Ocean-floor coring during Leg 167 provided a unique opportunity to examine the diatom assemblages from numerous near-continuous stratigraphic sequences from the California margin. Valuable middle 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 those middle through late Miocene diatom datum levels that have been widely used in the North Pacific for biostratigraphy appear to be isochronous within the level of resolution constrained by sample spacing. Reliable Miocene diatom datum levels in the North Pacific include the FCO of D. simonsenii (13.1 Ma), the FO of D. praedimorpha (12.9 Ma), the LCO of D. praedimorpha (11.5 Ma), the FO of D. dimorpha (9.9 Ma), the FO of Thalassionema schraderi (9.5 Ma), the LO of D. dimorpha (9.16 Ma), the LCO of D. simonsenii (8.6 Ma), both the FCO and LCO of T schraderi (7.6 Ma), the LO of Cavitatus jouseanus (6.7-6.8 Ma), the LCO of Rouxia californica (6.65 Ma), the FCO of Neodenticula kamtschatica (6.4 Ma), and the FO of T. oestrupii (5.49 Ma).
Within the Pliocene, there is nothing more reliable among datum levels along the California margin covered by Leg 167. The somewhat inconstant occurrence of Neodenticula kamtschatica in Leg 167 samples precluded the authorization of the N. kamtschatica Zone (NPD 7B). Pliocene diatom assemblages in the California Current basin are intermediate between those of subarctic and subtropical areas. Consequently, neither the present tropical nor the subarctic (high latitude) zonal schemes were applicable for this region.
At least 31 of the middle Miocene through Pleistocene diatom datum levels allow precise correlation along the length of the present-day California Current from Sites 1010 (30°N) to 1022 (40°N). The FO of Proboscia barboi (12.3 Ma) as a widely used Miocene diatom datum, however, is diachronous across latitude in the North Pacific. Within the Pliocene through Pleistocene, diachroneity is also documented for the FO of Neodenticula koizumii (3.53-3.95 Ma) and the LO of Actinocyclus oculatus (1.01-1.46 Ma).
A brief warming trend probably occurred between about 13.1 and 12.0 Ma in the area, as evidenced by the occurrence of relatively common warm-water taxa during this interval at Site 1010 (30°N) and Site 1022 (40°N). In addition, decreased diatom productivity along the California margin during most of the early Pliocene (~5.5-2.6 Ma) most likely reflects slackening of the California Current during a period of relative climatic warming. At Sites 1016, 1018, 1020, and 1022 off central-to-northern California, abundant and well-preserved diatoms returned at about 2.6 Ma near the base of Zone NPD 9, coincident with major cooling of high-latitude surface waters.