Volcanic ash drives contrasting redox shifts and biogeochemical feedbacks in ancient marine and lacustrine systems

Volcanic eruptions are major perturbations to Earth’s biogeochemical cycles, yet their long-term effects on nutrient dynamics and ecosystem recovery remain unclear. Here we present high-resolution geochemical records from Late Ordovician marine and Middle Triassic lacustrine successions to investigate how airborne volcanic ash influenced carbon and nitrogen cycling. Our results reveal that volcanic ash deposition triggered contrasting redox responses in marine and lacustrine systems. In marine systems, ash input promoted persistent anoxia, suppressed nitrogen cycling, and delayed ecosystem recovery for millennia, and was further amplified by enrichment in toxic elements. In lacustrine systems, ash enhanced water-column oxygenation, reduced toxic stress, which stimulated nitrogen fixation and the growth of cyanobacteria, facilitating ecological recovery within decades. These findings demonstrate that volcanic ash drives contrasting biogeochemical responses in marine and lacustrine ecosystems, provide mechanistic insights into past ecosystem resilience, and offer a framework for predicting recovery under future volcanic and climatic disturbances.