Since the pioneering works of Donahue (1970), Schrader (1973a), and Koizumi (1973a, 1973b) established the elementary framework of the Neogene diatom biostratigraphic zonation, successive revision and refinement of the North Pacific diatom stratigraphy has been developed rapidly by subsequent studies including Koizumi (1975a, 1975b, 1975c, 1975d, 1977, 1980, 1985, 1992), Burckle and Opdyke (1977), Barron (1980a, 1981, 1985b, 1992a), Akiba (1977, 1979, 1982b, 1983, 1986), Akiba, Hoshi, et al. (1982), Akiba, Yanagisawa, et al. (1982), Akiba and Ichinoseki (1983), Maruyama (1984b), Oreshkina (1985), Koizumi and Tanimura (1985), Yanagisawa (1996), and Watanabe and Takahashi (1997).
Neogene diatom zonations of Maruyama (1984b), Koizumi (1985), Akiba (1986), Barron and Gladenkov (1995), and Yanagisawa and Akiba (1998) essentially resemble each other. Akiba's (1986) paper is conceivably the best summary of the state of Miocene to Pleistocene North Pacific diatom stratigraphy, and his proposed stratigraphic zonation has been widely accepted as a standard workable scheme (Barron and Baldauf, 1995). In the high-latitude North Pacific transect (ODP Leg 145), Barron and Gladenkov (1995) succeeded in directly correlating diatom zones with magnetostratigraphy and in supplying precise ages for Neogene primary zonal marker biohorizons. Furthermore, Gladenkov and Barron (1995) confirmed an early Miocene through Oligocene diatom zonation by the nearly continuous documentation of diatom records. Recently, Yanagisawa and Akiba (1998) yielded some minor modifications to the Neogene North Pacific diatom zonation of Akiba (1986) to adjust the differences between the previously existing zonations.
The diatom zonation (Fig. 2) used for the Quaternary and Neogene closely follows the zonation of Yanagisawa and Akiba (1998) proposed for the northwest Pacific. Code numbers of Neogene North Pacific diatom (NPD) zones were also adapted to the above-mentioned zonation with contemporary rearrangement. There is a smaller change in zonal boundaries. The top of Denticulopsis katayamae Zone is marked in practical sense, by the last common occurrence (LCO) of Denticulopsis simonsenii as suggested by Akiba (1986). Relationships between the zone name, code label, and definition are shown in Table 2. Table 3 lists age estimates for the Neogene diatom datum levels that have been found to be useful in the middle-to-high latitudes of the North Pacific. Ages are presented according to both the geomagnetic polarity time scale of Berggren et al. (1985a, 1985c) and of Cande and Kent (1992, 1995).
On board the JOIDES Resolution, age vs. depth plots were constructed using paleomagnetic events and other microfossil datum levels, so that the accuracy of the diatom datum levels could be basically tested (Lyle, Koizumi, Richter, et al., 1997). Although the Neogene datum levels, especially Miocene datum levels, were not directly controlled by the magnetostratigraphy, they occur in the proper sequence and apparently at the proper intervals, thus implying that they may be isochronous with other parts of the North Pacific.
In this study, a correlation between magnetostratigraphy and diatom biohorizons follows mainly Koizumi and Tanimura (1985), Koizumi (1992), and Barron and Gladenkov (1995). The ages of the primary diatom biohorizons in the text are updated based on the revised geomagnetic polarity time scale of Cande and Kent (1995) by extrapolation of each horizon within each magnetic chron. Among the geomagnetic polarity time scales (Cande and Kent, 1992, 1995; Baksi, 1993; Wei, 1995; Berggren et al., 1995a, 1995b), the severe variance in age calibration points is relatively well known. Users have to be prepared for a maximum age difference of 1.3 m.y. in the latest early Miocene because the middle Miocene is deficient in reliable age calibration points (Motoyama and Maruyama, 1998; Yanagisawa and Akiba, 1998).
In addition, exceptional diachronism across latitude was documented for a number of diatom biohorizons such as the first occurrence (FO) of Actinocyclus ingens f. nodus, the FO of Proboscia barboi, the FO of Neodenticula kamtschatica, the FO of Actinocyclus oculatus, the FO of Neodenticula koizumii, and the last occurrence (LO) of N. koizumii (Burckle and Opdyke, 1985; Koizumi and Tanimura, 1985; Koizumi, 1992; Barron and Gladenkov, 1995).
Taxonomic studies, principally on a group commonly accepted as "marine Denticula" (Simonsen and Kanaya, 1961) or the genus Denticulopsis (Simonsen, 1979), made a remarkable advance in the Neogene North Pacific diatom biostratigraphy. Short-ranging species belonging to the three genera denominated Denticulopsis, Crucidenticula, and Neodenticula by Akiba and Yanagisawa (1986) provide many stratigraphically useful biohorizons (Schrader, 1973a, 1973b; Maruyama, 1984a, 1992; Akiba, 1977, 1979, 1982a, 1986; Akiba and Yanagisawa, 1986; Tanimura, 1989; Yanagisawa and Akiba, 1990).
The taxonomy used follows that of Koizumi (1980, 1992), Akiba (1986), Yanagisawa and Akiba (1990, 1998), Fenner (1991), Harwood and Maruyama (1992), and Akiba et al. (1993). Because it is not clear how to separate correctly preferable taxa of Crucidenticula and Denticulopsis, and because it is uncertain whether a number of secondary diatom biohorizons proposed by Yanagisawa and Akiba (1998) apply in the California margin, I have made the following groupings:
On board the JOIDES Resolution, strewn slides were prepared by placing a small amount of material on a slide glass, adding a few drops of distilled water, kneading the material, and extending it thinly with a toothpick. When, because of a low concentration of diatom skeletons or an induration of siliceous grains, age assignment control was required, selected samples were processed by boiling them in hydrogen peroxide and hydrochloric acid, followed by centrifuging at 1200 rpm for 2 min to remove the chemical solutions from the suspension.
Because a large amount of clay-rich sediments interfered with an identification of diatoms, almost all core-catcher samples had to be processed once again on land in the laboratory. Sample material was placed in an oven at 60°C for 24 hr, and about 2-3 g of dried material was boiled in a 200-mL beaker with about 30-50 mL of hydrogen peroxide solution (5%) for a few minutes and 10 mL of hydrochloric acid (5%) was added in small portions. Acid-treated material was made pH-neutral by repeatedly filling and decanting the beakers with distilled water and allowing 1 hr for settling. Strewn slides were prepared by spreading the pipette suspension onto a cover glass (24 × 32 mm), drying on a hot plate, and mounting in Photocuring Adhesive DB-855.
Diatoms are present throughout the sites, but with varying abundance and preservation. Whenever possible, all of the diatom taxa were tabulated without Chaetoceros spores. Strewn slides were examined in their entirety at a magnification of 600× for stratigraphic markers and paleoenvironmentally sensitive taxa. Identifications were checked routinely at 1000×. These abundances were recorded as follows:
Preservation of diatoms was determined qualitatively as follows: