Advances in global bioavailable strontium isoscapes

Abstract

Abstract Strontium isotope ratios (87Sr/86Sr) are a popular tool in provenance applications in archeology, forensics, paleoecology, and environmental sciences. Using bioavailable 87Sr/86Sr in provenance studies requires comparing the 87Sr/86Sr of a sample of interest to that of 87Sr/86Sr baselines. Historically, these baselines required building empirical datasets from plants or local animals to characterize the 87Sr/86Sr available to local ecosystems (bioavailable 87Sr/86Sr). However, researchers are increasingly relying on modeled bioavailable 87Sr/86Sr maps (called isoscapes). We review the advantages and limitations of existing approaches to mapping bioavailable 87Sr/86Sr for provenance studies and propose a globally applicable, scalable, and editable framework for creating bioavailable 87Sr/86Sr isoscapes. This framework relies on: 1) Compiling global bioavailable 87Sr/86Sr data; 2) Mapping 87Sr/86Sr variability in rocks; 3) Leveraging global environmental covariates; and 4) Applying a random forest regression method that integrates these data to predict bioavailable 87Sr/86Sr. When the random-forest model is applied at the global scale it performs well (explaining 60% of the variance of the global bioavailable 87Sr/86Sr dataset), and accounts for geological, geomorphological and atmospheric controls. In data-rich regions (e.g., Europe), the global bioavailable 87Sr/86Sr isoscape can be successfully extrapolated to broad regions without bioavailable 87Sr/86Sr data. However, we also show that this extrapolation may not be valid in exceptionally geologically complex and data-poor regions (e.g., Madagascar). We suggest research directions to improve the accuracy of global bioavailable 87Sr/86Sr isoscapes, which include: 1) Increasing the collection of bioavailable datasets in data-poor regions; 2) Harmonizing data management practices and metadata collection for bioavailable 87Sr/86Sr data; and 3) Relying on advances in remote sensing and geological mapping techniques to improve geological covariates. While significant potential to refine 87Sr/86Sr isoscapes remains, the data products provided in this review form a basis for using 87Sr/86Sr data in large-scale provenance studies, opening new research avenues in a range of fields.