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A Marine Focus
Causes: Marine Anoxia

Anoxia as a cause of the end-Permian extinction was first proposed by Arthur Hallam and Paul Wignall (Wignall and Twitchett, 2002). In the anoxic ocean scenario, waters of an anoxic deep ocean were carried onto continental shelves by a major transgression, producing the extinction.

The facies of marine strata during an anoxic event are quite unique. During the anoxic event, facies varied depending on bathymetry and latitude. Dark gray, uranium-enriched shales characterize deeper shelf locations over wide areas of northern Boreal seas, whereas the oceanic record consists of condensed, organic-rich, black shales. Finely laminated, pyrite-rich, micritic mudstones occur in equatorial Tethyan sections. Contemporaneous dolomitization in many shallow-marine settings provides further indirect evidence for widespread P-Tr anoxia. Similarly, common reports of unusual stromatolites in the earliest Triassic Griesbachian Stage could, in fact, reflect the widespread occurrence of direct calcite precipitation from carbonate-saturated anoxic bottom waters. (Twitchett, 2002)

framboidal pyrite
Figure: Framboidal pyrite grains similar to those
found at PT boundary (Pichler, 2004)