The standard zonal schemes of Martini (1971) and Okada and Bukry (1980) are used for Leg 181 Eocene and Oligocene sediments (Fig. F2). Middle Eocene (NP16/CP14a) through late Oligocene (NP25/CP19b) zones are represented with Zones NP17/CP14b, NP18/CP15a, NP23/CP17-18, and NP24/CP19a missing because of multiple hiatuses. Zones NP19 and NP20 (Martini, 1971) were combined because Sphenolithus pseudoradians was rarely present in only a few samples from Hole 1124C. The high-latitude zonal scheme of Wei and Wise (1990) was not used since Reticulofenestra oamaruensis was not identified because of poor preservation.
In the Martini (1971) and Okada and Bukry (1980) nannofossil zonal schemes, the Eocene/Oligocene boundary is approximated by the last occurrence (LO) of the large-bodied discoasters Discoaster barbadiensis (34.3 Ma) and Discoaster saipanensis (34.2 Ma). This classical nannofossil event is older than the global stratotype section and point (GSSP) recognized Eocene/Oligocene boundary (Coccioni et al., 1988; Premoli Silva and Jenkins, 1993) based on the extinction of the foraminiferal genus Hantkenina (33.7 Ma).