Permian-Triassic redox shift and its ferruginous aftermath in epicontinental seas

Abstract. Marine anoxia has been implicated as a key environmental driver of the end-Permian mass extinction (EPME) and the subsequent prolonged recovery. However, the spatial and temporal extent of oxygen limitation during the EPME interval remains contentious. Here, we present iron speciation, pyrite framboid and molybdenum-uranium (Mo-U) covariation data from two palaeogeographically distinct settings: the Tethyan Chibi section (South China) and the Panthalassian Ursula Creek section (Western Canada) to evaluate redox dynamics across the Permian-Triassic transition. Our data suggest that bottom waters were predominantly dysoxic during the late Changhsingian at both sites. Later, the prevalence of small pyrite framboids, elevated Mo and U enrichment factors (Mo_EF and U_EF), and high Mo_EF/U_EF 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 Mo_EF-U_EF covariation data reveal a shift to persistently 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, thereby contributing to the slow recovery.