The Paleoproterozoic metasedimentary rocks of the Zaonega Formation (Onega Basin, NW Russia) are important archives of inferred global environmental change following the initial oxygenation of the Earth’s atmosphere and oceans. However, the geochemical signals preserved in these exceptionally organic-and pyrite-rich metasedimentary rocks and their environmental meaning remain contested. In particular, the Zaonega Formation’s unusually high pyrite sulfur isotope ratios (δ³⁴S) have been explained by either global or local forcings acting on sulfur cycling processes. We tested former interpretations of the Zaonega Formation’s sedimentary pyrite record by integrating bulk and micro-scale δ³⁴S analysis to discriminate the isotopic signatures of different generations of pyrite and determine the underlying mechanisms contributing to δ³⁴S variability. We show that the prolonged genesis of pyrite occurred via multiple stages and included precipitation from early diagenetic fluids, organic matter pyritization, and late-stage alteration fluids. Our results demonstrate that early-stage pyrite typically carries more variable and lower δ³⁴S values than late-stage pyrite. Although the early pyrite captures pore water S isotope signatures least evolved from the seawater, their contribution to the bulk δ³⁴S results can be dwarfed by the greater volume of late-stage coarse pyrite. Consequently, determining the sequence of pyrite precipitation and δ³⁴S characteristics of individual generations in any given sample are fundamental to interpreting bulk δ³⁴S records. Our micro-scale results suggest that previous estimates based on bulk pyrite data (ca. 6 to 18‰) should not be related to the original seawater sulfate’s isotopic composition. These results demonstrate that a thorough understanding of the geological context and mechanisms associated with S-cycling, and pyrite formation is necessary to interpret bulk δ³⁴S records accurately.

Zaonega 组（Onega 盆地，俄罗斯西北部）的古元古代变沉积岩是推断地球大气和海洋初始氧化后全球环境变化的重要档案。然而，这些异常有机质和富含黄铁矿的变沉积岩中保存的地球化学信号及其环境意义仍然存在争议。特别是，Zaonega 组异常高的黄铁矿硫同位素比 (δ ³⁴ S) 可以通过作用于硫循环过程的全球或局部强迫来解释。我们通过整合体积和微尺度 δ ^{34测试了对 Zaonega 组沉积黄铁矿记录的先前解释}S 分析以区分不同代黄铁矿的同位素特征并确定导致 δ ³⁴ S 变异性的潜在机制。我们表明黄铁矿的长期成因经历了多个阶段，包括早期成岩流体的沉淀、有机质黄铁矿化和晚期蚀变流体。我们的研究结果表明，早期黄铁矿通常比晚期黄铁矿携带更多的可变性和更低的 δ ^{34 S 值。}尽管早期黄铁矿捕获的孔隙水 S 同位素特征是从海水中演化得最少的，但它们对大体积 δ ³⁴ S 结果的贡献可能会因晚期粗黄铁矿体积的增加而相形见绌。因此，确定黄铁矿沉淀的顺序和δ任何给定样本中各代的^{34 S 特征是解释大量 δ 34} S 记录的基础。我们的微观结果表明，先前基于大量黄铁矿数据（约 6 至 18‰）的估计不应与原始海水硫酸盐的同位素组成有关。这些结果表明，对与 S 循环和黄铁矿形成相关的地质背景和机制的透彻了解对于准确解释大量 δ ^{34 S 记录是必要的。}