Co-variation of nitrogen isotopes and Palaeo-redox conditions during the Middle Triassic Luoping biota preservation in South China

The early Middle Triassic (Anisian) Luoping biota serves as a significant example of marine ecosystem recovery following the Permian–Triassic mass extinction. However, the specific burial environment and mechanisms responsible for this exceptional preservation remain to be fully elucidated. The nitrogen isotopic composition of ancient sediments can provide valuable insights into the nitrogen biogeochemical cycle of the ocean and the redox conditions of the water column. In this study, we present bulk nitrogen isotopic compositions (δ¹⁵N_bulk), C/N_mol ratios, and the concentrations of Al and K₂O content from Member II of the Middle Triassic Guanling Formation at the Xiangdongpo (XDP) section in Luoping County, Yunnan Province, South China. The lower and upper fossiliferous layers of the Luoping biota exhibit distinct nitrogen isotopic compositions. The lower fossiliferous layers demonstrate relatively low nitrogen isotope values (−4.01‰ to +4.24‰, mean =  +1.61‰), indicating non-quantitative assimilation of upwelled NH₄⁺, while the upper layers display relatively high nitrogen isotope values (+3.04‰ to +8.91‰, mean =  +6.52‰) with a temporary moderately negative bias, suggesting assimilation of upwelled NO₃⁻ from non-quantitative denitrification. The δ¹⁵N_bulk shows a negative correlation with productivity and redox proxies (TOC/Al, P/Al, Mo_EF, and U_EF), indicating that the negative deviations of nitrogen isotopes correspond to high productivity and anoxic conditions, both of which are beneficial for the preservation of the biota. The concentrations of Al and K₂O, which indicate clay mineral content, show a significant increase in interval IV compared to interval II. This increase may further enhance the burial efficiency of the fossils. In summary, our results indicate that the negative nitrogen isotope deviations are consistent with relatively high productivity and anoxic water conditions, which facilitate the burial of the Luoping biota. Additionally, the increase in clay minerals can also promote fossil preservation to some extent, further enhancing the burial process.

早三叠世早期（安尼期（Anisian））罗平生物群是二叠纪-三叠纪大灭绝（Permian–Triassic mass extinction）后海洋生态系统复苏的典型范例。然而，造就该特异埋藏的具体沉积环境与保存机制仍有待全面阐明。古沉积物的氮同位素组成可为海洋氮生物地球化学循环及水体氧化还原条件提供重要见解。本研究针对中国南方云南省罗平县湘洞坡（XDP）剖面中三叠统关岭组二段的样品，分析了其全岩氮同位素组成（δ¹⁵N_bulk）、摩尔碳氮比（C/N_mol）以及铝（Al）和氧化钾（K₂O）的含量。罗平生物群的下部与上部含化石层位展现出显著不同的氮同位素组成特征：下部含化石层位的氮同位素值相对偏低（-4.01‰~+4.24‰，平均值为+1.61‰），指示对上升流携带的铵根离子（NH₄⁺）的非定量同化作用；上部含化石层位的氮同位素值则相对偏高（+3.04‰~+8.91‰，平均值为+6.52‰），且呈现暂时性的中等负偏特征，表明其氮源来自经非定量反硝化作用产生的上升流硝酸根离子（NO₃⁻）的同化过程。全岩氮同位素值（δ¹⁵N_bulk）与生产力代用指标及氧化还原代用指标（总有机碳/铝（TOC/Al）、磷/铝、钼富集因子（Mo_EF）及铀富集因子（U_EF））呈负相关关系，表明氮同位素的负偏现象对应着高生产力与水体缺氧环境，而这两类条件均有利于生物群的保存。指示黏土矿物含量的铝与氧化钾含量在第四段地层中较第二段地层显著升高，这一变化或可进一步提升化石的埋藏效率。综上，本研究结果表明，氮同位素的负偏现象与高生产力及水体缺氧环境相契合，而这些条件均有助于罗平生物群的埋藏；此外，黏土矿物含量的升高在一定程度上亦可促进化石保存，进一步优化了埋藏过程。