Lacustrine nitrogen cycling linked to redoxcline fluctuations during the Toarcian oceanic anoxic event

Marine nitrogen cycle perturbations during the Toarcian oceanic anoxic event (T-OAE, ca. 183 Ma) are recorded by the bulk nitrogen isotope compositions (δ¹⁵N_bulk) of sediments, which emphasize the impact of seawater redox changes on the biogeochemical nitrogen cycle. However, lacustrine nitrogen cycling during the T-OAE is unexplored. Here, we report the first δ¹⁵N_bulk data from the Anya section in the Ordos Basin (China) and establish a model of the lacustrine nitrogen cycle during the T-OAE. δ¹⁵N_bulk values covary with other redox proxies (C_org/P and Fe_HR/Fe_T ratios), and indicate varying phases of deposition linked to redox conditions. In particular, a positive shift in δ¹⁵N_bulk (ca. +3.1‰ to +7.4‰) prior to the T-OAE in black shale facies indicates nitrogen loss (via partial denitrification and/or anammox) under anoxic bottom water conditions. However, lower δ¹⁵N_bulk (mean = 3.1‰ ± 0.5‰) in the T-OAE itself likely indicates the predominance of nitrogen assimilation (nitrification and nitrate assimilation) and reduced nitrogen loss in oxygenated waters. The cause of redoxcline collapse may be linked to the destruction of a stable chemocline caused by enhanced hydrological cycling during T-OAE warming. The nitrogen cycle of the paleo-Ordos lake may have behaved differently to the nitrogen cycle in the ocean during the T-OAE. Our work illustrates that even during marked global warming, the regional nitrogen cycle is mainly controlled by localized depositional conditions. Our results suggest that redoxcline collapse played a significant role in biogeochemical nitrogen cycling in lacustrine systems, contributing to our understanding of the terrestrial response to the T-OAE.