Abstract

Nearly 57% of marine invertebrate families died out when the biggest mass extinction of the Phanerozoic took place at the Permo-Triassic (P/T) boundary. The lithologic change across the boundary recorded in a pelagic chert sequence in Japan suggests a remarkable oceanic anoxic event (OAE) in a deep-sea environment and its relevant effect on the extinction. The boundary unit in the deep-sea pelagic sequence occurs in the Jurassic accretionary complex in southwest Japan and consists of ca. 30 m of thick light gray to olive green siliceous claystone closely associated with jet black carbonaceous claystone (10-15 m thick). Stratigraphically under- and overlying units consist of red bedded radiolarian cherts. Paleozoic radiolarians were completely replaced by Mesozoic ones across the boundary. The boundary claystone is characterized by dark colors and ubiquitous occurrence of authigenic pyrite, indicating reductional depositional conditions, i.e., deep-sea anoxia, while hematite bearing red cherts above and below indicate a long lasting (>50 m.y.) oxic condition. As deep-sea sediments may normally record global oceanographic conditions on average, the present results suggest that an OAE occurred on a global scale across the P/T boundary and that this boundary deep-sea anoxia persisted for nearly 10 m.y. (from the Longtanian, early Late Permian to Anisian, early Middle Triassic), i.e., an extraordinarily long term in the Phanerozoic. A new term, “superanoxia”, is coined in order to discriminate this peculiar long term deep-sea OAE across the P/T boundary from other ordinary OAEs in the Phanerozoic. The P/T boundary mass extinction is probably related to this superanoxia and to the assembly of Pangea.