A detailed micropaleontologic analysis of sediments from Deep‐Sea Drilling Project site 577 from the Shatsky Rise, North Pacific, was undertaken to describe extinction and radiation patterns of plank‐tonic foraminifera in a continuous carbonate sequence spanning the Cretaceous/Tertiary (K/T) boundary. A 15‐m section containing the boundary was closely sampled and contained planktonic foraminiferal assemblages characteristic of the late Maastrichtian Abathomphalus mayaroensis zone through the Danian Globorotalia uncinata zone. The Cretaceous/Tertiary boundary was placed at the abrupt disappearance of almost all Maastrichtian planktonic foraminifera. Coincident with these extinctions was the presence of a large number of “microtektitelike” spherules. The most important faunal change in the late Maastrichtian was an abrupt increase in the relative abundance of the high‐latitude species Hedbergella monmouthensis, together with the appearance of diminutive populations of Guembelitria cretacea and Globigerina eugubina approximately 20,000 years before the boundary. These changes are interpreted as indicating a cooling of surface water conditions. Globigerina eugubina and G. cretacea were the only planktonic foraminiferal species to survive the Cretaceous/Tertiary boundary event at this locality. The δO‐18 and δC‐13 values of G. eugubina from below the K/T boundary are nearly the same as those from above the boundary, but in both cases the δO‐18 values indicate relatively cooler paleotemperatures than those from other coexisting planktonic foraminiferal species. In addition, the δC‐13 values of G. eugubina are lighter than those of the other planktonic foraminifera. Such a relationship might suggest that G. eugubina lived in cooler, shallow intermediate water masses advected from higher latitudes during a latest Maastrichtian cooling episode. Better preservation of planktonic foraminifera, a progressive change in the δC‐13 of benthic foraminifera, and a brief positive excursion in δC‐13 of the carbonate fine fraction and of planktonic foraminifera within 20 to 30 cm below the K/T boundary all suggest important paleoceanographic changes that preceded the extinction event. A large population of aberrant G. eugubina and Eoglobigerina was observed in the earliest Danian. These forms are characterized by the development of secondary apertures, bullae, and abnormal final chambers. These abnormal morphotypes are considered to be ecophenotypic variants reflecting ecologic stress or instability in the earliest Cenozoic marine environment.
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