Climate fluctuations drive periodic shifts in anoxic depositional environments: Mo availability regulates nitrogen cycling and organic carbon burial

Climate-induced changes in salinity and hydrological restriction can reshape ecological communities and biogeochemical cycles in anoxic water bodies, thereby altering the productivity–preservation balance and influencing organic carbon burial. This study distinguishes two anoxic depositional modes and employs lipid biomarkers, trace element indices, and C–N stable isotopes to elucidate their ecological and biogeochemical implications. During arid intervals (Mode A), characterized by hypersaline, restricted conditions and high TOC, elevated δ¹⁵N values (~6‰) indicate enhanced denitrification. Although cyanobacterial abundance is relatively high, low Mo/TOC ratios suggest limited Mo availability, which constrains nitrogen fixation. In humid periods (Mode B), corresponding to low‑salinity, open‑system settings with low TOC, δ¹⁵N values decrease (~4.5‰). Increased Mo/TOC ratios point to improved Mo availability that promotes nitrogen fixation, superimposing a nitrogen‑fixation signal on the δ¹⁵N record and causing a slight negative shift even under anoxic conditions. Differences in δ¹³C between the two modes further indicate that higher productivity during arid phases enriches the dissolved inorganic carbon pool in heavier carbon, whereas humid periods are marked by reduced productivity and greater input of terrestrially derived light carbon. Overall, the sensitivity of the nitrogen cycle to environmental perturbation is primarily governed by the supply of Mo—a key cofactor for nitrogenase—rather than cyanobacterial abundance. Meanwhile, aridity‑driven nutrient concentration combined with brief oxidative decomposition under a shallow halocline jointly enhances both organic matter input and preservation, ultimately promoting organic carbon burial. This framework highlights the coupling among climate, nutrient dynamics, trace‑metal limitation, and biological communities, offering an ecological‑process perspective for interpreting nitrogen‑cycle perturbations and carbon‑sink formation in anoxic systems.