Morphology and in-situ sulfur isotope characteristics of pyrite across the Ordovician-Silurian boundary marine shale in South China: Indicative significance for sedimentary environment

Pyrite plays a crucial role in the sulfur cycle, mirroring changes in global and local redox conditions within sedimentary environments across geological records. The grain size of framboidal pyrite effectively indicates the redox state of the sedimentary water column, while its sulfur isotope characteristics provide insights into early diagenetic history. However, variations in water column hydrodynamics can diminish the reliability of framboidal pyrite grain size distribution as an indicator of redox conditions. Bulk pyrite sulfur isotope measurements are often affected by later diagenetic processes. This study aims to investigate the impact of hydrodynamic conditions on framboidal pyrite grain size and to reconstruct the redox history across the Ordovician-Silurian boundary. To achieve this, we examined the morphology, grain size distribution of pyrite, total organic carbon, trace element abundances in the Wufeng (Ordovician)-Longmaxi (Silurian) formation in South China. The results indicate that enhanced hydrodynamic conditions driven by upwelling led to a significant increase in the average grain size of framboidal pyrite formed in reducing environments, as well as greater grain size variability. We also conducted in situ sulfur isotope analyses on two framboidal pyrite grains from the top of the Wufeng Formation and the base of the Longmaxi Formation. A response model was developed to illustrate the relationship between grain-scale δ³⁴S_pyr distribution and sea-level fluctuations. Ultimately, the redox evolution across the Ordovician-Silurian boundary in the Weiyuan area was reconstructed into five stages: (1) The upper Wufeng Formation experienced intensifying reducing conditions, culminating in euxinia at the top. (2) Oxidizing conditions briefly prevailed at the base of the Longmaxi Formation. (3) Oxygen levels in the sedimentary waters of the lower Longmaxi Formation decreased, stabilizing in a prolonged dysoxic-euxinic state. (4) The middle-lower Longmaxi Formation saw a gradual increase in the oxidative state of the sedimentary waters, transitioning to an oxic environment. (5) The middle Longmaxi Formation's sedimentary waters sustained a long-term dysoxic-oxic state.