Abstract Sulfate (SO42−) incorporated into calcium carbonate minerals enables measurements of sulfur (S) isotope ratios in carbonate rocks. This Carbonate Associated Sulfate (CAS) in marine carbonate minerals is thought to faithfully represent the S isotope composition of the seawater sulfate incorporated into the mineral, with little or no S isotope fractionation in the process. However, comparison between different calcifying species reveals both positive and negative S isotope fractionation between CAS and seawater sulfate, and a large range of S isotope ratios can be found within a single rock sample, depending on the component measured. To better understand the isotopic effects associated with sulfate incorporation into carbonate minerals, we precipitated inorganic calcite and aragonite over a range covering more than two orders of magnitude of sulfate concentration and precipitation rate. Coupled measurements of CAS concentration, S isotope composition and X-ray absorption near-edge spectra (XANES) permit characterization and explanation of the observed dependence of S isotope fractionation between CAS and aqueous sulfate (CAS-SO42− isotope fractionation) on sulfate concentration and precipitation rate. In aragonite, the CAS-SO42− isotope fractionation is 1.0 ± 0.3‰ and independent of the sulfate (and CAS) concentration. In contrast, the CAS-SO42− isotope fractionation in calcite covaries strongly with the sulfate concentration and weakly with the precipitation rate, between values of 1.3 ± 0.1 and 3.1 ± 0.6‰. We suggest that the correlation between aqueous sulfate concentration and CAS-SO42− isotope fractionation in calcite reflects a dependence of the equilibrium S isotope fractionation on the concentration of CAS, through the effect of the sulfate impurity on the carbonate mineral’s energetic state.

中文翻译：

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非生物方解石和文石中含水硫酸盐和碳酸盐相关硫酸盐之间的硫同位素分馏

摘要 加入碳酸钙矿物中的硫酸盐 (SO42−) 可以测量碳酸盐岩中的硫 (S) 同位素比率。海洋碳酸盐矿物中的这种碳酸盐伴生硫酸盐 (CAS) 被认为忠实地代表了结合到矿物中的海水硫酸盐的 S 同位素组成，在此过程中几乎没有或没有 S 同位素分馏。然而，不同钙化物质之间的比较揭示了 CAS 和海水硫酸盐之间的正负 S 同位素分馏，并且可以在单个岩石样品中发现大范围的 S 同位素比率，具体取决于测量的成分。为了更好地了解与硫酸盐掺入碳酸盐矿物相关的同位素效应，我们在硫酸盐浓度和沉淀速率超过两个数量级的范围内沉淀了无机方解石和文石。CAS 浓度、S 同位素组成和 X 射线吸收近边光谱 (XANES) 的耦合测量允许表征和解释观察到的 CAS 和硫酸盐水溶液之间的 S 同位素分馏（CAS-SO42-同位素分馏）对硫酸盐浓度和降水率。在文石中，CAS-SO42− 同位素分馏为 1.0 ± 0.3‰，与硫酸盐（和 CAS）浓度无关。相比之下，方解石中 CAS-SO42− 同位素分馏与硫酸盐浓度的协变强烈，而与沉淀速率的协变微弱，介于 1.3 ± 0.1 和 3.1 ± 0.6‰ 之间。