Walking through the landscape: A different approach to mobility through 87Sr/86Sr isoscapes

Strontium isotope analyses (87Sr/86Sr) represent a fundamental tool for reconstructing human mobility in the past. However, interpretations based on comparisons between the isotopic values of mineralised tissues and local baselines – reducing mobility to a simple “local” versus “non-local” dichotomy – may result in over-simplified views of complex phenomena. In fact, such an approach may blur the complexity of human movement and the non-linear relationships between individuals and the landscape in which they are ultimately buried. This study addresses these limitations by integrating 87Sr/86Sr isoscapes with two combined approaches: the use of isochrones, representing walking-time catchments around archaeological sites, and the Unusualness index, a statistical measure of deviation between human 87Sr/86Sr values and surrounding 87Sr/86Sr isoscape predictions. These methods are applied to two newly developed 87Sr/86Sr isoscapes of Italy, generated from an expanded dataset (n = 3096) of published environmental and archaeological samples, interpolated using Ordinary Kriging at 1 km grid size.We evaluate the impact of including or excluding sample categories with potentially high isotopic variability in the construction of isotopic maps (e.g., humans, medium-large mammals), and highlight the importance of consistent preliminary data processing.Two published Iron Age sites – Fermo (Esposito et al., 2023) and Monterenzio Vecchio (Sorrentino et al., 2018) – serve as case-studies to test this combined approach. By statistically evaluating the relationship between human samples and the isoscape values within the isochrones, we define three mobility classes, i.e., stable, circadian, and long-range ones, thereby contributing to the identification of more varied mobility patterns. This integrated framework demonstrates how the combination of isoscapes, isochrones, and the Unusualness index supports a nuanced, spatially sensitive understanding of past human mobility.