Stability up until the end: Disruption, recovery and the latest Permian nitrogen cycle at Penglaitan, China

Negative organic sedimentary nitrogen isotope excursions are a common feature of sedimentary records spanning the end-Permian mass extinction (EPME). These excursions likely reflect global-scale perturbations of the marine nitrogen cycle. However, most EPME sections offer too little stratigraphic resolution to assess the timing, nature, and local ecological impacts of these disruptions. The Penglaitan Northern Bank section in Guangxi, China, offers an opportunity to explore the end-Permian nitrogen cycle in over 600 m of Changhsingian strata deposited in marginal marine environments of the Nanpanjiang Basin. The succession contains numerous volcanogenic sandstones and crystalline tuffs, defining a pattern of disruption and recovery on ecological timescales that can be compared to the more impactful biogeochemical changes associated with the EPME.

Organic carbon and nitrogen isotopes in the Penglaitan Northern Bank section are stable throughout the majority of the Changhsingian, with nitrogen isotope values consistent with a background ‘greenhouse’ climate nitrogen cycle. This stability in the nitrogen cycle is interrupted by the latest Permian transgression, beginning at 252.0 Ma. During the transgression, nitrogen isotope values become considerably more variable, ranging from negative values indicative of N fixation in an oligotrophic environment to more enriched values reflecting the incursion of anoxic, denitrifying waters. Nitrogen isotope values in the earliest Triassic stabilize, but remain lower than the Changhsingian baseline. The negative δ¹⁵N excursions seen in other sections globally likely represent condensed expressions of the more complex nitrogen cycle processes revealed at Penglaitan.

At Penglaitan, δ¹⁵N instability precedes the paleontologically defined extinction at the EPME and demonstrates that changes to marine biogeochemical cycles were diachronous with respect to local species extinctions, which may have been proximally driven by local volcanism and facies changes. While unstable nutrient cycles may have stressed marine ecosystems in the latest Changhsingian, they did not immediately push these communities beyond their ability to recover from episodic disturbance by volcanism.