Ocean floor drilling during Leg 186 furnishes an opportunity to examine the diatom assemblages from almost-continuous stratigraphic sequences from the northwestern Pacific margin. Miocene through Pleistocene reference sections show that the standard diatom zonation of Yanagisawa and Akiba (1998) is of great use for North Pacific diatom biostratigraphy. When compared with one another and with published data, most of the middle through late Miocene diatom datum levels that have been widely accepted in the North Pacific for biostratigraphy appear to be isochronous within the level of resolution constrained by sample spacing. Reliable Miocene diatom datum levels proved in Leg 186 include the FO of D. lauta (15.9 Ma), the FO of D. hyalina (14.9 Ma), the FO of D. dimorpha (9.9 Ma), the LO of D. dimorpha (9.16 Ma), the LCsO of D. simonsenii (8.6 Ma), the LCO of T. schraderi (7.6 Ma), and the FCsO of N. kamtschatica (6.4 Ma). Such datum levels as the FO of D. simonsenii (14.4-14.6 Ma), the FCO of D. simonsenii (13.1 Ma), the FO of D. praedimorpha (12.9 Ma), and the LCO of D. praedimorpha (11.5 Ma) are compressed into a narrow stratigraphic interval because of a low sedimentation rate or hiatuses. The somewhat rare and inconsistent occurrence of R. californica in Leg 186 materials precluded the complete approval for its LCO as the base of the N. kamtschatica Zone (NPD7B).
Within the early Pliocene, there is nothing reliable among datum levels off northeast Japan covered by Leg 186. The rapid progress of taxonomic study on the T. trifulta group prohibits the FO of T. oestrupii (5.49 Ma) from dividing the N. kamtschatica Zone (NPD7B) into two subzones, and the authorization of the upper unit, named the T. oestrupii Subzone (NPD7Bb), lapses from the North Pacific zonal framework entirely. An incomplete biostratigraphy around the FO of T. bipora sensu amplificato precluded the replacement both of the T. oestrupii Subzone and the FO of T. oestrupii as the base datum in Leg 186 samples.
At least five of the late Pliocene through Pleistocene diatom datum levels allow precise correlation along the northwestern Pacific margin between Sites 1150 and 1151. The stable adoptions of datum levels are established on the FO of N. koizumii (3.53-3.95 Ma), the LCsO of N. kamtschatica (2.61-2.68 Ma), the LO of N. koizumii (2.0 Ma), the LO of A. oculatus (1.01-1.46 Ma), and the LO of P. curvirostris (0.3 Ma) during Leg 186. They are widely used as a zonal markers in the North Pacific Ocean; however, diachroneity across latitude is also documented for the FO of N. koizumii, the LCsO of N. kamtschatica, and the LO of A. oculatus.