During Leg 186, diatoms and calcareous nannofossils were studied to assess biostratigraphic constraints on the sedimentary sections at Sites 1150 and 1151. Age assignments were primarily made on core-catcher samples. However, additional samples from within the core were studied when a core-catcher sample was found to be inconclusive or otherwise unrepresentative of the core in its entirety.
Diatom datums of the North Pacific have been calibrated to Cande and Kent's (1995) geomagnetic polarity time scale ([GPTS] CK95) by Motoyama and Maruyama (1998) and Yanagisawa and Akiba (1998). All age estimates of nannofossil datums are based on the time scale of Berggren et al. (1995), which incorporates the GPTS of CK95. The diatom and nannofossil zones are correlated to each other and to the GPTS of CK95, as shown in Figure F6.
The diatom zonation (Fig. F6) used for the Neogene follows the zonation of Motoyama and Maruyama (1998). This zonation is based on Koizumi and Tanimura (1985) and Koizumi (1985) for the datums above the Neodenticula kamtschatica Zone (7.3-7.4 to 6.65 Ma) and on Barron and Gladenkov (1995) for the lower datums down to the Thalassiosira fraga Zone (20.1-18.4 Ma). The earliest Miocene Thalassiosira praefraga Zone (24.0-24.3 to 20.1 Ma) is defined based on Yanagisawa and Akiba (1998). Code numbers of North Pacific diatom zones by Yanagisawa and Akiba (1998) were adapted to the zonation.
Table T3 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 represented by Motoyama and Maruyama (1998), except for the first occurrence of T. praefraga (24.0-24.3 Ma) calibrated to CK95 by Yanagisawa and Akiba (1998). These ages are originally based on Barron (1980, 1992), Koizumi and Tanimura (1985), Akiba (1986), Gersonde and Burckle (1990), Baldauf and Barron (1991), Koizumi (1992), and Barron and Gladenkov (1995).
Smear slides were routinely examined for stratigraphic markers. When required (because of low concentration of specimens), selected samples were processed by boiling them in hydrogen peroxide and hydrochloric acid, followed by centrifuging at 1200 rpm for 2-4 min to remove the chemical solutions from the suspension.
These slides were examined in their entirety at a magnification of 500× for stratigraphic markers and paleoenvironmentally sensitive taxa. Identifications were checked routinely at 1250×. These abundances were recorded as follows:
Preservation of diatoms was qualitatively determined as follows:
The zonal schemes of Martini (1971; with modifications by Martini and Muller, 1986) and Bukry (1973, 1975; zonal code numbers added and modified by Okada and Bukry, 1980) were used for Cenozoic calcareous nannofossils (Fig. F6; Table T4).
Standard smear slides were made of all soft lithologies. Calcareous nannofossils were examined using standard light microscope techniques, under cross polarizers, transmitted light, and phase contrast at 1000× magnification.
Preservation and abundance of calcareous nannofossil species may vary significantly because of etching, dissolution, or calcite overgrowth. A simple code system to characterize preservation has been adopted and is listed below:
Six calcareous nannofossil abundance levels are recorded as follows: