Assessing Prehistoric Interaction Through Elemental Analysis of Stone Tool Raw Materials

My goal is to evaluate the degree to which neighboring prehistoric populations interacted through the exchange of obsidian in the Patagonia Region of Argentina (Rio Negro, Neuquen, Chubut and Santa Cruz Provinces) during the Mid to Late Holocene (6,500 BP – AD 1850) in order to investigate the social implications of economic interactions. Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) is the preferred method of elemental analysis because it is more sensitive and less destructive than other methods of artifact sourcing.

Obsidian (Volcanic Glass)

Long distance exchange of obsidian reflects social relationships between groups. The hypothesis I will test is that obsidian was more widely traded in the Mid-Holocene than the Late Holocene, because Mid-Holocene population levels were lower, and interaction between hunter-gatherer groups was important for access to information, marriage partners, and resources. Subsequent Late Holocene population increase due to immigration from neighboring regions reduced mobility and increased territoriality resulting in the intensified use of local resources. This posited shift in exchange and population suggests a social response to increasing competition over resources where residential groups became more protective of local resources, which archaeologists consider a factor leading to the emergence of social inequality. The identification of Mid-Holocene long-distance obsidian exchange, followed by a Late Holocene shift to local obsidian use would support this hypothesis. I will test this by assessing the distribution of obsidian artifacts from the Mid to Late Holocene in this region.

Data regarding stone raw material procurement shifts over time has been generated through provenance analysis using the geochemical technique of Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) at the California State University at Long Beach (CSULB). LA-ICP-MS is preferred because it is more sensitive and less destructive than other methods of artifact sourcing. I have collected data from obsidian samples from archaeological sites throughout Patagonia spanning the Holocene Period to determine the location of the material sources used. I have determined their elemental compositions using LA-ICP-MS at the California State University Long Beach (CSULB) Archaeometry Laboratory under the supervision of Dr. Hector Neff. Elements produce characteristic amounts of radioactive isotopes proportional to their concentration, and Dr. Neff and and Charles Stern, University of Colorado at Boulder, are showing me how to identify and quantify them by comparison with known standards to interpret the results. I have also analyzed samples collected from 7 modern obsidian sources. This will provide an elemental signature for regional obsidian sources, and should be similar to the elemental composition of the prehistoric obsidian stone tools made from them. I have made the results for the Santa Cruz Province obsidian sourcing study available on this website. The results for the Northern Patagonia and Southern Pampas obsidian sourcing study are not yet available.

Other Raw Material Sources

I plan to analyze more obsidian samples in 2007, as well as fine-grained basalt, chert, dacite, and chalcedony samples from sites across Patagonia, as part of my dissertation research. The results of this study will assess the scale and nature of prehistoric exchange and group interaction over the Mid to Late Holocene.

Comparative Study

In 2007, I will also collect and analyze samples from interior Patagonia and the U.S. Great Basin in order to conduct comparative analyses of long-distance obsidian exchange by high-latitude hunter-gatherers.