The western Pacific Ocean is a data-free region for the mass extinction event and the oxygen conditions at the Cenomanian/Turonian boundary. Taxonomic and stratigraphic studies of Cenomanian to Campanian intermediate-water (100-1000 m) benthic foraminifera at Oyubari, Hokkaido lead to the recognition of extinctions of the largest magnitude in the latest Cenomanian interval (94-91 Ma). The stepwise extinction event is marked by a 48% reduction of calcareous taxa, followed by a gradual orgination event during a late Turonian to Santonian interval. This sequence of events marks the greatest faunal turnover observed in the Late Cretaceous of Hokkaido. These benthic foraminiferal morphologies indicate a minimum dissolved oxygen content in the early Turonian, low during middle-late Turonian, medium during Coniacian to middle Santonian, and a high dissolved oxygen content during the late Santonian to Campanian. The establishment of prevailing dysoxic conditions around the Cenomanian/Turonian boundary is the main cause of the benthic foraminiferal mass extinction.
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