Quantitative methods of chert source identification as a tool for archaeological interpretation

Blue-gray chert from several sources in the Midwestern United States was a valued resource to ancient people of the Eastern Woodlands. It was transported or exchanged across great distances during the Paleoindian Period (11,500 to 9,000 B.C.), a segment of the Late Archaic Period (1,500-400 B.C.), and the Middle Woodland Period (150 B.C-A.D. 400). Identification of the source(s) exploited by specific populations can illuminate important cultural/behavioral patterns. Conventional means of relying on visual inspection of chert impedes accurate identification as some chert types share macroscopic traits. When culturally impacted samples are found a great distance from potential geologic sources, conventional means of classifying visually similar chert do not provide a scientifically viable process of distinguishing the sociocultural value of the samples. In an ongoing study, ISAS and ISGS researchers attempted to establish non-destructive means of source identification for Midwestern blue-gray chert from the Mississippian Ste. Genevieve and St. Louis Limestones, focusing on Cobden-Dongola chert, Wyandotte chert, and Kentucky Blue chert. Preliminary analyses included examination of elemental composition using energy dispersive x-ray fluorescence (EDXRF) and portable x-ray fluorescence (PXRF) instruments, standardized color measurement with a spectrophotometer, and initial isotopic analysis via laser ablation and mass spectrometer. Results varied from method to method in terms of identifying diagnostic properties, but did suggest further research would be beneficial. EDXRF revealed that the presence of Barium might be a distinguishing characteristic of Cobden-Dongola chert. Spectrophotometer data provided quantified color ranges that were largely shared between the various chert sources. Mass spectrometry indicated that Strontium is present in sufficient quantity to perform isotope ratio analysis, which may be effective for identifying sources. ISGS/ISAS efforts will result in a geochemical library of chert samples for comparison with artifacts from archaeological sites. The cultural patterns that source identification can illuminate will provide important insight into the lives of ancient native people of North America.