its weight and chemical makeup. And given the fact that the ratio of carbon 14 to carbon 12 in living organisms is approximately 1: In actually measuring these quantities, we take advantage of the fact that the rate of decay how many radioactive emissions occur per unit time is dependent on how many atoms there are in a sample this criteria leads to an exponential decay rate.

We have devices to measure the radioactivity of a sample, and the ratio described above translates into a rate of Voila, now you can tell how old a sample of organic matter is. Carbon 14 dating is not great for dating things like a year old because if much less than 1 half-life has passed, barely any of the carbon 14 has decayed, and it is difficult to measure the difference in rates and know with certainty the time involved.

On the other hand, if tons of half-lives have passed, there is almost none of the sample carbon 14 left, and it is really hard to measure accurately how much is left.

Since physics can't predict exactly when a given atom will decay, we rely on statistical methods in dealing with radioactivity, and while this is an excellent method for a bazillion atoms, it fails when we don't have good sample sizes. However it is possible, when dating very old rocks for instance, to use longer lived isotopes for dating on a longer time scale.

Isotopes of a particular element have the same number of protons in their nucleus, but different numbers of neutrons.

This means that although they are very similar chemically, they have different masses. The total mass of the isotope is indicated by the numerical superscript. While the lighter isotopes 12 C and 13 C are stable, the heaviest isotope 14 C radiocarbon is radioactive. This means its nucleus is so large that it is unstable. Over time 14 C decays to nitrogen 14 N. Most 14 C is produced in the upper atmosphere where neutrons, which are produced by cosmic rays , react with 14 N atoms.

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This CO 2 is used in photosynthesis by plants, and from here is passed through the food chain see figure 1, below. Every plant and animal in this chain including us! When living things die, tissue is no longer being replaced and the radioactive decay of 14 C becomes apparent. Around 55, years later, so much 14 C has decayed that what remains can no longer be measured.

Radiocarbon Dating and Archaeology

In 5, years half of the 14 C in a sample will decay see figure 1, below. Therefore, if we know the 14 C: Unfortunately, neither are straightforward to determine.

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The amount of 14 C in the atmosphere, and therefore in plants and animals, has not always been constant. For instance, the amount varies according to how many cosmic rays reach Earth.

Luckily, we can measure these fluctuations in samples that are dated by other methods. Tree rings can be counted and their radiocarbon content measured. A huge amount of work is currently underway to extend and improve the calibration curve. In we could only calibrate radiocarbon dates until 26, years. Now the curve extends tentatively to 50, years. Radiocarbon dates are presented in two ways because of this complication.

The uncalibrated date is given with the unit BP radiocarbon years before The calibrated date is also presented, either in BC or AD or with the unit calBP calibrated before present - before The second difficulty arises from the extremely low abundance of 14 C.

How Carbon-14 Dating Works

Many labs now use an Accelerator Mass Spectrometer AMS , a machine that can detect and measure the presence of different isotopes, to count the individual 14 C atoms in a sample. Australia has two machines dedicated to radiocarbon analysis, and they are out of reach for much of the developing world. In addition, samples need to be thoroughly cleaned to remove carbon contamination from glues and soil before dating.

This is particularly important for very old samples. Because of this, radiocarbon chemists are continually developing new methods to more effectively clean materials.

What is Radiocarbon Dating?

These new techniques can have a dramatic effect on chronologies. With the development of a new method of cleaning charcoal called ABOx-SC , Michael Bird helped to push back the date of arrival of the first humans in Australia by more than 10, years. Moving away from techniques, the most exciting thing about radiocarbon is what it reveals about our past and the world we live in. Radiocarbon dating was the first method that allowed archaeologists to place what they found in chronological order without the need for written records or coins.

In the 19th and early 20th century incredibly patient and careful archaeologists would link pottery and stone tools in different geographical areas by similarities in shape and patterning.