Abstract Anomalous sulfur isotope compositions present in Archean rocks have been intensely scrutinized over the last 20 years because they record key aspects of Earth's atmospheric composition prior to the appearance of free molecular oxygen ca. 2.3 billion years ago. These isotopic compositions can be described as mass anomalous fractionations (MAF) and are produced in the atmosphere as UV light interacts with SO2 molecules. Most interpretations suggest that atmospheric processes generate a reduced S-phase with a positive (33S-enriched) MAF signature, as measured in pyrites, and an oxidized S-phase with a negative anomaly, as measured in bedded barite deposits. However, recent data for carbonate-associated sulfate (CAS) — a direct proxy for the isotopic composition of sulfur from seawater sulfate — in Neoarchean rocks showed no such negative values, but rather the opposite. To understand if the positive MAF anomalies we measured in Neoarchean CAS reflect secondary processes (diagenetic, metamorphic, handling) instead of original signals of Archean seawater sulfate, we collected additional sample suites with various degrees of preservation and metamorphic alteration across the Campbellrand-Malmani platform in South Africa. Results illustrate that within this comprehensive suite, less-altered samples all contain positive MAF values while secondary processes tend to either remove CAS from the sample and/or decrease the 33S-enrichment. This positive MAF signal in sulfate is therefore reasonably interpreted as a primary depositional origin, and implies that the assumption that sulfate always carries a negative MAF anomaly throughout the Archean rock record needs to be reconsidered. Our CAS observations suggest that future experiments and calculations should also consider atmospheric and/or sulfur cycling processes that can produce oxidized sulfur with a positive MAF signature.

中文翻译：

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新太古代中具有正Δ33S的硫酸盐气溶胶的沉积

摘要 在过去的 20 年里，太古宙岩石中存在的异常硫同位素组成受到了严格的审查，因为它们记录了大约在自由分子氧出现之前地球大气组成的关键方面。23亿年前。这些同位素组成可被描述为质量异常分馏 (MAF)，并在大气中产生，因为紫外线与 SO2 分子相互作用。大多数解释表明，大气过程产生了具有正（富含 33S）MAF 特征的还原 S 相，如黄铁矿所测量，以及具有负异常的氧化 S 相，如层状重晶石沉积物所测量。然而，新太古代岩石中碳酸盐相关硫酸盐 (CAS)（海水硫酸盐中硫的同位素组成的直接代表）的最新数据没有显示出这样的负值，而是相反。为了了解我们在新太古代 CAS 中测量的正 MAF 异常是否反映了次生过程（成岩、变质、处理）而不是太古代海水硫酸盐的原始信号，我们在 Campbellrand-Malmani 平台上收集了具有不同程度保存和变质改变的额外样本组在南非。结果表明，在这个综合套件中，较少改变的样品都包含正 MAF 值，而二次加工倾向于从样品中去除 CAS 和/或减少 33S 富集。因此，硫酸盐中的这种正 MAF 信号被合理地解释为主要沉积起源，并意味着需要重新考虑硫酸盐在整个太古代岩石记录中始终带有负 MAF 异常的假设。我们的 CAS 观察表明，未来的实验和计算还应考虑大气和/或硫循环过程，这些过程可以产生具有正 MAF 特征的氧化硫。