The Manumiella seelandica global spike: Cooling during regression at the close of the Maastrichtian

Manumiella seelandica (Lange 1969) Bujak and Davies 1983, emend. Firth 1987 occurs in great abundance, together with a closely related and possibly conspecific form, Isabelidinium? sp., virtually at the end of the Maastrichtian. The abundance of M. seelandica immediately adjacent the Cretaceous/Paleogene (KPg) boundary, and its global distribution, serve as an excellent biostratigraphic marker for dating it. The abundance spike is widely distributed in both Northern and Southern hemispheres, and ranges from locations as far south as Seymour Island, Antarctica to as far north asWest Greenland. The global distribution of the M. seelandica spike in such widely separated areas, and the ease with which it is recognized in closely sampled sections, underscores its value in dating. Until recently, this short-lived event was correlated within an interval of global sea level fall and regression which extended from the latest Maastrichtian into the earliest Danian; that is, that the species became numerically abundant as the shoreline approached. However, based on our study of the boundary interval at Bass River in southern New Jersey USA, we propose that an additional factor may have contributed to its origin. Our investigation of this section shows that there is a prominent abundance spike in the 20 cm (7.8 in.)-interval immediately beneath the boundary, and that this interval correlates precisely with the delta18O isotope evidence of cooling. At Bass River, the last 500 ky of the Maastrichtian was a period of global warmth, as revealed by geochemical evidence. However, based on the geochemical evidence, a mild cooling period began within tens of thousands of years of the close of the Cretaceous Period. Based on this correlation, we present the hypothesis that the cooling event played an important role in producing this global assemblage of dinoflagellates. Study of a large number of fossils in the sample situated 15 cm (5.85 in.) beneath the boundary shows that there is considerable variation in overall form and archeopyle excystment in Manumiella seelandica. The same degree of variation was not observed in the samples with fewer specimens.