Permian-Triassic Redox Shift and Its Ferruginous Aftermath in Epicontinental Seas

Marine anoxia has been hypothesised as a primary environmental stressor during the end-Permian mass extinction (EPME) and in the subsequent prolonged biotic recovery. However, the spatial and temporal extent of oxygen restriction through the EPME remains contentious. Here, we present iron speciation, pyrite framboid and molybdenum-uranium covariation data from two palaeogeographically distinct settings: the Tethyan Chibi section (South China) and the Panthalassian Ursula Creek section (Western Canada), which capture redox dynamics across the Permian-Triassic transition. Our data suggest that bottom waters were predominantly dysoxic during the late Changhsingian at both sites. The prevalence of small pyrite framboids, elevated Mo and U enrichment factors (MoEF and UEF), and high MoEF/UEF ratios near the EPME horizon implicate seafloor anoxia as a key trigger for marine extinctions in epicontinental seas. In the post-extinction Early Triassic, iron speciation and MoEF-UEF covariation data reveal a shift to persistent ferruginous conditions in both locations. A global compilation of iron speciation data indicates that anoxic conditions fluctuated between ferruginous and euxinic in epicontinental seas during the Permian-Triassic crisis, with ferruginous conditions expanding significantly in the earliest Triassic. The expansion of a ferruginous seafloor would have limited phosphorus bioavailability, suppressing primary productivity in the immediate aftermath of the EPME.