Controversies on the placement of Cretaceous-Paleogene boundary and the K/P mass extinction of planktonic foraminifera

Examination of recently reported K/P boundary sections indicates that the placement of the K/P boundary is based on equivocal criteria and that the boundary as placed is not synchronous. The conclusion that the K/P boundary in several U.S. Gulf Coast sections is complete and within a condensed section is simply the artifact of delineating the K/P boundary on disparate paleontologic datum planes and preservational bias of the microfossil assemblages. The upper slope and outer shelf sections at El Kef, Gredero, and Agost as well as the DSDP Site 577 section are continuous. The non-separation of Zone P0 at some deep sea sites is due to very low sedimentation rates and the short time interval of Zone P0. All Late Cretaceous planktonic foraminiferal species in the boundary sections included in this study, except Guembelitria cretacea Cushman, Hedbergella monmouthensis (Olsson) and H. holmdelensis Olsson, became extinct at the end of the Late Cretaceous. Species population ratios and species ranges in six K/P boundary sequences of different stratigraphic settings indicate that all other Cretaceous species occurring in the lower Paleocene are reworked specimens. Consequently, the loss of planktonic foraminifera at the end of the Cretaceous is extremely heavy. The stepwise extinction of Keller (1988,1989) and the "foreshadowed" Late Cretaceous extinction and prolonged Paleocene extinctions proposed by Brinkhuis and Zachariasse (1988) are due to equivocal placement of the K/P boundary and treatment of reworked Cretaceous species occurring in the lower Paleocene as survivor species. The terminal Cretaceous mass extinction is most likely a geologically instantaneous single event which eliminated many groups of organisms. As for planktonic foraminifera, 50 out of 53 species became extinct at the end of Cretaceous. Only 3 species survived the mass extinction event and were the stem forms for the subsequent radiation of all Paleocene planktonic foraminifera.