Place the sample into the glass chamber
Electrically excited oxygen is used to oxidize or convert organic material in the paint sample to carbon dioxide (CO2) and water.
The glass tube is immersed in liquid nitrogen in order to freeze the gas CO2 to dry ice.
Seal off the tube of CO2 using a blow torch.
Send glass tube of CO2 off for radiocarbon dating at an Accelerator Mass Spectrometry (AMS) laboratory.
The Plasma Oxidation Advantage
One beneficial aspect of plasma oxidation over other sample preparation techniques is that only the organic carbon is extracted, leaving intact the inorganic portion of the paint sample because the system remains below the decomposition temperature of these minerals. This allows us to avoid the extensive acid treatments that traditional sample preparation and combustion use. And, we are able to analyze much smaller samples with high mineral content, which is ideal for dating paint that is still adhering to rock! Once the carbon dioxide is extracted, it is sent for radiocarbon dating using AMS. I have collaborated with Lawrence Livermore National Laboratory’s Center for Accelerator Mass Spectrometry (CAMS) for over 20 years.
Ongoing Shumla Radiocarbon Research
Since the 1990s, the focus of rock art dating in the Lower Pecos has been on dating Pecos River Style paintings, with 33 radiocarbon assays from 9 different sites. The radiocarbon dates for the Pecos River Style range from approximately 2700 B.C. to A.D. 600 (Bates et al. 2015). With this data set, the Lower Pecos Canyonlands is considered one of the best-dated rock art provinces in the world. However, most of these samples were collected in the 1990s, and in many ways this early work was experimental — Marvin and his students were demonstrating that plasma oxidation could be utilized to directly date rock paintings. But, because the Lower Pecos Canyonlands was the first place that plasma oxidation was used to radiocarbon date rock paintings, and the focus was on developing the technique, there was very little focus on answering archaeological research questions. For most of the dated samples, we know nothing more about the sample than the radiocarbon date itself. In other words, we have no information on the rock figures which were dated. This is a crucial piece of missing information because one of the research projects I and other Shumla researchers are most interested in pursuing is determining the chronological distribution of specific rock art images and motifs across the Lower Pecos landscape.
The Shumla Chemistry Lab Part 2: Plasma Oxidation and Eagle Cave Pictographs
Stay tuned for Part 2 of the Shumla Chemistry Lab blog which will detail how we used plasma oxidation to date the rock art at Eagle Cave, and how we plan to use radiocarbon dating alongside The Alexandria Project to enhance our knowledge of Lower Pecos chronology.