Global-ocean redox variations across the Smithian-Spathian boundary linked to concurrent climatic and biotic changes

The Smithian-Spathian boundary (SSB) was an interval characterized by a major global carbon cycle perturbation, climatic cooling from a middle/late Smithian boundary hyperthermal condition, and a major setback in the recovery of marine necto-pelagic faunas from the end-Permian mass extinction. Although the SSB has been linked to changes in oceanic redox conditions, key aspects of this redox variation (e.g., duration, extent, and triggering mechanisms) and its relationship to coeval climatic and biotic changes remain unresolved. Here, we report a high-resolution middle Smithian to middle Spathian U isotope (δ²³⁸U) record based on marine carbonates of the Zuodeng (South China) and Jesmond (British Columbia) sections to investigate the timing and global extent of ocean-redox variation across the SSB. Our δ²³⁸U record reveals values similar to or slightly heavier than modern seawater (-0.39‰) during the middle Smithian, a rapid negative shift to highly negative values during the early late Smithian, a positive shift at the SSB, and a rapid shift back to more negative values in the early-middle Spathian. A simple U-isotope mass balance modeling suggests that the global area of anoxic seafloor expanded strongly during the late Smithian and the early-middle Spathian (covering ~11% of total seafloor area), but that it contracted sharply during the SSB (~2%). The redox pattern documented by our δ²³⁸U record shows a good first-order correspondence to tropical sea-surface temperature (SST) data for the Smithian-Spathian. In particular, peak anoxia coincided with the middle/late Smithian boundary hyperthermal event, and diminished anoxia with a pronounced decline in SSTs at the SSB. The temporal correlation between anoxia and low biodiversity levels of many marine clades (e.g., conodonts and ammonoids) during the late Smithian indicates that oceanic anoxia may have played a role in the SSB biocrisis.