From the uppermost Miocene (Messinian) to the Holocene six planktonic foraminifer zones have been recognized in tropical and subtropical latitudes. Using overlapping segments from deep-sea cores previously dated by use of planktonic foraminifers, the calcareous nannofossils for this same interval were analyzed in order to relate nannofossil ranges to established foraminiferal zones.

With the light microscope nine calcareous nannofossil zones are readily distinguishable for this interval, one for the uppermost Miocene (Messinian), four for the Pliocene, and four for the post-Pliocene. The planktonic foraminifer zones N-18 through N-20 have roughly corresponding nannoplankton zones, although zone N-20 appears to represent a shorter stratigraphic interval than the corresponding nannoplankton zone. Zone N-21 is divisible into two nannofossil zones of about equal duration. The top of zone N-21, which is marked by the first evolutionary occurrence of Globorotalia truncatulinoides, corresponds rather closely to the extinction of discoasters in deep-sea sediments. Above this horizon for additional nannofossil zones can be recognized, based on partial ranges or concurrent ranges of three placolith genera that successively dominate the nannofossil assemblages.

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