Main controls on nitrogen cycling and Mo-U enrichment of the Middle Triassic Chang 7 Member in the Ordos Basin: Implications for redox and ultra-organic enrichment in a giant highly productive lake

During the deposition of the Chang 7 Member (∼241.5 Ma) black shale from the Yanchang Formation in the Ordos Basin, a giant and deep perennial lake developed. This deposition represents a substantial organic carbon burial, and this lake serves as a significant focus for investigating the ecosystem recovery and environmental evolution following the end-Permian mass extinction (EPME). Due to potential influences from volcanic-hydrothermal activity and terrigenous detrital input, the primary controls on nitrogen (N) and redox-sensitive trace element enrichment remain unclear, leading to ongoing debates regarding the lake's redox conditions inferred from these proxies. This study examines the main controls on the different isotopic compositions of N species and the enrichment of redox-sensitive trace elements (RSTEs) in deep lake facies drill cores. The following conclusions can be drawn: (1) The high total N content (averaging 1.11 %) and inorganic N content (accounting for 65 % of total N) are likely influenced by terrigenous detrital input. This input also affects N isotope composition, leading to summarized results of bulk δ¹⁵N_bulk values being lower than δ¹⁵N_ker (δ¹⁵N of the kerogen) values (average 2.4 ‰). Other post-depositional factors, including thermal maturation and retained oil, do not significantly affect the organic N isotopes, thereby enhancing their reliability for N cycling analysis. (2) The notable enrichments of Mo and U in the Chang-7 source rocks, with Mo_EF values reaching 90.8 and U_EF values of 13.9, were primarily controlled by strong adsorption of metal sulfides and organic matter under anoxic-euxinic bottom water conditions, with additional input from hydrothermal activities. (3) Under anoxic-euxinic bottom water conditions, moderate δ¹⁵N_ker values (∼4.5 ‰) and decreasing δ¹⁵N_ker in deeper facies indicate a deep redox chemocline with an metalimnetic oxygen minimum near the lake's margin. N cycling was primarily dominated by denitrification but with a notable contribution from nitrogen fixation, while nitrates and ammonium were the primary bio-limiting nutrients in the metalimnetic oxygen minimum. Factors such as global cooling, which facilitated ecological recovery after the EPME, and local volcanic-hydrothermal activities that enhanced weathering and nutrient input, may have promoted primary production and development of anoxic bottom water in this large lake, ultimately fostering the anomalous accumulation of organic material.