Large Nitrogen Cycle Perturbations During the Early Triassic Hyperthermal

Delta Ocean temperature, redox state, circulation, and nutrient levels regulate the marine nitrogen (N) cycle, yet their specific impacts during greenhouse intervals remain poorly understood. Here, we examined the Smithian-Spathian hyperthermal event (similar to 250.5 Ma) during the Early Triassic greenhouse using stable N isotopes (delta N-15) from sedimentary records in the Nanpanjiang Basin and Northern Yangtze Basin of South China. The delta N-15 profiles in both basins reveal consistent trends that correspond to fluctuations in temperature across the Smithian-Spathian transition. The Smithian hyperthermal interval exhibited low delta N-15 values (mostly <+2 parts per thousand), indicating N deficiency and enhanced biological N-2 fixation. Bacterial blooms and the release of the potent greenhouse gas N2O, enhanced by high temperatures, may have triggered positive feedback mechanisms that sustained the warming and contributed to the late Smithian extinction. During subsequent cooling across the Smithian-Spathian transition, delta N-15 increased to a range of +3 parts per thousand to +7 parts per thousand, likely reflecting signals of partial denitrification based on reconstruction of the NO3- inventory associated with oceanic cooling and oxygenation. The prevailing increases in sedimentary TOC/TN ratios signify heightened deamination (removal of amino groups) and N recycling across the Smithian-Spathian boundary. This transition is likely attributed to cooling-driven amelioration of seawater stratification and anoxia from the Smithian to the Spathian, which resulted in increased nitrate availability in the photic zone. Eukaryotic algae thrived while prokaryotes declined during the Spathian, evidenced by elevated delta N-15 and Delta C-13(carb-org) (the difference between carbonate and organic carbon isotopes). The proliferation of eukaryotic algae had a positive impact on environmental conditions and facilitated biotic recovery due to more efficient burial of organic particles. A notable latitudinal gradient in the N cycle response was observed, with low-latitude regions showing a more pronounced response to cooling compared to mid-latitude areas. This significant gradient may suggest that the rapid recovery of nutrient cycles, despite relatively small decline in high temperature, was a key factor in the amelioration of climate and recovery of life in low latitudes. These findings highlight that the marine N cycle is highly sensitive to temperature changes, particularly in low-latitude regions, and that changes in N cycle under high-temperature conditions may be a critical life-limiting factor.