Abstract Local carbonate cycling in lagoon-estuarine systems, involving processes such as inorganic and biogenic carbonate precipitation/dissolution, represents an important but poorly constrained component of the coastal carbon budget. This study investigates the sensitivity of stable Sr isotope tracer (δ88/86Sr) with respect to carbonate saturation and salinity of local waters in the Coorong, Lower Lakes and Murray Mouth (CLLMM) estuary in South Australia. The CLLMM has an extensive range of salinity from fresh to hypersaline (from ∼0 to over 100 PSU), with corresponding variations in water chemistry and major ion composition that in turn controls mineral saturation states, and thus CaCO3 precipitation/dissolution in local waters. Here we use the novel δ88/86Sr tracer in tandem with the more established radiogenic Sr isotope ratio (87Sr/86Sr), where the latter is a robust proxy for Sr sources and thus water provenance. We also produced a geochemical (PHREEQC) model of calcium carbonate (CaCO3) saturation changes across this unique lagoon-estuarine system. The results indicate a systematically increasing trend of δ88/86Sr (from ∼0.25‰ to ∼0.45‰) with increasing salinity and CaCO3 (aragonite, calcite) saturation indices of the coastal waters, which in turn suggest an overall control of carbonate dissolution/precipitation processes on the stable Sr isotope composition in the CLLMM system. This was further corroborated by Ca isotope data (δ44/40Ca) published previously on the same samples from the Coorong, as well as a quantitative simulation of local carbonate removal in the lagoon based on Rayleigh modelling and Sr isotope data. Overall, our results confirm that a coupled Sr isotope approach (combining 87Sr/86Sr and δ88/86Sr) can be used to constrain not only the main water sources (continental versus marine Sr) but also local CaCO3 dissolution/precipitation processes, and thus inorganic carbon and coastal carbonate cycling in the CLLMM system. Finally, this coupled δ88/86Sr and 87Sr/86Sr approach can be potentially applied to fossil carbonate archives to reconstruct paleo-hydrology and salinity changes in the CLLMM and/or other carbonate-producing coastal systems.

中文翻译：

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盐度和碳酸盐饱和度对泻湖-河口系统稳定 Sr 同位素 (δ88/86Sr) 的影响

摘要泻湖 - 河口系统中的局部碳酸盐循环，涉及无机和生物碳酸盐沉淀/溶解等过程，是沿海碳预算的一个重要但缺乏约束的组成部分。本研究调查了稳定 Sr 同位素示踪剂 (δ88/86Sr) 对南澳大利亚 Coorong、Lower Lakes 和 Murray Mouth (CLLMM) 河口当地水域碳酸盐饱和度和盐度的敏感性。CLLMM 的盐度范围很广，从新鲜到高盐度（从 0 到 100 PSU 以上），水化学和主要离子组成的相应变化反过来控制矿物饱和状态，从而控制当地水域的 CaCO3 沉淀/溶解。在这里，我们将新型 δ88/86Sr 示踪剂与更成熟的放射性 Sr 同位素比 (87Sr/86Sr) 结合使用，其中后者是 Sr 来源和水源的可靠代表。我们还制作了这个独特的泻湖-河口系统中碳酸钙 (CaCO3) 饱和度变化的地球化学 (PHREEQC) 模型。结果表明，随着沿海水域盐度和 CaCO3（文石、方解石）饱和度指数的增加，δ88/86Sr 有系统的增加趋势（从~0.25‰ 到~0.45‰），这反过来表明碳酸盐溶解/沉淀的总体控制CLLMM 系统中稳定 Sr 同位素组成的过程。先前发表的关于 Coorong 相同样本的 Ca 同位素数据 (δ44/40Ca) 以及基于瑞利模型和 Sr 同位素数据对泻湖中局部碳酸盐去除的定量模拟进一步证实了这一点。全面的，我们的结果证实，耦合 Sr 同位素方法（结合 87Sr/86Sr 和 δ88/86Sr）不仅可用于限制主要水源（大陆与海洋 Sr），还可用于限制局部 CaCO3 溶解/沉淀过程，从而限制无机碳和CLLMM 系统中的沿海碳酸盐循环。最后，这种耦合的 δ88/86Sr 和 87Sr/86Sr 方法可以潜在地应用于碳酸盐化石档案，以重建 CLLMM 和/或其他碳酸盐生产沿海系统的古水文和盐度变化。