251-8 The Marine Nitrogen Cycle and Arc Volcanism at Penglaitan: Environmental Disturbance at the Junction of Ecological and Geological Timescales

Session: Phanerozoic Earth System Shifts in the Marine Sedimentary Record

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Negative nitrogen isotope excursions are a widespread feature of sedimentary records spanning the end-Permian mass extinction, and likely reflect reorganization of the global marine nitrogen cycle, with persistent nutrient stress throughout the Early Triassic inhibiting biological recovery. In most end-Permian sections, the transitional interval is sufficiently condensed that the exact timing of the change cannot be readily distinguished from other aspects of the mass extinction cascade. The Penglaitan Northern Bank section, in Guangxi, China, offers unprecedented resolution, with over 600 meters of well-dated Changhsingian strata. This detailed view of the end-Permian shows that the baseline Late Permian nitrogen cycle mode, dominated by recycling of marine nitrate, persisted until the onset of rapid transgression in the latest Permian, at ~252.0 Ma. Nitrogen isotopes became increasingly unstable in the final 50-100 kyr of the Permian, with negative values indicative of N fixation in an oligotrophic environment and more positive values reflecting the incursion of anoxic, denitrifying waters.

This succession also contains massive volcanogenic sandstones and crystalline tuffs, recording arc volcanism on the subduction margin of the South China Block. Each of these depositional events would have resulted in extreme dislocation of the local benthic fauna, with a recurrence interval on the order of thousands to tens of thousands of years. The repeated recovery of characteristically Permian benthic faunas (and an apparently healthy latest Permian tropical flora) from these volcanic events, which persisted into the interval of nitrogen cycle instability, sheds light on the connection between proximal and ultimate kill mechanisms during mass extinctions. While increasing nutrient stress was not itself sufficient to extinguish the Permian biota of the Nanpanjiang Basin, ratcheting global oceanographic changes made recovery from mesoscale environmental disasters increasingly difficult, and eventually impossible. A similar interplay between local and global environmental stresses likely operated in other parts of the Permian world, partly concealed by stratigraphic condensation.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10025

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