Isotopic Methods for Sourcing Shell Beads in California

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With support from the National Science Foundation, Drs. Jelmer Eerkens and Howard Spero will build on previous research to develop a robust method for tracing the geographic origin of ancient shell beads in Central California. The approach is based on stable isotope signatures preserved in the shells used to make beads. There is significant variation along the California Coast in seasonal water temperature, salinity, and upwelling of cold and nutrient-rich seawater. Together, these effects impart distinctive isotopic signatures to shells growing along different sections of the coast. When a shell is collected on the coast, turned into a bead, and traded inland, it retains the signature of where it originally grew in the calcium carbonate structure of its shell. When archaeologists recover shell artifacts far away from the coast, they can work backwards, from the artifact and through the isotopic signature, to determine where the shell originally grew.Archaeologists use these types of techniques to trace the movement of raw materials across the landscape and this allows them to reconstruct ancient trading and interaction networks. These trading networks, by extension, allow evaluation of how people adapted to their ever-changing social and natural environments. As new technologies were innovated and spread, demand for raw materials to support them also increased, and reliable access to raw materials that were not locally available would have been important. Archaeologists can trace the history and development of different technologies by examining where raw materials were procured, the style of artifacts produced, and how the two co-evolved over time. Although the specific case study to be examined here addresses ancient trade and exchange of marine shell artifacts in Central California, the approach can be applied to shell artifacts almost anywhere in the world.In California, indigenous peoples commonly made beads out of the purple olive shell, Olivella biplicata. At the time of contact, in the late 1700s, these shell beads were used in a manner akin to currency and they are ubiquitous in archaeological sites. An exact tally is not available, but over the last 100 years archaeologists have recovered well over a million Olivella beads from sites in Central California. Even the earliest sites in California, dating to over 10,000 years ago, have beads made out of Olivella biplicata. Why Native Californians selected this particular species is not known, but is something the proposed research may be able to address.This project will support one part-time post-doctoral researcher and one to two UC Davis undergraduate students, who will be trained in geochemical and archaeometric analysis. The data and resulting analysis will be shared with the Central California community through outreach efforts at UC Davis, including public displays at the UCD Anthropology Museum, programs with school children at Bodega Bay Marine Laboratory, and presentations at professional conferences.