Large Nitrogen Cycle Perturbations During the Early Triassic Hyperthermal Interval

Ocean temperature, redox state, circulation, and nutrient levels regulate marine nitrogen (N) cycle, yet their impacts during greenhouse intervals remain poorly understood. Here, we examined the Smithian–Spathian hyperthermal event (~250.5 Ma) during the Early Triassic greenhouse using stable N isotopes (δ15NDCN) from two basins in South China. The Smithian hyperthermal interval exhibited low δ15NDCN (+1‰ – +3‰), indicating N deficiency and enhanced N2 fixation. Subsequent cooling across the Smithian–Spathian boundary led to discrete δ15NDCN changes, likely reflecting signals of partial NH4+ assimilation in anoxic and NH4+-enriched conditions. In the early Spathian with milder temperatures, biological N gradually changed to nitrate-dominated sources from partial denitrification, as indicated by increased δ15NDCN (up to +3‰ – +6‰). Eukaryotic algae thrived while prokaryotes declined during the Spathian, evidenced by elevated δ15NDCN and Δ13Ccarb-org (carbonate-organic carbon isotope difference). These findings highlight significant turnovers in N cycle in response to temperature changes in low-latitude South China.