Late Cretaceous (Maestrichtian) Calcareous Nannoplankton Biogeography withEmphasis on Events Immediately Preceding the Cretaceous/Paleocene Boundary

摘要

Well preserved late Cretaceous (Maestrichtian) nannoplankton assemblages fromhigh southern and mid-latitudes were investigated to delineate paleoenvironmental conditions in the marine photic zone prior to the Cretaceous/Paleocene boundary. A pronounced turnover from early to late Maestrichtian occurred in high southern latitudes with about one third of the early Maestrichtian nannoplankton species becoming extinct around the early/late Maestrichtian boundary (-72.4-70.4 Ma). These extinctions are not a consequence of temperature decrease alone but also of increased surface water fertility. Another third of all species disappeared from high southern latitudes during the same time interval but persisted until the end of the Maestrichtian at lower latitudes. This is by the isotopic record. Abundance increases of certain high latitude taxa (e.g. Nephrolithus frequens, Cribrosphaerella daniae, Kamptnerius magnificus, and Gartnerago spp.) in the Southern Ocean correlate roughly with decreasing temperature while abundance changes of others (e.g. Ahmuellerella octoradiata) do not, indicating that latitudinal biogeographic patterns do not necessarily reflect temperature differences. During the last 500 ky of the Maestrichtian high abundance fluctuations occurred in a few taxa in the South Atlantic and Southern Oceans. However, no such fluctuations seem to have occurred during the same time in the Indian Ocean. There is no evidence for gradual or stepwise extinctions of nannoplankton during the late Maestrichtian. Calcareous nannoplankton, Maestrichtian, Biogeography.