END-CENOMANIAN BENTHIC FORAMINIFERAL EXTINCTIONS AND OCEANIC DYSOXIC EVENTS IN THE NORTHWESTERN PACIFIC-OCEAN

The second-largest Cretaceous extinction event coincided with an oceanic anoxic event at the Cenomanian/Turonian (CIT) boundary. This high resolution analysis of upper bathyal benthic foraminifera in central Hokkaido, Japan indicates that the extinction event occurred in the latest Cenomanian over a period of similar to 0.2 m.y. (93.6-93.4 Ma). Analysis of the dissolved oxygen index based on calcareous benthic foraminiferal wall thickness, size, and morphology shows that low oxygen, 0.1-1.5 ml/l conditions developed during a period of similar to 0.15 m.y. before the C/T boundary and similar to 1.5 m.y. after. The minimum oxygen condition (0.1-1 ml/l) prevailed twice during this interval. Similarly, benthic foraminiferal extinctions occurred twice in concert with initiation of the two minimum oxygen levels. Coincidence of the two extinction horizons with development of low oxygen conditions suggests that the main cause of the extinctions was formation of dysoxic intermediate-water. This report suggests that the global C/T boundary extinction event occurred in a stepwise fashion during the latest Cenomanian 0.2-0.5 m.y. due to development of widespread oceanic anoxic-dysoxic conditions within intermediate water masses.