Multiple sulphur (S) isotope ratios are powerful proxies to understand the complexity of S biogeochemical cycling through Deep Time. The disappearance of a sulphur mass‐independent fractionation (S‐MIF) signal in rocks <~2.4 Ga has been used to date a dramatic rise in atmospheric oxygen levels. However, intricacies of the S‐cycle before the Great Oxidation Event remain poorly understood. For example, the isotope composition of coeval atmospherically derived sulphur species is still debated. Furthermore, variation in Archaean pyrite δ³⁴S values has been widely attributed to microbial sulphate reduction (MSR). While petrographic evidence for Archaean early‐diagenetic pyrite formation is common, textural evidence for the presence and distribution of MSR remains enigmatic. We combined detailed petrographic and in situ, high‐resolution multiple S‐isotope studies (δ³⁴S and Δ³³S) using secondary ion mass spectrometry (SIMS) to document the S‐isotope signatures of exceptionally well‐preserved, pyritised microbialites in shales from the ~2.65‐Ga Lokammona Formation, Ghaap Group, South Africa. The presence of MSR in this Neoarchaean microbial mat is supported by typical biogenic textures including wavy crinkled laminae, and early‐diagenetic pyrite containing <26‰ μm‐scale variations in δ³⁴S and Δ³³S = −0.21 ± 0.65‰ (±1σ). These large variations in δ³⁴S values suggest Rayleigh distillation of a limited sulphate pool during high rates of MSR. Furthermore, we identified a second, morphologically distinct pyrite phase that precipitated after lithification, with δ³⁴S = 8.36 ± 1.16‰ and Δ³³S = 5.54 ± 1.53‰ (±1σ). We propose that the S‐MIF signature of this secondary pyrite does not reflect contemporaneous atmospheric processes at the time of deposition; instead, it formed by the influx of later‐stage sulphur‐bearing fluids containing an inherited atmospheric S‐MIF signal and/or from magnetic isotope effects during thermochemical sulphate reduction. These insights highlight the complementary nature of petrography and SIMS studies to resolve multigenerational pyrite formation pathways in the geological record.

中文翻译：

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新太古代微生物垫中的硫循环。


多种硫 (S) 同位素比是了解深时硫生物地球化学循环复杂性的有力指标。 <~2.4 Ga 岩石中硫质量无关分馏 (S-MIF) 信号的消失已被用来确定大气中氧气水平急剧上升的时间。然而，人们对大氧化事件之前的 S 循环的复杂性仍然知之甚少。例如，同时代大气中衍生的硫物质的同位素组成仍然存在争议。此外，太古宙黄铁矿δ ³⁴ S 值的变化被广泛归因于微生物硫酸盐还原（MSR）。虽然太古宙早成岩黄铁矿形成的岩相证据很常见，但 MSR 存在和分布的结构证据仍然是个谜。我们使用二次离子质谱 (SIMS) 结合了详细的岩相学和原位高分辨率多种 S 同位素研究（δ ³⁴ S 和 Δ ³³ S），记录了页岩中保存异常完好的黄铁矿微生物岩的 S 同位素特征来自南非 Ghaap 群 ~2.65‐Ga Lokammona 组。这种新太古代微生物垫中 MSR 的存在得到了典型的生物成因纹理的支持，包括波状皱纹纹层和含有 <26 id=3>34 S 和 Δ ³³ S = -0.21 ± 0.65‰ (±1σ) 的早成岩黄铁矿。 δ ³⁴ S 值的这些巨大变化表明在高 MSR 速率期间对有限的硫酸盐池进行了瑞利蒸馏。此外，我们还确定了第二个形态独特的黄铁矿相，在岩化后沉淀，δ ³⁴ S = 8.36 ± 1.16 ‰ 和 Δ ³³ S = 5.54 ± 1.53 ‰ (±1σ)。 我们认为，次生黄铁矿的 S-MIF 特征并不反映沉积时的同期大气过程；相反，它是由含有继承的大气 S-MIF 信号的后期含硫流体的流入和/或热化学硫酸盐还原过程中的磁同位素效应形成的。这些见解强调了岩相学和 SIMS 研究的互补性，以解决地质记录中多代黄铁矿的形成途径。