This short video aims to explain how archaeologists date historical sites. This question is very general, because a historical site can be destroyed and rebuilt after hundreds of years, meaning that in this case it is possible to look at a site as a set of several sites, each from a different period. We will assume that archaeologists know how to separate the different periods. Since I’m guessing you’re all chemistry lovers, so a few minutes about carbon 14 and its importance for determining the date of historical sites.
Carbon 14 allows us to date organic materials such as plant and animal bones. A common organic material used in Israel are olive kernels.
All elements in nature are made of atoms. The atom is built, in general, from a nucleus, which consists of protons and neutrons, and from the electrons that revolve around the nucleus. The atomic property that distinguishes one substance from another is the number of protons in the nucleus. This number – called the atomic number of the element – determines the chemical properties of the material, and from it derives its mechanical and other properties.
Last term for today – Isotopes. Isotopes of a chemical element are atoms that have the same atomic number but have a different mass, that is, the number of neutrons is different. Isotopes of the same element have the same chemical properties.
An example related to our case – Isotopes of carbon are carbon-12 which makes up approx. 98.9% of the carbon in nature, it has six protons, like all carbon, and six neutrons. Carbon-13, which is approx. 1.1% of the carbon in nature, has also 6 protons but 7 neutrons, and carbon-14, which is one part in trillion of the carbon in nature, has 6 protons and 8 neutrons. All the isotopes together make up 100%, so the ratio between them is known to us.
Carbon 14, like the other carbons, is assimilated in plants from the atmosphere through photosynthesis (photosynthesis is the conversion of light into chemical energy), from the plants it passes to animals because they eat plants, which means that every organic substance has carbon 14.
As we have already seen, because all three have the same number of protons, their atomic number is the same and so are the chemical properties of the substance, but…The first two isotopes, carbon-12 and carbon-13, are stable, maintaining their structure and chemical properties, while carbon-14 is radioactive, which means that it decays, breaks down, changes its composition and properties, and turns into another element over time: nitrogen-14. The most important is that the rate of decay of carbon 14 is constant, and based on this fact, a parameter called – “half-life” was determined.
If you take a certain amount of carbon-14 and measure how long it took for half of it to become nitrogen-14, you will find that 5,730 years have passed. Broadly speaking, this means that, for example, by comparing the ratio between carbon 14 and carbon 12 in the atmosphere compared to the ratio between carbon 14 and carbon 12 in organic matter, the age of the sample can be deduced.
For example, if in the atmosphere we expect to find that out of the total carbon isotopes, one part in 1 trillion is carbon 14 and 98.9% is carbon 12, and in the organic matter we find that there is only 1 part in two trillion is carbon 14, it means that half has decayed, so the age of the organic matter is 5730 years and so on.
Two final comments. I just presented a general example and it is accurate only on the assumption that the atmosphere has a constant amount of regular carbon, which is what was initially assumed, but over the years it was discovered that this is not such an accurate assumption, so calibration methods were introduced that I will not elaborate on. Second note – as long as the organic being is a live, it “breath” and thus the three isotopes maintain a constant level of the ratios as in the atmosphere. When it dies, the carbon no longer penetrates, the decay process of carbon 14 begins and everything I explained occurs.
Bottom line, by measuring the amount of carbon 14, it is possible to estimate the age of the measured material with an accuracy of say +- 20 years as long as its age is less than 60’000 years. Carbon 14 was discovered in 1950 by physicist Willard Libby and this won him the Nobel Prize in 1960.
And to conclude as usual a question for you. In the absence of organic materials on the site: what is another method of determining the age of historical sites? The answer will appear at the end of the video.