How do you know that?
I generally reply: It's my job. But in response to a diminishing number of requests I will try to remove some of the mystery from the processes archeologists use to determine the age of objects recovered in excav
ations. Unless you have been a resident of the planet Zork for your entire life you have certainly heard of radiocarbon dating. Unfortunately the process is somewhat complicated, which tends to put people like me in a category of bystanders who don't have a real good grasp of physics and chemistry. Nevertheless, if you want to understand how archeologists come up with dates for artifacts you will just have to grin and bear it.
It is hard to imagine what archeology must have been like before the invention of radiocarbon dating in the late 1950's. Scholars had figured out the process of stratification early in the twentieth century, which provided them with a relative chronology of objects recovered from archeological sites throughout Mesoamerica. In other words, they concluded that materials found deep in the ground were, almost without exception, to be older than those objects nearer the surface. Sounds rather simplistic now, but for those of us who have routinely flown thousands of miles at speeds in excess of five hundred miles per hour at altitudes of eight miles above the earth, the Wright Brothers must seem a little simplistic as well.
So, having established the idea of stratification, archeologists could then assign things like distinctive kinds of pottery or special stone tools to periods in the past. Still a crude system of classification, but now at least pottery type "A" could be shown to be in all cases to be older than pottery type "B" because it was always found below type "B" in excavations. So archeologists, especially in the Maya area, could now put together "floating chronologies" where it could be demonstrated that certain classes of artifacts could be shown to be older or more modern than other types. What was desperately needed was a way to fix the "floating chronology" to a hard date in the past. Alas, it was to take many years to come up with such a fixed date.
By the early 1960's, scientists experimenting with radioactive materials discovered that certain naturally occurring substances such as uranium and thorium would gradually give off particles and eventually become stable and non-radioactive. Thus certain radioactive materials could serve as clocks, since the rate of decay of the materials were constant and the rate could be measured with extreme accuracy.
Radiocarbon dating derives its name from the measurement of the amount of particles remaining in a certain form of...you guessed it...carbon. High in the upper atmosphere of the earth a lot of very strange things go on that most of us never hear about (or to tell the truth, really care about). Even as you are reading this column (every word a pearl) gamma rays from outer space are bombarding the upper atmosphere. Sort of like X-rays, these very powerful Gamma rays strike nitrogen molecules and cause then to burst apart and form other substances, one of which is a radioactive form of carbon, called Carbon-14. Now Carbon-14 will, over time, decay into a non-radioactive form of carbon, but it is a slow process. It takes around 3700 years for Carbon-14 to lose half of the particles it must lose before becoming neutral. We therefore say that Carbon-14 has a "half-life" of 3700 years or so.
Meanwhile Carbon-14 molecules have linked up with oxygen molecules to form carbon dioxide. As you know, all plants take in carbon dioxide to make oxygen so all plants are constantly maintaining a level of measurable radioactivity due to Carbon-14. This does not excuse children from eating spinach, as the levels of radioactivity are so low as to be undetectable with all but the most sensitive instruments. However, since every living thing on earth is a part of a food chain that contains Carbon-14, all plants and animals, as long as they are living, will maintain a level of radioactivity. When that plant or animal dies, it no longer takes in Carbon-14 and the decay process of the existing Carbon-14 begins. Since we can measure the level of radioactivity generated in modern samples of Carbon-14, we can take samples of ancient materials (as long as they are organic) and compare levels of radiation with the modern sample. If the piece of ancient bone or charcoal contains only half as much radioactivity as the modern sample, the ancient material should be around 3700 years old, right? (You might want to read the last paragraph again).
The key word is "organic", because inorganic materials such as pottery or stone tools can't be dated by radiocarbon means, since they never took in Carbon-14. However, if one finds organic substances at the same depth or level as the pottery, for example, then by inference the date of the organic material must be the same as the date for the pottery found at the same spot.
Many advances have been made in the Carbon-14 process over the years. The latest device that utilizes the same principal of dating through a count of radioactive particles is called a tandem-accelerated mass spectrometer, but I think we've had enough physics for one day.
Curious about something to do with Archeology? Dr. Smith welcomes suggestions for future articles. Please send suggestions to P.O. Box 35 or e-mail: email@example.com
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