A Bioavailable Strontium Isoscape of Australia

Abstract. Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) at the Earth’s surface offer powerful tools for geological, environmental, and archaeological applications. In minerals and biological materials, ⁸⁷Sr/⁸⁶Sr reflects the isotopic composition of the local bedrock and derived soils. In Australia, however, large regional-scale surveys of bioavailable ⁸⁷Sr/⁸⁶Sr remain scarce. Here, we present a new dataset of bioavailable ⁸⁷Sr/⁸⁶Sr ratios from 278 catchment outlet (floodplain) sediment samples, spanning inland southeastern Australia (South Australia, New South Wales, Victoria), northern Western Australia, the Northern Territory, Queensland (north of 21.5° S), and the Yilgarn Craton in southern Western Australia. Combined with more than 20,000 global Sr isotope measurements, this dataset was used to generate a high-resolution isoscape of Australia using random forest regression (Bataille et al., 2020).

Australian bioavailable ⁸⁷Sr/⁸⁶Sr values span a narrower range (0.70501–0.78121) compared to co-located bulk sediment values (0.70480–1.09089) (Caritat et al., 2022, 2023, 2025b), reflecting the influence of soluble and exchangeable mineral phases and atmospheric inputs such as rain and dust. The predicted isoscape reproduces major geological patterns, with higher values over ancient crustal provinces like the Yilgarn Craton and eastern Palaeozoic orogens, and lower values across younger sedimentary basins and coastal margins. Model uncertainty, assessed via prediction standard deviations, is lowest across well-sampled, geologically stable regions and highest in coastal and lithologically complex zones. Compared to existing global and regional isoscapes, our model offers significantly improved coverage and resolution for Australia. This isoscape provides a robust baseline for applications in provenance research, palaeoecology, and environmental geochemistry.