Oxic conditions in shallow marine settings during the Permian-Triassic Mass Extinction

Anja B Frank , Elisabeth Warncke-Rüting, Stephen Grasby, Evelyn Kustatscher, Herwig Prinoth, Simon W Poulton, Yijun Xiong, Dieter Korn, Jana Gliwa, William Foster

The concept that ultra-shallow marine anoxia was a major cause of the Permian-Triassic mass extinction was partly based on sections from the Dolomites (Italy). We test this hypothesis by re-examining the Dolomites record, utilizing an updated redox sensitive trace metal (V, U and Mo) approach, combined with Fe speciation and Th/U ratios, and paleontological observations. Redox sensitive trace metal and Fe speciation data reveal fluctuating redox conditions prior to the extinction, with periodic enrichments in highly reactive Fe likely reflecting Fe2+ mobilization in anoxic ferruginous deeper waters. Oxic conditions are indicated during and after the extinction event, suggesting that local redox conditions did not control biodiversity changes. However, this redox reconstruction contrasts somewhat with traditional interpretation of Th/U ratios, which indicate more persistent shallow water anoxia, likely due to a strong regional influence of the detrital sediment composition. A comparison with other shallow marine successions reveals similar redox trends, suggesting that shallow water anoxia was not global. Instead, redox conditions across the Permian/Triassic boundary were likely variable, rendering it vital to assess the role of anoxia on a site-specific basis to understand its role in the shallow marine Permian-Triassic mass extinction.