Genesis of carbonate concretions in Early Silurian shales of the Upper Yangtze area: Multi-phase constraints from deposition to diagenesis

Abundant carbonate concretions in Early Silurian Longmaxi Formation shales of the Upper Yangtze archive key palaeoenvironmental and diagenetic information. In this study we integrate petrography, inorganic geochemistry and carbon–oxygen isotopes to constrain their origin. The concretions are dominated by low-Mg calcite with minor dolomite, pyrite and diagenetic barite. Moderately 13 C-depleted carbonate (δ 13 C = −10.7‰ to −14.7‰) and palaeotemperatures of 64–66 °C indicate early-diagenetic formation at shallow burial within the sulfate reduction zone (SRZ) and overlying sulfate–methane transition zone (SMTZ). These signatures and associated geochemical data show that dissolved inorganic carbon (DIC) was mainly supplied by organoclastic sulfate reduction (OSR) and sulfate-driven anaerobic oxidation of methane (SD-AOM). Core–rim elemental zoning records dominantly concentric growth with subordinate pervasive pore-filling cementation. Rare-earth element (REE) patterns distinguish volcanic Eu anomalies in Hirnantian–Rhuddanian host shales from subtler Eu–Yb anomalies in Aeronian concretions, consistent with modification by reducing, Fe-rich pore fluids rather than by basin-wide hydrothermal overprint. High Ba and 34S-enriched barite document internal Ba recycling in a paleo-SMTZ. Enhanced Hirnantian–Rhuddanian productivity and anoxia, followed by more oxygenated Aeronian conditions and greater carbonate–clay input, together with sustained OSR and SD-AOM, controlled concretion nucleation and cementation. Overall, these concretions offer a useful record of how ancient marine environments, carbon cycling and early burial processes interacted in organic-rich sediments.