A combination of the nannofossil zonal schemes of Martini (1971) and Okada and Bukry (1980) is used for Leg 173 Eocene sediments (Fig. F3). All early and middle Eocene zonal markers and boundaries of Martini (1971) or Okada and Bukry (1980) except for Coccolithus crassus and Rhabdosphaera gladius can be recognized, although some of the zonal markers are rare. All early and middle Eocene zones from NP10/CP9a through NP16/CP14a are present.
The Paleocene/Eocene boundary is defined in this study by the first occurrence (FO) of Discoaster diastypus, which marks the CP8/CP9 boundary in Okada and Bukry's (1980) zonal scheme. The FO of Tribrachiatus bramlettei, used by Martini to define this boundary (NP9/NP10), is difficult to distinguish in the lower Eocene at these sites because of poor preservation and was not used. The FO of Discoaster sublodoensis determines the early/middle Eocene boundary (NP13/NP14 or CP11/CP12a). Although preservation of this delicate nannofossil is poor, its distinctive form is easily identified.