Carbon Dating System
Edited by StephWrites, Jen Moreau
Is carbon dating reliable?
Almost everyone has heard on the news about archaeological findings claiming to have discovered a 12,000 years old bone, or a tomb with a skeleton of an individual who died 18,000 years ago. Do you remember when they found the famous tomb of Tutankhamun in Egypt? We have certainly been to a Natural History museum and saw exhibits of dinosaur or mammoth skeletons weather they are real or replicas. We might have seen displays of how our pre-historic ancestors looked like, and every display would indicate in the information box the date they were alive. But have you ever wonder how scientists can tell how old they are? The most common answer is by using Carbon 14 dating method!
What is Carbon 14?
Carbon 14 is an isotope of carbon with eight neutrons instead of the more common six neutrons. It is unstable, and it radioactively decays by electron emission to Nitrogen 14.
Carbon-14 is produced every day when cosmic rays enter the earth's atmosphere. If one of these rays collides with an atom, it would create a secondary cosmic ray and become an energetic neutron. If these newly created energetic neutrons encountered a nitrogen-14 atom (seven protons, seven neutrons), it would turn into a carbon-14 atom (six protons, eight neutrons) and a hydrogen atom (one proton, zero neutrons). When these carbon-14 atoms are combined with oxygen, they form carbon dioxide, which plants absorb naturally and incorporate into plant fibers by photosynthesis.
How does it work?
There is a very constant ratio of carbon 12 to carbon 14 within all living things and in the air. It is a fact that carbon-12 and carbon-13 are stable, and that Carbon-14 decays slowly due to beta decay caused by nitrogen-14 with a half-life of approximately 5,730 years.
After the organism dies it stops taking in new carbon. Carbon 14 is created in the atmosphere, and every living organism is exposed to it and will acquire a certain amount after receiving it continuously through his life. The moment this organism dies, it will stop receiving Carbon 14, and the one already contained in its body will begin a slow and precise decay process.
The most important fact scientists have learned after many experiments with Carbon 14 is that it has a half-life of 5730 years. Since all living organisms contain carbon 14, this finding is an incredible aid to calculate the organism´s age by examining how much carbon 14 has decayed, and how much is still intact.
What this means is that if we had an organism with carbon 14 and left it for 5,730 years, half of those carbons would have decayed and the other half would still be carbon 14. With this phenomenon, it is possible to calculate an organism's age, depending on its overall proportion of carbon 14.
Carbon dating formula
All living things have carbon 14 in their tissue. In plants, carbon 14 is incorporated through photosynthesis; in animals or humans, it is acquired when eating plants. When the animal, human or plant dies, there is no more absorption of Carbon 14, and the amount it already contains begins to decrease as it undergoes radioactive decay.
Measuring the amount of Carbon 14 in a sample such as a piece of wood, bone, charcoal, marine or freshwater shell, peat, and organic-bearing sediments; provides the necessary information to calculate when the animal or plant died.
For example: If an archaeologist found a bone, took it to the lab and analyzed it, then discovered that it contains one-half of C14, it means it has decayed one-half life and is 5,730 years old. Then after digging lower on the site, another bone is found and taken to be studied. It turns out to contain only one-fourth of C14; it means it has undergone two half lives: after one half-life it would have lost 50% of C14, and after its second half-life it would end up with half of that. So 2 half-lives are 5730 x 2 = 11,460 years.
- Carbon 14 dating method was developed by Willard Libby in 1940. It is stated on an autobiography by Emilio Segre that the concept was suggested to Libby by Enrico Fermi at a seminar in Chicago. Libby was able to prove the accuracy of this method by calculating and guessing the age of a series of samples of wood for which the age was known, including an ancient Egyptian royal barge dating from 1850 BCE.
- Libby was granted the Nobel Prize for his work in 1960; the method then became a standard tool for archaeologists.
- This method will only work on dead organic material like animals or human bones and tissue, or plants and wood.
- Carbon 14 is not a great method for dating relatively new organisms as none of the carbon 14 would have decayed in such short time. This method is mostly used on organic material which died between 1,000 to 1,000,000 years ago.
- It will not be useful either if the item is older than 1,000,000 years because there would be no carbon 14 left, and there is no way to determine how long has it been gone.
Every time a sample is manipulated, it is absolutely important to keep it from getting contaminated. Any addition of carbon to a sample would change the results of the measured date dramatically. Archaeologists must use specific tools and follow a meticulous sampling methodology to keep their samples pure.
- Contamination with modern age carbon would cause a sample to appear to be younger than it really is.
- The older the sample, the more it would be affected. For example, if a sample that is 17,000 years old is contaminated so that 1% of the sample is carbon from a modern age, it will turn out to be 600 years younger. However, for a sample that is 34,000 years old, the same 1% of contamination would cause an error of 4,000 years.
- On the other hand, if the sample would be contaminated in 1% with an older carbon, so old that it has no carbon 14 remaining, the sample would appear to be about 80,000 years older than it actually is, regardless of the date of the sample.
Improvements over time
- Previously, the measurement of Carbon 14 was done by a beta-counting device, which would count the amount of beta radiation emitted by decaying C 14 atoms in a sample.
- In the latest years, the accelerator mass spectrometry (AMS) has become the method of choice. AMS is a technique for measuring long-lived organisms that have acquired Carbon 14 naturally. AMS works with a particle accelerator along with large magnets, ion sources, and detectors to count single atoms in the presence of a thousand million, millions of stable atoms. It works by counting all the 14 C atoms in the sample and not just the few that happened to decay during the measurements; it can, therefore, be used with much smaller samples, as small as one particular plant seed, plus it also gives results much faster. AMS is used for a wide variety of dating and tracing applications in the geological and planetary sciences, archaeology, and biomedicine.
Radiocarbon dating method development has made a serious impact on archaeology and universal history. It has allowed a more accurate dating of archaeological sites than previous methods, and it has also allowed comparing dates of events across great distances. They have called it "The Radiocarbon Revolution" amongst archaeologists.
This proven dating technique has been of extreme value to anthropologists, scientists, and archaeologists. It is thanks to this method that we have been able to date the most significant transitions in prehistory like the end of the ice age and the Neolithic and Bronze age.
Human history research has been more accurate since the discovery of Carbon 14 method. Before that, most of the dates were estimated, based on information found in ancient books or what was deciphered on hieroglyphics. However, there was no scientific method to be certain.
Next time you visit an archaeological site or go to a Natural History museum, keep in mind that most of the dates previewed on the exhibits were calculated using Carbon 14 method. You might even use this information to start an interesting conversation with your companions.
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