The Neogene through Quaternary diatom sequences of the North Pacific Ocean have been the subject of investigation since the pioneering work of Donahue (1970), Schrader (1973a), and Koizumi (1973a, 1973b). These authors established the fundamental basis for the Neogene biochronostratigraphic zonation. Successive revision and refinement of the North Pacific diatom stratigraphy have been developed rapidly by subsequent studies including those by Koizumi (1975a, 1975b, 1975c, 1975d, 1977, 1980), Burckle and Opdyke (1977), and Akiba (1977, 1979).
Barron (1980) presents a major advance toward the understanding of the timing of Neogene paleoclimatic and paleoceanographic evolution of the North Pacific Ocean based on diatom biostratigraphy. Later investigators (e.g., Barron [1981, 1985], Barron and Baldauf [1986], Akiba [1982b, 1983], Akiba et al. [1982a, 1982b], Akiba and Ichinoseki [1983], Maruyama [1984b], Koizumi [1985], Koizumi and Tanimura [1985], and Oreshkina [1985]) extended and modified the biostratigraphic framework of Barron (1980). These studies documented the late Neogene diatom biostratigraphy for a number of North Pacific deep-sea sites drilled that have provided data necessary to correlate the Neogene marine sequences exposed on land in Japan with those elsewhere in the North Pacific Ocean.
A significant advance in Neogene biochronostratigraphy was achieved by Akiba (1986), whose paper is conceivably the best summary of the Miocene to Pleistocene North Pacific diatom stratigraphy, and his proposed stratigraphic zonation has been widely accepted as a standard practicable scheme (Koizumi, 1992; Barron, 1992a; Barron and Baldauf, 1995, Yanagisawa, 1996; Watanabe and Takahashi, 1997). For the high-latitude North Pacific transect (ODP Leg 145), Barron and Gladenkov (1995) succeeded in directly correlating diatom stratigraphic events with magnetostratigraphy and in supplying precise ages for the Neogene zonal markers. Furthermore, Gladenkov and Barron (1995) presented an early Miocene through Oligocene diatom zonation based on a near-continuous stratigraphic record.
Recently, minor modifications to the Neogene North Pacific diatom zonation of Akiba (1986) were proposed in order to adjust the differences between the previously existing zonations (Yanagisawa and Akiba, 1998; Maruyama, 2000) and to integrate the diatom zonation with the radiolarian zonation (Motoyama and Maruyama, 1998). The ages of the primary diatom events are updated based on the revised geomagnetic polarity timescale of Cande and Kent (1995) by extrapolation of each horizon within each magnetic chron. Among the geomagnetic polarity timescales (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.
The diatom zonation (Fig. F2) used for the late Cenozoic closely follows the zonation of Yanagisawa and Akiba (1998) proposed for the northwest Pacific Ocean. There are small changes in zonal boundaries. For example, the top of the Denticulopsis katayamae Zone is marked by the last consistent occurrence (LCsO) of Denticulopsis simonsenii as suggested by Akiba (1986) and the base of the Neodenticula kamtschatica Zone is decided not by the last common occurrence (LCO) of Rouxia californica, but by the first consistent occurrence (FCsO) of N. kamtschatica. Moreover, the base of the Neodenticula koizumii Zone is recognized by the LCsO of N. kamtschatica instead of the strict last occurrence (LO) of N. kamtschatica. A consistent occurrence (CsO) is important for determining a taxon's stratigraphic consistency in comparison to a common occurrence (CO) by placing special emphasis on a change in relative abundance. Code numbers of Neogene North Pacific diatom (NPD) zones were also modified. Relationships between the zone name, code label, and definition are shown in Table T1.
Although the Neogene datum levels, particularly Miocene datum levels, were not directly calibrated with the magnetostratigraphy, they occur in the proper stratigraphic sequence and apparently at the proper intervals, thus suggesting that they may be isochronous with other parts of the North Pacific Ocean. In addition to the ODP Leg 186 zonation, exceptional diachronism across latitude has been well known for a number of diatom events such as the first occurrence (FO) of N. kamtschatica, the FO of N. koizumii, the LO of N. kamtschatica, the LO of N. koizumii, and the LO of Actinocyclus oculatus.
