Volcanogenic controls on coupled Fe-S-P cycling in a lacustrine basin: Deciphering mechanisms of extreme organic carbon enrichment

Black shales serve as primary archives of organic carbon, preserving critical records of elemental cycling dynamics within carbon sequestration systems. Ancient lakes are critical nodes in the carbon-element coupling system, yet full understanding of this remains challenging. This investigation employs an integrated stratigraphic-geochemical methodology to elucidate volcanic material–induced perturbations in Fe-S-P coupling mechanisms and their consequent impacts on organic carbon preservation within Upper Triassic lacustrine shales of the Ordos Basin, North China Platform. Petrographic analysis reveals that organic carbon enrichment demonstrates genetic associations with volcanic inputs, though peak carbon accumulation intervals stratigraphically lag behind tuff deposition events. The volcanic inputs introduced substantial Fe, P, and S into the lacustrine basin. Volcanogenic atmospheric fertilization stimulated proliferation of coccolithophore, lamellibranch, and actinopterygian assemblages within the lacustrine ecosystem, yet sustained nutrient replenishment over million-year time scales proved geochemically untenable. Crucially, volcanic-derived phosphorus became effectively sequestered via iron oxide adsorption and biomineralization processes in sedimentary matrices during this stage. Sulfate influx modulated the dominant bacterially mediated redox pathway from dissimilatory iron reduction to bacterial sulfate reduction, thereby transitioning the redox regimes to possibly euxinic conditions. During this process, Fe3+ in phosphorus-bearing iron (oxides) was reduced to Fe2+ and subsequently removed via pyrite formation. Subsequent phosphate remobilization from phosphorus-rich sediments under anoxic bottom-water conditions established a positive feedback mechanism that sustained lacustrine productivity over millions of years, ultimately facilitating exceptional organic matter preservation. This research highlights lacustrine nutrient cycling dynamics as critical amplifiers of carbon burial following volcanic material perturbations, providing mechanistic insights into the functioning of lacustrine carbon sinks and long-term carbon sequestration.