Evaluation of palaeo-oxygenation of the ocean bottom across the Permian–Triassic boundary

The ‘long-lasted anoxia’ of a deep ocean bottom which may be related to the Permian–Triassic (PT) mass extinction event has been accepted without critical evidence. The degree of bioturbation is examined for radiolarian chert successions in central Japan with the reassessment of geochemical proxies to evaluate the degree of palaeo-oxygenation of a pelagic oceanic bottom. No evidence for anoxia is found from Wuchiapingian and lower Changhsingian chert by the reassessment of geochemical proxies. Anoxia started from the late Changhsingian stage that was interrupted by an oxygenation event at the supposed PT boundary. Griesbachian to the earliest Smithian, being characterised by the occurrence of black carbonaceous claystone, is an anoxic interval with rare bioturbation in the early part and is oxygenated in the later part. Succeeding grey siliceous claystone of early Smithian represents largely an anoxic interval, but it is oxygenated in the later part of this stage. Grey siliceous claystone with black carbonaceous claystone interbeds of Spathian was suboxic to oxic, and temporal anoxia being caused mainly by high productivity intermittently was established within a short time interval. Red chert is rather common in Anisian, but unusual sedimentation of organic matter still continued until the latest Anisian.