Biostratigraphically useful nannofossil markers were investigated by means of quantitative methods in Oligocene through middle Miocene sediments from the western tropical Indian Ocean. In the Oligocene a distinct shift in dominance between Sphenolithus predistentus and Sphenolithus distentus represents a better marker event than the first occurrence (FO) of the latter for recognition of the CP17/CP18 boundary. Similarly, the shift in dominance between S. distentus and Sphenolithus ciperoensis at 28.8 Ma provides a more reliable indication for the CP19a/CP19b (NP24/ NP25) boundary than the last occurrence (LO) of the former species, which may occur as late as 27.4 Ma. Because it is rare in the critical interval and occurs well within the early Miocene, the LO of Helicosphaera recta (NP25/NN1 boundary) provides unreliable biostratigraphic information In the Miocene, an acme interval of Sphenolithus delphix that ranges in age from 23.6 to 24.7 Ma, is an important addition to the poorly resolved biostratigraphy of the early Miocene. The midpoint of this short acme interval of S. delphix would form a suitable criterion for replacing the formal CNla/CNlb boundary definition (end acme Cyclicargolithus abisectus), which has never been defined quantitatively. No overlap in the ranges of Sphenolithus belemnos and Sphenolithus heteromorphus was observed. The LO of Coronocyclus nitescens provides a more reliable biostratigraphic signal than the FO of Discoaster kugleri for subdivision of the CN5 (NN6-NN7) interval. Biometric analysis of Calcidiscus placolith size revealed a clear bimodal distribution into forms larger or smaller than 10 ^m, in sediments of late Pliocene-early Pleistocene age. The middle Miocene populations, however, are clearly unimodal. Morphotypes > 11 ^m appear rather distinctly in Subzone CN5b between about 11 and 12 Ma.
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