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 an important role in the sulfur cycle, reflecting changes in both global and local redox conditions within sedimentary environments. The grain size of framboidal pyrite is an effective indicator of 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. Additionally, bulk sulfur isotope measurements of pyrite are often influenced by later diagenetic processes. In this study, we investigated the redox sensitive elements content, morphology and in-situ sulfur isotopic characteristics of pyrite in the Wufeng (Ordovician)-Longmaxi (Silurian) Formation shales in South China. The results indicate that bottom currents, by altering the hydrodynamic conditions of the sedimentary water column, leads to larger and more dispersed grain sizes of framboidal pyrite formed in anoxic water column. Moreover, framboidal pyrite formed during the Late Ordovician and Early Silurian exhibits distinctly different sulfur isotope distribution characteristics at the particle scale, which appears to reflect the response of sedimentation rate changes to sea level fluctuations. Ultimately, we systematically reconstructed the redox evolution of the sedimentary water column during the Ordovician-Silurian transition in South China, dividing it into five stages: (1) The upper Wufeng Formation experienced increasingly 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, s stabilizing in a prolonged dysoxic to euxinic state. (4) The middle-lower Longmaxi Formation experienced a gradual increase in the oxidative state of the sedimentary waters, transitioning to an oxic water column. (5) The middle Longmaxi Formation sustained a long-term dysoxic to oxic water column.