A bioavailable strontium isoscape of Australia. Initial contribution

Strontium isotopes (87Sr/86Sr) are useful in the earth sciences (e.g. recognising geological provinces, studying geological processes) as well as in archaeological (e.g. informing on past human migrations), palaeontological/ecological (e.g. investigating extinct and extant taxa’s dietary range and migrations) and forensic (e.g. validating the origin of drinks and foodstuffs) sciences. Recently, Geoscience Australia and the University of Wollongong have teamed up to determine 87Sr/86Sr ratios in fluvial sediments selected mostly from the low-density National Geochemical Survey of Australia (NGSA; www.ga.gov.au/ngsa). Three regional projects were focussed on: inland southeastern, northern, and southwestern Australia, together covering over 2.5 million km2 of catchment area. Whilst results on <em>total</em> Sr isotopic analyses have been reported previously (Caritat et al. 2022, 2023, 2024), a pilot study targeting the isotopic composition of <em>bioavailable</em> (or labile) Sr was conducted in parallel and this dataset is released here. In contrast to the total Sr isotope analyses, which were conducted mostly on NGSA Bottom Outlet Sediments (BOS; taken on average from ~60-80 cm depth), the bioavailable Sr work used Top Outlet Sediments (TOS; taken from 0-10 cm depth) to make the results more relevant to soil-, plant- and animal-focused applications. Approximately 1 g of air-dried, coarse-sieved (&lt;2 mm) soil sample was reacted with 2.5 M ammonium acetate (buffered to pH 7) for 24 hr on a mixing table, to extract operationally defined plant-available cations (after Stewart et al. 1998). The solution was filtered at 0.45 µm and dried down to incipient dryness. The residue was re-dissolved in 2M nitric acid. The Sr was separated by chromatography and its 87Sr/86Sr ratio determined by multicollector-inductively coupled plasma-mass spectrometry. Results for 278 samples across all three regions demonstrate a wide range of bioavailable Sr isotopic values (0.7050 to 0.7812, median 0.7191) across Australia, reflecting a large diversity of source rock lithologies, geological, pedogenic and biogeochemical processes, and, ultimately, bedrock ages. Modelling and interpretation of this dataset will be presented elsewhere. The resulting bioavailable Sr isoscape for Australia, although sparse at the moment, and models to be derived therefrom, may have applications in archaeological, paleontological and ecological studies that aim to investigate past and modern animal (including humans) dietary habits and migrations. The new spatial dataset is publicly available through the Geoscience Australia portal (https://portal.ga.gov.au/).<br>References citedCaritat, P. de, Dosseto, A., Dux, F., 2022. A strontium isoscape of inland southeastern Australia, Earth System Science Data, 14, 4271–4286, https://doi.org/10.5194/essd-14-4271-2022 Caritat, P. de, Dosseto, A., Dux, F., 2023. A strontium isoscape of northern Australia, Earth System Science Data, 15, 1655–1673, https://doi.org/10.5194/essd-15-1655-2023 Caritat, P. de, Dosseto, A., Dux, F., 2024. A strontium isoscape of southwestern Australia and progress toward a national strontium isoscape, Earth System Science Data Discussion [non peer-reviewed preprint], https://doi.org/10.5194/essd-2024-352 Stewart, B. W., Capo, R. C., Chadwick, O. A., 1998. Quantitative strontium isotope models for weathering, pedogenesis and biogeochemical cycling, Geoderma, 82, 173–195, https://doi.org/10.1016/S0016-7061(97)00101-8

锶同位素(87Sr/86Sr)在地球科学领域（如识别地质省域、研究地质作用）、考古学领域（如揭示古人类迁徙规律）、古生物与生态学领域（如探究已灭绝和现存类群的饮食范围与迁徙模式）以及法医学领域（如验证饮品与食品的来源）均具有重要应用价值。

近日，澳大利亚地球科学局（Geoscience Australia）与卧龙岗大学展开合作，依托澳大利亚国家地球化学调查（National Geochemical Survey of Australia, NGSA; www.ga.gov.au/ngsa）的低密度采样点，选取河流沉积物开展87Sr/86Sr比值测定工作。本次研究聚焦澳大利亚内陆东南部、北部与西南部三个区域，覆盖流域总面积超250万平方千米。

此前已有研究团队报道了全量Sr同位素分析结果（Caritat等，2022、2023、2024），本研究同步开展了针对生物可利用态（或称不稳定态）Sr同位素组成的先导性研究，本次数据集即公开发布于此。与主要采集自约60~80cm深度的NGSA底出口沉积物（Bottom Outlet Sediments, BOS）的全量Sr同位素分析不同，本次生物可利用态Sr分析采用顶出口沉积物（Top Outlet Sediments, TOS，采集于0~10cm深度），以提升结果对土壤、植物及动物相关研究应用的适配性。

实验流程为：称取约1g经风干、粗筛（粒径<2mm）的土壤样品，与2.5M乙酸铵溶液（pH缓冲至7）在混匀台上反应24小时，以提取操作定义下的植物有效态阳离子（参照Stewart等，1998）。将反应液经0.45μm滤膜过滤后蒸发至近干，残渣用2M硝酸溶液重新溶解。通过色谱法分离Sr元素，并采用多接收器电感耦合等离子体质谱仪（multicollector-inductively coupled plasma-mass spectrometry）测定其87Sr/86Sr比值。

对三个区域共278个样品的分析结果显示，澳大利亚境内生物可利用态Sr同位素值分布范围广泛（0.7050~0.7812，中位数为0.7191），这一分布特征反映了源岩岩性、地质作用、成土过程与生物地球化学循环过程的巨大多样性，归根结底也体现了基岩年代的显著差异。本数据集的建模与解读工作将另行发表。

本次构建的澳大利亚生物可利用态锶同位素等值线图目前采样点仍较为稀疏，后续基于该数据集衍生的模型，可应用于旨在探究古今动物（包括人类）饮食习性与迁徙规律的考古学、古生物学与生态学研究。

本次新增的空间数据集可通过澳大利亚地球科学局官方门户（https://portal.ga.gov.au/）公开获取。

参考文献
Caritat, P. de, Dosseto, A., Dux, F., 2022. 澳大利亚内陆东南部锶同位素等值线图，《地球系统科学数据》，14卷，4271–4286，https://doi.org/10.5194/essd-14-4271-2022
Caritat, P. de, Dosseto, A., Dux, F., 2023. 澳大利亚北部锶同位素等值线图，《地球系统科学数据》，15卷，1655–1673，https://doi.org/10.5194/essd-15-1655-2023
Caritat, P. de, Dosseto, A., Dux, F., 2024. 澳大利亚西南部锶同位素等值线图与全国锶同位素等值线图研究进展，《地球系统科学数据讨论》[非同行评议预印本]，https://doi.org/10.5194/essd-2024-352
Stewart, B. W., Capo, R. C., Chadwick, O. A., 1998. 用于风化、成土与生物地球化学循环的定量锶同位素模型，《Geoderma》，82卷，173–195，https://doi.org/10.1016/S0016-7061(97)00101-8