Taxonomic studies, especially on a group commonly accepted as "marine Denticula" (Simonsen and Kanaya, 1961) or the genus Denticulopsis (Simonsen, 1979), resulted in a remarkable advance in the Neogene North Pacific diatom biostratigraphy. The three dominant genera, Denticulopsis, Crucidenticula, and Neodenticula (Akiba and Yanagisawa, 1986), which consist of various short-ranging species, provide many stratigraphically useful biohorizons (Schrader, 1973a, 1973b; Maruyama, 1984a, 1992; Akiba, 1977, 1979, 1982a, 1986; Tanimura, 1989; Yanagisawa and Akiba, 1990).
The taxonomy used in this study follows that of Koizumi (1980, 1992), Akiba (1986), Yanagisawa and Akiba (1990, 1998), Fenner (1991), Harwood and Maruyama (1992), and Akiba et al. (1993). However, several groups are defined because it is not clear how to correctly separate taxa of Crucidenticula and Denticulopsis and because it is uncertain whether a number of secondary diatom biohorizons proposed by Yanagisawa and Akiba (1998) are applicable in the North Pacific Ocean.
Moreover, on the basis of the available taxonomic studies by Shiono (2000, 2001), and Shiono and Koizumi (2000, 2001), an identification and an age assignment of the Thalassiosira trifulta group were carefully conducted for documenting the taxon ranges accurately from the northwest Pacific deep-sea holes.
On board the JOIDES Resolution, supplementary smear slides were prepared from appropriate core intervals for determining placement of zonal boundaries and for calibrating sedimentation rates (Sacks, Suyehiro, Acton, et al., 2000). Postcruise observations are based on examination of core catcher samples that were newly processed in the shore-based laboratory. Sample material was placed in an oven at 60°C for 24 hr, ~1-2 g of dried material was boiled in a 200-mL beaker with ~30-50 mL of hydrogen peroxide solution (H2O2, 5%) for a few minutes, and 10 mL of hydrochloric acid (HCl, 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 (22 mm × 22 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. Strewn slides were examined in their entirety at a magnification of 600x, and identifications were checked routinely at 1000x for stratigraphic markers and paleoenvironmentally sensitive taxa. Semiquantative estimates were made of the relative abundance of stratigraphically important taxa for each sample. These abundances were recorded as follows:
Preservation of diatoms was determined qualitatively as follows:
Diatom analysis on board the JOIDES Resolution presumed the Miocene/Pliocene boundary to be expected between Samples 186-1150B-5R-CC and 15R-CC and 186-1151A-50R-CC and 60R-CC, respectively. Particular attention was focused on morphology and stratigraphic distribution of the T. trifulta group. In an effort to resolve the diatom biostratigraphy for the epoch boundary interval, 11 core catcher samples were picked for analysis of this group from each site, respectively. Not only light microscope (LM) observations but also scanning electron microscope (SEM) observations were made on selected samples in order to settle the Miocene/Pliocene epoch boundary in both Holes 1150B and 1151A.
Samples were boiled in a solution of hydrogen peroxide (H2O2, 15%) and hydrochloric acid (HCl, 15%), which washed the organic matter from the diatom skeleton, and then the residue was suspended in a solution of sodium diphosphate decahydrate (Na4P2O7·10H2O, 0.02%-0.03%) for reducing the clay minerals. The LM pictures were taken using a Pixera camera system, and the SEM pictures were produced using an S-2250N Hitachi SEM and Quartz PCI image management system. Diatom valves of all species including the genus Chaetoceros were counted in each sample under the LM until the number of valves totaled 1000 or 1000.5. Whenever we encountered valve specimens, each end of a pennate diatom was taken as a half valve (0.5). With regard to centric diatoms, the central area was taken as one valve (1).