A bioavailable strontium isoscape of Australia

<p>Strontium isotope ratios (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi/><mn mathvariant="normal">87</mn></msup><mi mathvariant="normal">Sr</mi><msup><mo>/</mo><mn mathvariant="normal">86</mn></msup><mi mathvariant="normal">Sr</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="49pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="cb03c700d9f7b4c21eab0ba899e5ee93"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-17-4865-2025-ie00001.svg" width="49pt" height="15pt" src="essd-17-4865-2025-ie00001.png"/></svg:svg></span></span>) at the Earth's surface offer powerful tools for geological, environmental, and archaeological applications. In minerals and biological materials, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi/><mn mathvariant="normal">87</mn></msup><mi mathvariant="normal">Sr</mi><msup><mo>/</mo><mn mathvariant="normal">86</mn></msup><mi mathvariant="normal">Sr</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="49pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="d2b7982e9addb6d3b31bd74956c21674"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-17-4865-2025-ie00002.svg" width="49pt" height="15pt" src="essd-17-4865-2025-ie00002.png"/></svg:svg></span></span> reflects the isoztopic composition of the local bedrock and derived soils. In Australia, however, large regional-scale surveys of bioavailable <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi/><mn mathvariant="normal">87</mn></msup><mi mathvariant="normal">Sr</mi><msup><mo>/</mo><mn mathvariant="normal">86</mn></msup><mi mathvariant="normal">Sr</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="49pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="88798747f73600ea5163394c59f0f3bd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-17-4865-2025-ie00003.svg" width="49pt" height="15pt" src="essd-17-4865-2025-ie00003.png"/></svg:svg></span></span> remain scarce. Here, we present a new dataset of bioavailable <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi/><mn mathvariant="normal">87</mn></msup><mi mathvariant="normal">Sr</mi><msup><mo>/</mo><mn mathvariant="normal">86</mn></msup><mi mathvariant="normal">Sr</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="49pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="7e62ec102cca3841d532ffdfea7f62a2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-17-4865-2025-ie00004.svg" width="49pt" height="15pt" src="essd-17-4865-2025-ie00004.png"/></svg:svg></span></span> 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 a well-established random forest spatial regression framework (Bataille et al., 2020).</p> <p>Australian bioavailable <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi/><mn mathvariant="normal">87</mn></msup><mi mathvariant="normal">Sr</mi><msup><mo>/</mo><mn mathvariant="normal">86</mn></msup><mi mathvariant="normal">Sr</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="49pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="437a9ac3b8bbbcb62857f95dd7de3c1c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-17-4865-2025-ie00005.svg" width="49pt" height="15pt" src="essd-17-4865-2025-ie00005.png"/></svg:svg></span></span> values span a narrower range (0.70501–0.78121) compared to co-located bulk sediment values (0.70480–1.09089) (de Caritat et al., 2022, 2023, 2025b), reflecting the influence of soluble and exchangeable mineral phases and atmospheric inputs such as rain and dust/seaspray. 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 quantile random forest regression, is lowest across well-sampled, geologically stable regions where the model is well-trained and highest in poorly-sampled regions and lithologically complex zones. Despite remaining spatial gaps and areas of high prediction uncertainty, our model offers significantly improved coverage and resolution for Australia compared to other global or regional isoscapes. It also provides a scalable framework for updating the Australian isoscape as sampling density increases. This isoscape establishes a robust baseline for applications in provenance research, palaeoecology, and environmental geochemistry. The bioavailable Sr isotope dataset is available from the Geoscience Australia e-Catalogue entry by de Caritat et al. (2025a) on <a href="https://doi.org/10.26186/150024">https://doi.org/10.26186/150024</a></p>