Mapping ⁸⁷Sr/⁸⁶Sr variations in bedrock and water for large scale provenance studies

Although variation in ⁸⁷Sr/⁸⁶Sr has been widely pursued as a tracer of provenance in environmental studies, forensics, archeology and food traceability, accurate methods for mapping variations in environmental ⁸⁷Sr/⁸⁶Sr at regional scale are not available. In this paper, we build upon earlier efforts to model ⁸⁷Sr/⁸⁶Sr in bedrock by developing GIS-based models for Sr isotopes in rock and water that include the combined effects of lithology and time. Using published data, we fit lithology-specific model parameters for generalized equations describing the concentration of radiogenic Sr in silicate and carbonate rocks. The new model explained more than 50% of the observed variance in measured Sr isotope values from independent global databases of igneous, metaigneous, and carbonate rocks, but performed more poorly (explaining 33% of the variance) for sedimentary and metasedimentary rocks. In comparison, a previously applied model formulation that did not include lithology-specific parameters explained only 20% and 8% of the observed variance for igneous and sedimentary rocks, respectively, and exhibited 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 strontium content. The resulting water model was compared to data from 68 catchments and shown to give more accurate predictions of stream water ⁸⁷Sr/⁸⁶Sr (R²=0.70) than models that did not include lithological weathering parameters. We applied these models to produce maps (“isoscapes”) predicting ⁸⁷Sr/⁸⁶Sr in bedrock and water across the contiguous USA, and compared the mapped Sr isotope distributions to data on Sr isotope ratios of US marijuana crops. Although the maps produced here are demonstrably imperfect and leave significant scope for further refinement, they provide an enhanced framework for lithology-based Sr isotope modeling and offer a baseline for provenance studies by constraining the ⁸⁷Sr/⁸⁶Sr in strontium sources at regional scales.