MAPPING 87Sr/86Sr VARIATIONS IN BEDROCK AND WATER FOR REGIONAL MIGRATION STUDIES

Although ⁸⁷Sr/⁸⁶Sr variations in bones and tooth enamel have been used to trace provenance of individuals in archeology, methods for mapping the variations in archeological ⁸⁷Sr/⁸⁶Sr at regional scale are not available. In this paper, we build upon earlier efforts to model ⁸⁷Sr/⁸⁶Sr in bedrock developing GIS-based models for Sr isotopes in rock and water that include the combined effects of lithology and time. We fit lithology-specific model parameters for generalized equations describing the concentration of radiogenic Sr in silicate and carbonate rocks using published data. The new model explains more than 50% of the observed variance in measured Sr isotope values from global databases of igneous, metaigneous, and carbonate rocks, but performs less well (33% of the variance) for sedimentary and metasedimentary rocks. In comparison, a previously applied model formulation that does not include lithology-specific parameters explains only 20% and 8% of the observed variance for igneous and sedimentary rocks, respectively, and exhibits an inverse relationship with measured carbonate rock values. Building upon the bedrock model, we also developed and applied equations to predict the contribution of different rock types to ⁸⁷Sr/⁸⁶Sr variations in water as a function of their weathering rates and Sr content. The resulting surface water model was compared to data from 68 catchments and shown to give more accurate predictions of surface water ⁸⁷Sr/⁸⁶Sr (R²=0.70) than models that do not include lithological weathering parameters. We applied these models to produce maps predicting ⁸⁷Sr/⁸⁶Sr in bedrock and surface water across the contiguous USA. In many areas where geology is heterogeneous over short distances, bedrock and water isoscapes display large ⁸⁷Sr/⁸⁶Sr variations over a range of spatial scales that are promising for provenance studies. We compared the mapped Sr isotope distributions with two recent datasets gathering ⁸⁷Sr/⁸⁶Sr measurements in plants and archeological samples from the USA and found the strongest correlations in comparisons with the water model . Although the maps produced here are demonstrably imperfect and leave significant scope for refinement, they provide a baseline for archeological studies by constraining the ⁸⁷Sr/⁸⁶Sr in strontium sources at regional scales.