Shell Bead Sourcing: A Comparison of Two Techniques on \u3cem\u3eOlivella biplicata\u3c/em\u3e Shells and Beads from Western North America

We compare two methods for tracking the geographic source of Olivella biplicata shell beads along the California and Oregon Pacific coast; bulk element composition by inductively coupled plasma-mass spectrometry (ICP-MS) and stable carbon and oxygen isotopes by isotope ratio-mass spectrometry (IR-MS). Both techniques hold some promise for reconstructing prehistoric trading systems, but neither is perfect. Currently, oxygen isotopes can reliably differentiate recent shell beads (post A.D. 1500) derived from points north versus south of Point Conception. We are working to extend the time depth of this technique. Elemental composition will help to divide the northern isotopic zone into finer geographic sections by providing chemical signatures for certain regions, such as Monterey Bay and extreme Northern California. Additional sampling, particularly on prehistoric specimens that are subject to post-depositional chemical alteration, will be necessary to make this technique reliable for sourcing ancient beads. In concert, the two techniques could provide archaeologists with a dependable means for identifying where the shell used to make a bead was originally collected