Pleistocene through Miocene calcareous nannofossils from ODP Leg 138 holes

The calcareous nannofossil biostratigraphy of the sediments retrieved during Leg 138 in the eastern equatorial Pacific Ocean is presented and discussed. The sedimentary sequences stidoed, at 10 of the 11 sites drilled, span the stratigraphic interval from Pleistocene to upper (Sites 847 and 848) and middle Miocene (Sites 851, 852, and 853), and three extend to the upper part of the lower Miocene (Sites 844, 845, and 846). Most of the zonal boundaries of the 1973 zonation of Bukry and standard 1971 zonation of Martini are recognized and used for the biostratigraphic classification of these low-latitude sediments. Additional biostratigraphic events are discussed and in some intervals are used as secondary criteria for improving the biostratigraphic resolution. A further subdivision of upper Miocene Subzone CN9b of Okada and Bukry (1980) is proposed using the lowest and highest occurrences of Amaurolithus amplificus.
Comments on the biochronology of calcareous nannofossils are given, with special reference to Miocene events, taking advantage of the very good magnetostratigraphy and orbitally tuned time scale produced for the Leg 138 sites.