A large-scale environmental strontium isotope baseline map of Portugal for archaeological and paleoecological provenance studies

Strontium isotopes (Sr-87/Sr-86) provide valuable information to help reconstruct past mobility. For the analysis of archaeological tooth enamel to provide a direct assessment of mobility, a comparison to the baseline Sr-87/Sr-86 in a region is required. In this study, a large-scale Sr-87/Sr-86 baseline of Portugal is created based on 151 paired plant and soil leachate samples combined with previously published data (20 additional plant and 33 additional soil leachate sites). Spatial patterns of Sr-87/Sr-86 are evident, following Portugal's geology and terrain, with higher Sr-87/Sr-86 in the granite dominated north and further inland. Influences from sea spray are observed along the coastal regions of the country. The bioavailable strontium range for Portugal is 0.70575-0.73487, and paired plant-soil leachate site measurements show a strong positive relationship. Empirical Bayesian Kriging (EBK) alongside mean Sr-87/Sr-86 per geological unit are used to provide predictive surfaces for bioavailable Sr-87/Sr-86. We find that the addition of archaeological site-specific measurements is required in archaeological mobility studies to ensure local-scale Sr-87/Sr-86 variation is captured, illustrated in this study using the Late Middle Neolithic to Early Bronze Age site of Perdigoes. The bioavailable strontium isoscape for Portugal provides a baseline map for future archaeological and palaeoecological studies in this region and contributes to the global efforts to map strontium isotope variability.