BIOSTRATIGRAPHY

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.

Diatoms

Zonation

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.

Datum Levels

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).

Methods

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:

D (dominant) = more than five specimens per field of view,

A (abundant) = two or more specimens per field of view,

C (common) = one specimen per two fields of view,

F (few) = one specimen per each vertical traverse,

R (rare) = one specimen per a few traverses,

T (trace) = one specimen per several or more vertical traverses, and

B (barren).

Preservation of diatoms was qualitatively determined as follows:

VG (very good) = finely silicified forms present, with no alteration of frustules, and some colonies of frustules preserved;

G (good) = finely silicified forms present and no alteration of frustules observed;

M (moderate) = finely silicified forms present with some alteration; and

P (poor) = finely silicified forms absent or rare and fragmented, and the assemblage is dominated by robust forms.

Calcareous Nannofossils

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).

Methods

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:

VG = very good preservation (no evidence of dissolution and/or overgrowth; no alteration of primary morphological characteristics, and specimens appear diaphanous; specimens are identifiable to the species level);

G = good preservation (little or no evidence of dissolution and/or overgrowth; primary morphological characteristics only slightly altered; specimens are identifiable to the species level);

M = moderate preservation (specimens exhibit some etching and/or overgrowth; primary morphological characteristics somewhat altered; however, most specimens are identifiable to the species level); and

P = poor preservation (specimens are severely etched or exhibit overgrowth; primary morphological characteristics largely destroyed; fragmentation has occurred; specimens cannot be identified at the species and/or generic level.

Six calcareous nannofossil abundance levels are recorded as follows:

V = very abundant (10-100 specimens per field of view);

A = abundant (1-10 specimens per field of view);

C = common (1 specimen per 2-10 fields of view);

F = few (1 specimen per 11-100 fields of view);

R = rare (1 specimen per 101-1000 fields of view); and

B = barren.