Half-Life: Radiocarbon Dating Tehuacán Carbon Samples

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.

Jars of Carbon Samples from various sites in the Tehuacán Valley.
Jars of Carbon Samples from various sites in the Tehuacán Valley.

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 Examples of objects that can be used to find absolute dates are historical documents and calendars.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.

Part of this carbon sample sent to Isotopes lab for radiocarbon dating.
Part of this carbon sample sent to Isotopes lab for radiocarbon dating.

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.

Map of the Tehuacán Valley and some of the sites the carbon samples came from.
Map of the Tehuacán Valley and some of the sites the carbon samples came from.

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.

Further Reading

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.

Ceramic Inventory Complete!

For the past year and a half, I have spent the majority of my time inventorying drawers as part of the Peabody’s Inventory and Reboxing Project. As exciting as that project has been, every once in a while I have needed to take a break from it to recharge my brain. In order to recharge while simultaneously staying productive, I was tasked with photographing and inventorying the ceramic vessel collection that is housed on the second floor of the Peabody. In order to complete this project, I printed out inventory sheets for the ceramic cabinets in Second Floor Storage and went shelf by shelf making sure each vessel was there and photographing it. This was great because I enjoy photography and try to do it often in my spare time. Once the vessels were photographed, I edited them in Photoshop and then uploaded the finished photos to each object’s catalog record in our museum software, PastPerfect. Editing the photos was very enjoyable for me because I was able to expand my Photoshop skills, which were pretty limited before taking on this project.

265_119
One of the ceramics that was inventoried and photographed

The original goal was to take one day a week to do these tasks. However, as the inventory project got rolling and certain collections needed to be cataloged faster than others, the ceramic inventory ended up getting slightly pushed aside in order to accommodate more pressing tasks at the Peabody. The main point of this blog post is this: the ceramic inventory has officially been completed!! Each vessel is logged into PastPerfect and has a photograph attached to it. The other exciting bit of information is that all of these vessels (except for NAGPRA sensitive ones) are available to view in our online catalog which can be found HERE. Take a look at the link to see what I have been working on, and feel free to peruse the online collections even further to see what else is housed at the Robert S. Peabody Institute of Archaeology!

Changing Roles and Responsibilities of Museums

The past five years have been a busy time for museums- most notably in the image department.  Following a number of high profile controversies, a lot of people–audiences, and museum professionals alike–asked what role museums play in our society?  Here are a couple of recent articles dealing with this subject head on.

Why Museum Professionals Need to Talk About Black Panther

black panther
Killmonger in the Museum (Photo courtesy of article)

Last month saw the release of Marvel’s newest blockbuster, Black Panther.  Besides being a fantastic movie, this film offers a unique chance to open dialogues on a large scale about many topics- least of which are museums as mechanisms of colonialism.  This article discusses how and why museum professionals especially should look at their roles in this and the effects they have on the audiences we try to reach.  The piece ends by laying out suggestions for how museums can move forward incorporating and working towards more diverse and open dialogues between communities.

Two Museum Directors Say It’s Time to Tell the Unvarnished History of the U.S.

Ggover and Bunch
Gover and Bunch at Symposium (Photo courtesy of Smithsonian Magazine)

This article opens with the quote, “history matters because it has contemporary consequences,” and it just gets better from there.  Directors Kevin Gover (National Museum of the American Indian) and Lonnie Bunch (National Museum of African American History and Culture) participated in a day long symposium titled, “Mascots, Myths, Monuments and Memory,” in which they talked about confronting the historic and continued racist ideologies that are entrenched in contemporary American society and the role of museums.  They specifically discuss the example of the concurrent rise of confederate statues and racist mascots.

How the Dana Schultz Controversy- and a Year of Reckoning- Have Changed Museums Forever

protest
Election protest (photo courtesy of Quartz)

Chronicling a series of high profile controversies, this article looks at the combination of factors that have led to these, as well as the changes they are bringing to museums and their operation.  It also discusses why museums have become ground zero for explosive cultural encounters stating, “We’re in a time when these issues are real, these controversies are part of public space and public discourse, and museums are going to become the places where these issues get played out.”

Native Voices, Accurate History Forge Deeper, Better Understanding of American Indians in Nations Schools

NMAI education
Students using NK360 (photo courtesy of Smithsonian Insider)

This article showcases the role museums have within their respective walls and how they are branching out to have far reaching impacts in classrooms all over the nation.  Similar to classes taught at the Peabody by Curator of Education, Lindsay Randall, this article follows the creation and implementation of National Museum of the American Indian’s newest initiative, Native Knowledge 360. NK360 is a “long-term initiative to integrate the Native American experience into social studies, language arts and other curriculum in kindergarten through 12th-grade classrooms across the country.”   This program works with the inclusion and cooperation of Native communities and educators as well as provides educational materials for teachers.

 

 

Updated Collections Online

A storage bay with a mixture of drawers and boxes

The Peabody is excited to share our updated collections website!

Curious about the scholarly depth of the Peabody collections?  Looking for material from a particular site for your research?  Interested in simply browsing through the artifacts?

The site’s new and improved format is more user-friendly and provides easier access to our object records.  Enter a key word to search, browse a full list of sites, and click through random images of artifacts.

In addition to a streamlined interface, the updated website also includes information about the archival collections housed at the Peabody.  Temporary Archivist Irene Gates’s recent blog post  highlights the completion of the first step in processing our archives.  Or, explore the archival collections here.

While we do not yet have our full collection online, we add new records regularly – so come back often.  I hope that you enjoy exploring the Peabody’s collections as much as I do!

The Peabody Collection Online is made possible in part by a grant from the Abbot Academy Association, continuing Abbot’s tradition of boldness, innovation, and caring.

Girl Power! in the Archaeological Record

Contributed by Samantha Hixson

Follow these links to read some awesome stories of how women are helping unlock the secrets of the archaeological record.

Elite ‘Dynasty’ at Chaco Canyon Got It’s Power From One Woman, DNA Shows

Once again DNA analysis of sites is opening up our understanding of how societies operated historically.  By testing bone samples from Room 33 in Pueblo Bonito of Chaco Canyon, scientists were able to shed more light on the inner workings of power, class, wealth and status of ancestral Puebloans, and the major role women played within these.

New Discoveries from Cahokia’s ‘Beaded Burial’ May Rewrite Story of Ancient American City

Mound 72 of the Cahokia culture complex, when originally excavated in 1967, was thought to be a shining example of a burial of elite male warriors.  Fast forward almost 50 years and imagine archaeologist’s surprise when one third of the skeletons found were in fact female!  These findings call into question the idea that Cahokia was a male warrior-led patriarchy.

Grave of Disabled Young Woman Reveals Touching Tale of Care in Prehistoric Arizona

The excavation of young Hohokam woman’s grave is an example of what the excavators and author call the “Bioarchaeology of care.”  The young woman, who lived about 800 years ago had scoliosis, rickets, and tuberculosis. Through looking at this site, archaeologists are able learn more about the community in which the girl lived, and how they supported and cared for her, giving a decidedly human lens to a science that can sometimes become disconnected.