image: The marine nitrogen cycle box model adapted from Kang et al. (2023), which considers two reservoirs: one containing organic nitrogen produced by diazotrophs and ammonium generated via remineralization (the fixer-ammonium reservoir, Nfixer/ammonium), and another including nitrate and organic nitrogen produced by nitrate-assimilating organisms (the assimilator-nitrate reservoir, Nassimilator/nitrate). The primary input flux to the marine nitrogen pool is nitrogen fixation (Ffix). Major output fluxes include water column denitrification (Fwcd) and sedimentary denitrification (Fsd), while only a small fraction of organic nitrogen originating from nitrogen fixation and nitrate assimilation is buried in sediments (Ffixer_burial and Fassimilator_burial). Internally, nitrogen is transferred from the fixer-ammonium reservoir to the assimilator-nitrate reservoir via remineralization (Fremin).
Credit: ©Science China Press
The Early Triassic represents a critical interval for ecosystem recovery following the end-Permian mass extinction. This interval was marked by extreme and highly volatile global conditions, including widespread ocean stratification and pervasive anoxia. As a key proxy for tracing past marine nitrogen cycle, nitrogen isotopes provide critical evidence for reconstructing the paleoenvironments during the Early Triassic. However, prior research has lacked quantitative constraints on nitrate availability during the Early Triassic, hindering a comprehensive understanding of nitrogen cycling processes and their ecological impacts.
Graduate student Youren Ma, from the School of Earth and Space Sciences at Peking University, systematically collected and integrated nitrogen isotope data from 11 Early Triassic sections worldwide. Through statistical analysis, the study reconstructed the overall evolutionary trends of nitrogen isotopes across different regions. Building on this dataset, Youren Ma applied a nitrogen cycle box model to quantitatively estimate changes in marine nitrate availability across regions. By comparing regional patterns and correlating them with environmental proxies, the study investigated the primary controls on nitrate dynamics in South China and suggested mechanisms that may underlie interregional differences.
Throughout the Early Triassic, nitrate availability in South China (~0.06–0.2) remained significantly lower than the modern ocean value (~0.7). Comparative analysis with environmental proxies suggests that before the Smithian–Spathian boundary (SSB), sea surface temperatures rose from 32 °C to 39 °C, intensifying ocean stratification, suppressing upwelling, and leading to widespread anoxia. During this phase, nitrate availability remained consistently low at ~0.06. Following the SSB, temperatures decreased to around 35 °C, reducing stratification and enhancing upwelling, which promoted reoxygenation and increased nitrate availability to ~0.2. Subsequently, a return to stronger stratification and weakened upwelling led to renewed anoxia and a decline in nitrate availability to ~0.09. These trends suggest that temperature changes may have governed the evolution of nitrate availability in the Early Triassic by modulating ocean stratification, upwelling intensity, and redox conditions.
Regional comparisons reveal significant differences in nitrate availability between South China and northwestern Pangea during the Early Triassic. Prior to the SSB, the differences may be primarily controlled by the initial nitrate inventories, which were influenced by three key environmental factors. South China was characterized by warm currents, restricted connections with the Panthalassa, and episodic upwelling, leading to a limited nitrate reservoir and consistently low nitrate availability (~0.06). In contrast, northwestern Pangea was influenced by cold currents, strong connections with Panthalassa, and persistent upwelling, resulting in a larger initial reservoir with nitrate availability values decreasing from ~0.48 to ~0.06. After the SSB, regional differences likely arose from regional responses to global cooling. In South China, weakened ocean stratification and intensified upwelling promoted ocean oxygenation and increased nitrate availability to approximately 0.2. Meanwhile, northwestern Pangea remained strongly stratified with suppressed upwelling and persistent anoxia, leading to continuously low nitrate availability maintained at around 0.06.
This study demonstrates that the low-nitrate environment during the Early Triassic led to a bacteria-dominated primary producer community, thereby limiting overall marine primary productivity. Concurrently, ammonium accumulation exerted toxic effects on marine fauna. Together, these factors delayed ecosystem recovery following the end-Permian mass extinction. This finding not only enhances the understanding of the ecological role of the nitrogen cycle but also offers valuable insights into the mechanisms of the co-evolution of the environment and the biosphere following mass extinctions.
See the article:
Ma Y, Ge Z, Shen J. 2025. Prolonged nitrate depletion delayed marine ecosystem recovery after the end-Permian mass extinction. Science China Earth Sciences, 68(9): 3035–3049, https://doi.org/10.1007/s11430-025-1629-8
Journal
Science China Earth Sciences
Method of Research
Data/statistical analysis