EVALUATING THE POTENTIAL OF STABLE ISOTOPE AND X-RAY FLUORESCENCE (XRF) ANALYSES IN PROVENANCE STUDIES OF YAPESE STONE MONEY

Large disks-shape artifacts, commonly referred to as the “stone money” (rai) of Yap (Federated States of Micronesia) are highly prized as important exchange valuables and symbols of cultural tradition. Most disks were carved between c. AD 1400-1900 from flowstone precipitates in caves on an uplifted limestone terrain that forms the Rock Islands of the Republic of Palau and were transported by watercraft >400 km to Yap. Various scholars surmise that some may have even come from more distant western Pacific islands such as Guam, the Philippines, or Taiwan. The sampling of rai for petrographic or geochemical analyses is not an option, and our previous work evaluated the use of a non-destructive portable handheld XRF technique in provenance studies of stone money.

Flowstone samples from four locations on Palau were examined and compared to a sample of broken stone money acquired from Yap Historical Preservation Office staff. Fresh interior and weathered exterior surfaces of all samples were analyzed in the laboratory using a Bruker Tracer III-V XRF elemental analyzer. The study concluded that samples from Palau are compositionally identical to the analyzed stone money piece from Yap and that due to surficial contamination by soil and organic material, there is more compositional variation between the interior and exterior of individual samples than between samples from different localities. This analysis could not discriminate between the four source rock sites on Palau and thus did not prove very useful.

For this reason, the current study also evaluated the suitability of stable isotope analysis for provenance determination. This preliminary study included only 5-6 data points per each sample. Out of four flowstone samples, only two overlap on the δ¹³C vs δ¹⁸O cross-plot and the other two form individual compositional fields. Data points for Yapese stone money overlap with only one of these individual fields, suggesting that stable isotope analysis may potentially be useful in provenance studies of stone money. Since these artifacts could not be easily sampled for this type of analysis, their provenance determination using this technique remains a promising, but to date only theoretical possibility.