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.