STRONTIUM ISOTOPIC ANALYSIS AS A TOOL FOR CERAMIC PROVENANCE RESEARCH

Although variations in strontium (Sr) isotopes have been used in archaeometric studies to fingerprint obsidian, shell, wood, man-made glass, and human remains, the method has never been applied to provenance studies of archaeological ceramics.  The application of Sr isotopic techniques for ceramics builds upon a solid foundation of geological research on: 1) the Sr isotopic compositions of siliciclastic sediments and rocks as indicators of sedimentary provenance; 2) the Sr isotopic systematics of clays in relation to their parent materials; and 3) the behavior of Sr during weathering of feldspars and biotites.  As a test case, this study focuses on ceramics of the prehistoric Kayenta Anasazi and Cohonina cultural groups from the eastern Grand Canyon area of northern Arizona.  The archaeological area of interest is well-suited to a Sr isotopic provenance study due to the geologic diversity of the eastern Grand Canyon region, which includes Paleoproterozoic crystalline rocks and Mesoproterozoic and Paleozoic sedimentary rocks of the inner Grand Canyon as well as Paleozoic and Mesozoic sedimentary rocks and Cenozoic volcanics of the Coconino Plateau.

We have applied thermal ionization mass spectrometry (TIMS) to characterize the bulk Sr isotopic compositions of: 1) Kayenta Anasazi and Cohonina ceramics from the Coconino Plateau; and 2) potential geologic sources of ceramic raw materials from the inner Grand Canyon and the Coconino Plateau.  Initial results document a dramatic contrast in the Sr isotopic compositions of the main types of Cohonina gray ware (⁸⁷Sr/⁸⁶Sr values around 0.769) and Anasazi gray ware (⁸⁷Sr/⁸⁶Sr values around 0.719).  Clay-rich geologic materials (shales and mudstones, alluvium, and soils) in the eastern Grand Canyon area exhibit a broad range of Sr isotopic compositions (⁸⁷Sr/⁸⁶Sr values from 0.709 to 0.842), and similarities between the ⁸⁷Sr/⁸⁶Sr values of certain geologic and ceramic specimens merit further investigation.  Experiments suggest that the effect of secondary calcium carbonate on the Sr isotopic compositions of ceramics is negligible.  Heterogeneity in the Sr isotopic composition of a given ceramic, as gauged through analyses of multiple samples from a ceramic specimen, appears to be at least one order of magnitude lower than the variation between different types of ceramics.