Shell bead sourcing

Shell Bead Sourcing A Comparison of Two Techniques on Olivella biplicata Shells and Beads from Western North America... 

In sum, the ICP-MS analyses suggest that Olivella shells are subject to fairly significant post-depositional alteration as shells are exposed to midden soils over hundreds to thousands of years. While discouraging, it does not preclude the application of elemental compositional studies for shell-bead sourcing. Some elements, such as Sc, Zr, and Al, among others not discussed in detail here, remain fairly constant when taken as ratios against Ca. It is these elements that should be the subject of provenance analyses and that we will target in future research with Olivella shell. 

It is clear that shell-bead sourcing using chemical means is still in its infancy. If shell-bead sourcing is to play a more important role in archaeological... 

Carbon stable isotope ratios in Olivella biplicata shell beads sourcing, 180-185/... 

Despite the potential to track the spatial movement of marine shell beads, archaeometric attempts at sourcing have been minimal. We are aware of only three other groups that have taken this approach including a neutron activation analysis (NAA) of Busycon sp. shells and artifacts in the American Southeast (/), the use of strontium isotopes as a measure of geologic age of fossil shells in Southwest Europe (2), and the use of oxygen isotope ratios to source Sporufylus sp. beads in Central Europe (5). 

To refine potential Olivella source zones to more specific sections of the coastline, we have employed two different archaeometric techniques, including determination of elemental composition by inductively coupled plasma-mass spectrometry (ICP-MS) and the use of carbon and oxygen stable isotopes using an isotope ratio-mass spectrometer (IR-MS) (38). Each of these techniques is described below, and the final section evaluates and compares their utility in sourcing Olivella shell beads. 

We have applied two distinct techniques, analysis of bulk chemical composition and the ratios of oxygen and caibon stable isotopes, in an effort to determine source provenance of Olivella shell beads in Western North America. These techniques produce varying degrees of success. 

Prior to the current analyses we had predicted that most of the remaining 38 prehistoric Delta Olivella beads would be made from shells procured in northern California. This was based purely on an economic model that minimizes transport distance. In other words, the closest source (by foot) of Olivella shell to Owens Valley is southern California, while the closest source of Olivella in the Delta region is the coast immediately north and south of San Francisco Bay. Moreover, saddle (F2/F3) and sequin (M1/M2) beads are found exclusively in central and northern California (12, 13), again implying a more northern source. On the other hand Hlb and G2 beads are found all over California. Results of the carbon and oxygen isotopic analyses are presented in Table II. 

Table II. Results of Oxygen Isotope Sourcing for Historic and Prehistoric Delta Beads Based on Modern Shell Data...

Yet we are not without hope for future applications of bulk chemical analyses. More thorough chemical cleaning of the shell to remove inorganic components may reduce some of the variability among shells collected from a region, allowing us to better define potential source zones and assign unknown beads to these source zones. 

For these and similar systems the original source, resin or solution, of the counter-ion being chemically consumed and the nature of the co-ion greatly influence the observed kinetics. The association-dissociation of weakly functional resins is of particular practical interest since in these instances a reactive and non-reactive core respectively forms within the resin which shrinks towards the bead centre as exchange proceeds. This Shrinking Core or Shell Progressive mechanism is usually particle diffusion controlled and explains why exchange on weakly functional resins is invariably flow-rate sensitive under column operation. 

Shellfish belong to the biological group of species known as mollusks, animals with a shell and a foot. Univalve mollusks have one shell and include many species of snails, conchs, and others. Bivalve mollusks have two shells and include clams, scallops, and oysters. There are many species of mollusks and both freshwater and salt water varieties. In addition to their use as food, shells were often used to make beads, fishhooks and other objects. Crushed shells have been used as a source of lime, for pottery temper, and as construction material. In some areas such as the South Pacific, shells of species like the cowrie have been traded as valued items over long distances and even used as money. 

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