Provenance analysis of Olivella biplicata shell beads from the California and Oregon Coast by stable isotope fingerprinting

Abstract

While Olivella beads are a common component of archaeological sites in California, and were widely traded in prehistory, no method has been developed to trace individual beads to a point of origin. This study examines the potential of stable carbon and oxygen isotopes to source Olivella beads from the Pacific coast. The study shows that 1) the oxygen isotopic composition of modern Olivella biplicata shells faithfully varies with ambient sea surface temperature and local upwelling, lending themselves to sourcing studies; 2) oxygen isotope ratios in modern shells can be used to identify shells that grow north versus south of Point Conception, California; and 3) shell carbon isotope ratios may further subdivide these two regions into more spatially restricted source zones. Analyses on a small sample of 10 beads found at various archaeological sites within the interior of California suggest that all were made in southern California.

Stable Isotope Provenance Analysis ofOlivellaShell Beads From the Los Angeles Basin and San Nicolas Island

“…This is because temperature is the primary environmental control on δ 18 O, and our previous work (Eerkens et al, 2005) showed that O. biplicata faithfully records a SST signal in its aragonitic shell. Sequential samples taken along growth lines of shells show seasonal patterns of increasing (springsummer) and decreasing (winter-spring) oscillations that track seasonal changes in SST.…”

“…Our previous research using oxygen and carbon stable isotope data (Eerkens et al, 2005(Eerkens et al, , 2007 sought to develop such a method for Olivella beads. Our approach takes advantage of the fact that ocean temperatures show a north-south gradient along the Pacific Coast, and that these temperatures affect the fractionation of oxygen isotopes in shell carbonate.…”

Molluscs emergent, Part II: themes and trends in the scientific investigation of molluscs and their shells as past human resources

“…O. biplicata is distributed along the coasts of California and Oregon, making the alignment of trade or exchange routes rather tricky. Eerkens et al (2005) showed that the d 18 O composition of a small sample of shell beads from central California pointed to a southern California origin for the shells. Subsequently, Eerkens et al (2010) reinforced this southern Californian focus using d 18 O, d 13 C and 87 Sr/ 86 Sr, although differences between various forms of shell beads showed that some were made in a range of locations and others in more restricted localities, suggesting some form of intra-regional specialization.…”

A Programmatic Approach to Determine Eligibility of Prehistoric Sites in the San Diego Subregion, Southern Coast Archeological Region, California, for the National Register of Historic Places

“…Sourcing of Olivella shells may help to trace prehistoric and historical period shell bead exchange networks as well. Carbon isotope studies have proven to be effective in identifying Olivella sources based on general geographic locations (i.e., northern California and southern Ore-gon, Santa Barbara mainland and Santa Rosa Island, and Santa Cruz Island) (Eerkens et al 2005). Continued research has the potential to refine spatial resolution, perhaps to an island specific level.…”