BIOSTRATIGRAPHY

During Leg 204, diatoms and calcareous nannofossils were studied to assign preliminary ages to core catcher samples. Samples from within the cores were examined when a more refined age determination was necessary. The timescale of Berggren et al. (1995a, 1995b) was used. The biostratigraphic zones of calcareous nannofossils and diatoms are summarized in Figure F9.

During Leg 204, we used the Neogene diatom zonations and code number of the North Pacific Diatom Zone (NPD) by Yanagisawa and Akiba (1998) (Fig. F9).

Diatom bioevents that define the zones used in Leg 204 are summarized in Figure F9. Ages for the events are based on Barron (1992), Barron and Gladenkov (1995), Koizumi (1992), Koizumi and Tanimura (1985), and Yanagisawa and Akiba (1998).

Smear slides were prepared for each sample using Pleulax as a mounting medium. The slides were examined on a light microscope at magnifications of 400x, 630x, and 1000x. The abundance of diatoms was estimated based on the number of specimens observed per field of view, with a magnification of 630x and the following definitions:

A = abundant; five or more valves per field of view.

C = common; one to five valves per field of view.

F = few; more than five valves per each vertical traverse but less than one specimen per field of view.

R = rare; more than one valve per two vertical traverses but less than five valves per each vertical traverse.

T = trace; for sparser occurrences.

B = barren; no valves observed on the slide.

The quantitative estimation of preservation of diatoms was recorded as follows:

G = good; both thinly silicified forms and heavy robust forms are present, and no alteration of the frustules was observed.

M = moderate; thinly silicified forms are present but exhibit some alteration.

P = poor; thinly silicified forms are rare or absent, and robust forms dominate the assemblage.

During Leg 204, calcareous nannofossils were studied to obtain biostratigraphic constraints for the sediments recovered on Hydrate Ridge. Ages for calcareous nannofossil events in the Pliocene and Pleistocene follow the geomagnetic polarity timescale (GPTS) of Cande and Kent (1995). We employed the nannofossil zonations of Martini and Müller (1986) for late Cenozoic sediments. Numerical ages used are those compiled by Berggren et al. (1995a, 1995b) (Fig. F9) to facilitate easy comparison with other studies.

Calcareous nannofossil assemblages were examined using smear slides prepared from each core catcher sample and from additional core samples. Standard smear slides were made for all samples using Norland Optical Adhesive as a mounting medium. Examination was performed with a light microscope. Unless otherwise noted, we followed taxonomic concepts summarized in Perch-Nielsen (1985).