Comment on essd-2022-446

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<strong class="journal-contentHeaderColor">Abstract.</strong> Strontium isotopes (⁸⁷Sr / ⁸⁶Sr) are useful in the Earth sciences as well as in forensic, archaeological, palaeontological, and ecological sciences. As very few large-scale Sr isoscapes exist in Australia, we have identified an opportunity to determine ⁸⁷Sr / ⁸⁶Sr ratios on archive fluvial sediment samples from the low-density National Geochemical Survey of Australia (<a href="https://www.ga.gov.au/about/projects/resources/national-geochemical-survey" target="_blank" rel="noopener">www.ga.gov.au/ngsa</a>; last access: 15 December 2022). The present study targeted the northern parts of Western Australia, the Northern Territory and Queensland, north of 21.5&deg; S. The samples were taken mostly from a depth of ~60&ndash;80 cm in floodplain deposits at or near the outlet of large catchments (drainage basins). A coarse (&lt; 2 mm) grain-size fraction was air-dried, sieved, milled then digested (hydrofluoric acid + nitric acid followed by aqua regia) to release total Sr. The Sr was then separated by chromatography and the ⁸⁷Sr / ⁸⁶Sr ratio determined by multicollector-inductively coupled plasma mass spectrometry. Preliminary results demonstrate a wide range of Sr isotopic values (0.7048 to 1.0330) over the survey area, reflecting a large diversity of source rock lithologies, geological processes and bedrock ages. Spatial distribution of ⁸⁷Sr / ⁸⁶Sr shows coherent (multi-point anomalies and smooth gradients), large-scale (&gt; 100 km) patterns that appear to be broadly consistent with surface geology, regolith/soil type, and/or nearby outcropping bedrock. For instance, the extensive black clay soils of the Barkly Tableland define a &gt; 500 km-long northwest-southeast-trending unradiogenic anomaly (⁸⁷Sr / ⁸⁶Sr &lt; 0.7182). Where carbonate or mafic igneous rocks dominate, a low to moderate Sr isotope signature is observed. In proximity to the outcropping Proterozoic metamorphic basement of the Tennant, McArthur, Murphy and Mount Isa geological regions, conversely, radiogenic ⁸⁷Sr / ⁸⁶Sr values (&gt; 0.7655) are observed. A potential correlation between mineralisation and elevated ⁸⁷Sr / ⁸⁶Sr values in these regions needs to be investigated in greater detail. Our results to-date indicate that incorporating soil/regolith Sr isotopes in regional, exploratory geoscience investigations can help identify basement rock types under (shallow) cover, constrain surface processes (e.g. weathering, dispersion), and, potentially, recognise components of mineral systems. Furthermore, the resulting Sr isoscape and future models derived therefrom can also be utilised in forensic, archaeological, paleontological and ecological studies that aim to investigate, e.g., past and modern animal (including humans) dietary habits and migrations. The new spatial Sr isotope dataset for the northern Australia region is publicly available (de Caritat et al., 2022a; <a href="https://dx.doi.org/10.26186/147473" target="_blank" rel="noopener">https://dx.doi.org/10.26186/147473</a>; last access: 15 December 2022).