Recovery from persistent nutrient-N limitation following the Permian–Triassic mass extinction

Global warming, widespread oceanic anoxia and stagnation, and large perturbations of the global carbon cycle characterized the end-Permian to Middle Triassic interval. Nitrogen isotopes of marine sediments (δ15Nbulk) decreased through the Permian–Triassic transition, implying development of nitrate-limited and ammonium-dominated conditions (i.e., anaerobic marine N cycle) in Early Triassic oceans, which may have contributed to the delay in marine biotic recovery following the end-Permian mass extinction. However, the temporal evolution of the nitrogen cycle and the role of nutrient supply in marine ecosystem recovery during this interval remain poorly understand. Here, we present a new high-resolution Permian–Triassic nitrogen isotope curve from the Nanpanjiang Basin of South China. Low δ15N (−2‰ to +2‰) during the Griesbachian-to-Smithian substages (i.e., first ∼2 Myr of Early Triassic) reflects enhanced nitrogen fixation concurrently with climatic hyperwarming and expansion of oceanic anoxia. A large rise in δ15N (to +8‰) followed by a decline (to −2‰) reflects an aborted recovery of the marine N cycle during the Spathian substage of the Early Triassic. During the Middle Triassic, δ15N fluctuations between +1‰ and +4‰ during the Anisian Stage, followed by stabilization around +4‰ in the Ladinian Stage, suggest a slow stepwise re-establishment of the aerobic marine N cycle. Although both South China and northwestern Pangea experienced a transition to anaerobic N cycling during the Early Triassic, South China experienced an earlier and more rapid onset of this event as well as larger N-cycle fluctuations during the recovery interval than northwestern Pangea. Overall, N cycle changes coincided with those in paleotemperature and ocean-redox state, demonstrating an integrated response of the marine system to the extreme environmental perturbations of the Early Triassic. In summary, our results support persistence of nutrient-N-limited conditions and strong microbial N-fixation throughout the Early Triassic, with a full return to the aerobic N cycle only after stabilization of oceanic environmental conditions during the Middle Triassic.