Iron mineral evolution as an important geobiological driver of deep-time carbon sequestration in coal-bearing systems

• Microbially driven iron mineral transformations regulate organic and inorganic carbon preservation in coal-forming systems. • Freshwater conditions favor Fe-carbonates via iron reduction, whereas marine influence promotes sulfate reduction and pyritization. • Redox-controlled Fe–C coupling records early diagenetic evolution and long-term carbon stabilization. Although coal-forming environments have been widely studied, how iron mineral transformations interact with microbial processes to control organic and inorganic carbon preservation during peat accumulation remains unclear, particularly under varying redox conditions in marine-influenced versus freshwater seams. The interplay between iron mineralogy and carbon preservation in coal-forming environments reflects a complex suite of biogeochemical processes driven by microbial activity and redox dynamics. This study investigates the coupled cycling of iron and carbon during the formation and early diagenesis of Carboniferous–Permian coal seams from the Qinshui Basin, with emphasis on the roles of mineral transformations and microbial mediation. Geochemical proxies (e.g., Al 2 O 3 /TiO 2 , Zr/TiO 2 , Nb/Yb) indicate that detrital iron was primarily derived from felsic granitoids of the Neoarchean–Paleoproterozoic Yinshan Orogenic Belt. Organic carbon preservation varied with plant tissue composition and depositional redox conditions, with microbial communities facilitating the selective degradation of labile compounds and the persistence of recalcitrant materials. In the No. 3 seam, freshwater-dominated, sulfate-poor conditions promoted dissimilatory iron reduction (DIR) and the formation of Fe-carbonates (siderite, ankerite), thereby contributing to inorganic carbon accumulation through co-precipitation with organic matter. In contrast, the Nos. 9 and 15 seams experienced intermittent marine incursions, promoting microbial sulfate reduction (MSR) and the widespread formation of framboidal and massive pyrite. These mineralogical transitions reflect a redox-controlled diagenetic evolution and reveal a tightly coupled Fe–C system. The co-precipitation of iron minerals with both organic and inorganic carbon indicates a dual preservation mechanism by microbial driven mineralization and authigenic minerals stabilization. This study provides new insights into the mechanisms underlying carbon stabilization in coal-bearing environments and their implications for paleoenvironmental reconstruction and the deep-time carbon cycle.