Abstract

A continuous pelagic record of the late Neogene was studied in four deep-sea cores for which the magnetic stratigraphy had been worked out previously. Within the past 8 m.y., eleven distinctive calcareous nannofossil datum levels are recognizable, and these are related to the absolute chronology of the magnetic time scale. Two datum levels are recognizable within the late Pleistocene, at about 170,000 and 350,000 yrs B.P. The earlier Pleistocene did not yield comparable marker species. The Pliocene-Pleistocene boundary, which is correlated on the geomagnetic scale with the base of the Olduvai event within the Matuyama reversed epoch, is closely approximated by the extinction of the last true discoaster species at about 1.8 m.y. B.P. Within the Pliocene, six datum levels are recognized, and correlation with Mediterranean Pliocene deposits indicates an age of about 5 m.y. for the Miocene-Pliocene boundary. The uppermost Miocene seems to be devoid of distinctive markers, but this may be due to inadequate data.

Of the many nannoplankton which became extinct in the late Neogene, only two species, Cydococcolithina macintyrei and Discoaster brouweri, have their last occurrence near a magnetic reversal, and of these the extinction of Discoaster brouweri probably cannot be related to a magnetic reversal. It appears, therefore, that calcareous nannoplankters were not adversely affected by collapse or reversals in the Earth's magnetic field.

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