The contrast between the high extinction rates of calcareous marine plankton and low extinction rates of deep-water calcareous benthic foraminifera across the Cretaceous/Tertiary (K/T) boundary is now recognized. However, the water depth at which the vertical change in extinction rates of marine organisms occurred at the K/T boundary is not yet known because of the lack of data on the extinction rates of intermediate-water organisms. The extinction rate of intermediate-water benthic foraminiferal species from Kawaruppu, Hokkaido, Japan, was determined by distinguishing locally extinct species. A distinctly low extinction rate (10%) for calcareous benthic foraminiferal fauna from the upper part of the intermediate-water is documented for the first time. This rate is almost equal to the extinction rate of deep-water benthic foraminifera (∼10%) and is much smaller than the planktic foraminiferal extinction rate (81%). The extinction rate curve for calcareous foraminifera in a vertical water column indicates that a rapid change from a high extinction rate (∼80%) to a low extinction rate (∼10%) took place at a water depth of about 150 m. This depth of change is equivalent to the boundary depth between the surface and intermediate waters and between the euphotic and aphotic zones. Furthermore, this coincidence is consistent with the hypothesis that acid rain and a darkness-induced breakdown of the marine food-chain, produced by a cometary or asteroidal impact at the K/T boundary, were the main causes of the mass extinction of calcareous planktic organisms in surface waters.
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