A nutrient-driven tipping point catalysed marine anoxia during the end-Permian mass extinction

The global spread of marine anoxia has been proposed as a major driver of the end-Permian mass extinction—the most devastating biotic crisis of the Phanerozoic. However, the mechanism that drove large stretches of the ocean to an anoxic state is unclear. Here, we utilize a combined geochemical and palaeontological approach to show that, prior to the extinction, enhanced chemical weathering driven by Siberian Traps volcanism increased the oceanic influx of phosphorus. This promoted increased primary productivity and thus the development of oxygen depleted waters in proximal settings. However, bioavailable phosphorus was also efficiently sequestered in the sediment in association with iron minerals. This limited phosphorus recycling back to the water column and hence restricted the intensity and spatial extent of oxygen-depleted waters. The collapse of vegetation on land immediately prior to the extinction horizon changed the weathering regime, thereby crossing a critical threshold and significantly enhancing redox-controlled remobilization of phosphorus. This resulted in a positive productivity feedback that stimulated the spread of sulfidic waters across large portions of the shelf, which was a potent kill mechanism for benthic taxa. This model reconciles an apparent contradiction between the timing of enhanced chemical weathering and the development of widespread ocean anoxia, while the spatial redox structure would have affected benthic species that were unable to migrate to shallow and deeper shelf environments.