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 (MoEF and UEF), and high MoEF / UEF ratios near the EPME horizon implicate seafloor anoxia as a key trigger for marine extinctions in the Ursula Creek section. In the post-extinction Early Triassic, iron speciation and MoEF–UEF covariation data reveal a shift to persistently ferruginous conditions in both locations. A global compilation of iron speciation data indicates redox variation between ferruginous and euxinic conditions 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.