Contributed by Emma Cook
In my last blog I discussed soil analysis and its importance in dating and understanding a site. Another form of analysis used in the Tehuacán Archaeological-Botanical Project by Richard “Scotty” MacNeish was a process called radiocarbon dating, a technique developed by University of Chicago physicist Willard Libby. Carbon samples were collected during excavation and sent for carbon dating to be used for the Tehuacán Chronology Project.
There are two techniques for dating in archaeological sites: relative and absolute dating. Relative dating, in a stratigraphic context, is the idea that objects closer to the surface are more recent in time relative to objects found deeper in the ground. This relates to the law of superposition, which in its plainest form, states soil layers in undisturbed sequences will have the oldest materials at the bottom of the sequence and the newest material closer to the surface. Although this form of dating can work well in certain cases, it does not work for all.
Many sites include soil layers that have been disturbed and this can happen several ways. Natural disasters, such as floods, can erase top layers of sites. Rodents can move around layers in a site as they burrow underground, sometimes moving items from one context to another. Even current human activity can change the stratigraphy of a site through construction, post holes, and pits.
This takes us to our second dating technique. Absolute dating represents the absolute age of a sample before the present. Examples of objects that can be used to find absolute dates are historical documents and calendars. However, when working in an archaeological site without documents, it is hard to determine an absolute date. If a site has organic material present, radiocarbon dating can be used to determine an absolute date. Radiocarbon dating is a universal dating technique that is used around the world and can be used to date materials ranging from about 400 to 50,000 years old. Radiocarbon dating may even work on very recent materials.
Organisms such as plants and animals all contain radiocarbon (14C). When these organisms die, they stop exchanging carbon with the environment. When this occurs, they begin to lose amounts of 14C overtime through a process called radioactive decay. The half-life of 14C is about 5,730 years. Radiocarbon dating measures the amount of stable and unstable carbon in a sample to determine its absolute date. As a result, the older the organic material, the less 14C it has relative to stable versions of the isotope.
The carbon samples recovered from the Tehuacán Valley were collected specifically with this in mind. Many of these samples had labels or notes stating that some of each sample was sent to labs for radiocarbon testing. The carbon samples are organic material and their properties of radiocarbon were used to determine the age of the material, which in turn, helped date each site.
The following sites are represented in some of the jars of carbon samples I catalogued from the Tehuacán Archaeological-Botanical Project.
Site Number Site Name Radiocarbon years
Tc 35 El Riego 6800 to 5000 B.C.
Tc 50 Coxcatlan Cave 5000 to 3400 B.C.
Tc 307 Abejas 3400 to 2300 B.C.
Tc 272 Purron Cave 2300 to 1500 B.C.
Ts 204 Ajalpan 1500 to 800 B.C.
These results were published in Volume Four of MacNeish’s Prehistory of the Tehuacan Valley: Chronology and Irrigation and can be found on Page 5. MacNeish and Tehuacán Chronology Project director, Frederick Johnson, selected carbon samples to be sent for testing, which resulted in the determination of 218 radiocarbon dates. Johnson played a prominent role in radiocarbon dating, serving as the chair of the Committee on Radioactive Carbon 14 set up by the American Anthropological Association. This project not only produced a chronology for the Tehuacán sequence of excavated sites, but later contributed (along with 400 additional radiocarbon dates) to the chronology for all of Mesoamerica. The dates, however, were made within the first two decades of radiocarbon dating and lack the accuracy and precision now available with newer techniques, especially with the older dates.
To read more about the Tehuacán Archaeological-Botanical Project and the Tehuacán Chronology Project visit Internet Archive.
Libby, Willard F. Radiocarbon Dating, 2nd ed., University of Chicago Press, Chicago, IL, 1955. Print.
MacNeish, Richard S. et al., The Prehistory of the Tehuacan Valley: Chronology and Irrigation. Vol. 4. University of Texas Press, Austin, TX, 1972. Print.
Stromberg, Joseph. “A New Leap Forward for Radiocarbon Dating.” Smithsonian.com, October 18, 2012. Web. https://www.smithsonianmag.com/science-nature/a-new-leap-forward-for-radiocarbon-dating-81047335/
Taylor, R.E. and Ofer Bar-Yosef. Radiocarbon Dating: An Archaeological Perspective. 2nd ed., Routledge, New York, NY, 2016. Print.