In-situ sulfur isotope and microscopic analyses reveal multiple pyrite genesis mechanisms in the early Ediacaran Doushantuo Formation, South China

Sedimentary pyrite constitutes an invaluable geological archive for reconstructing Earth's ancient environments, yet interpreting its origins within deep geological records is often complicated by subsequent diagenetic modification. Here, we examine four distinct pyrite morphologies (framboidal pyrites hosted in chert nodules and matrix, euhedral pyrites within calcite rims of nodules, and pyrite aggregates within surrounding matrix rocks) preserved in the Ediacaran Doushantuo Formation of South China, utilizing a combination of high-resolution electron microscopy and in-situ secondary ion mass spectrometry sulfur isotope analysis. Our microscopic observations identify well-preserved framboidal pyrites, variably altered by post-depositional processes, alongside large euhedral pyrite crystals encapsulated by calcite, and irregular pyrite aggregates. Integrated nanoscale microscopic and in situ isotopic analyses consistently demonstrate that framboidal pyrites exhibit negative δ 34 S values, indicative of microbial sulfate reduction within the water column or at the sediment-water interface during early diagenesis. Conversely, euhedral pyrites and irregular aggregates from nodule rims and surrounding matrix yield significantly positive δ 34 S values, consistent with thermochemical sulfate reduction during late-stage burial diagenesis. This finding contrasts with previous interpretations that attribute positive δ 34 S values to a diminished marine sulfate reservoir within a restricted paleoceanographic setting. Our results emphasize that post-depositional thermochemical sulfate reduction processes can significantly elevate δ 34 S values of pyrites. This study underscores the critical necessity of deconvolving diagenetic overprinting from primary signatures by integrating in-situ isotopic and microscopic analyses, thereby refining paleoenvironmental reconstructions from pyrite archives. • Integrated nano-microscopy and in-situ SIMS analysis reveal distinct pyrite origins. • Framboidal pyrites (negative δ 34 S) originate from microbial sulfate reduction. • Non-framboidal pyrites (positive δ 34 S) form via thermochemical sulfate reduction. • Diagenetic overprinting significantly alters primary pyrite δ 34 S signals. • Reevaluates Ediacaran sulfur cycle and ocean redox conditions.