For the present study, 34 samples were selected to cover the uppermost lower Pliocene to middle Miocene of Hole 1151A. The shallowest is Sample 186-1151A-16R-1, 45-48 cm (downhole depth = 209.35-209.38 meters below seafloor [mbsf]), whereas the deepest is Sample 186-1151A-109R-3, 41-46 cm (1107.31-1107.36 mbsf). For a detailed description of the studied samples, see "Appendix A." Downhole depths of a certain sample, where given, are indicated by the depth of the top of the sampled interval.
According to the diatom biostratigraphy by Maruyama and Shiono (this volume), the shallowest sample was located at ~5 m below the lowest occurrence (LO) of Neodenticula koizumii (3.53-3.95 Ma), whereas the deepest sample was located between the LO of Denticulopsis lauta (15.9 Ma) and the LO of Denticulopsis praelauta (16.3 Ma). Thus, our samples range from the uppermost lower Pliocene down to the lowermost middle Miocene. The studied section represents a time interval of between 3.5 and 16 Ma and constitutes a stratigraphic record of about 12.5 m.y. in duration within a mid-latitude forearc setting in the northwestern Pacific. It should be noted, however, that the section includes a significant hiatus between Samples 186-1151A-101R-1, 71-73 cm, and 186-1151A-101R-1, 90-91 cm (1027.31-1027.50 mbsf), in which an interval of upper Miocene (12-14 Ma) sediments is missing. This interval corresponds to the lower Denticulopsis praedimorpha Zone (NPD5B) to the upper Denticulopsis hyalina Zone (NPD4B) based on the diatom biostratigraphy by Yanagisawa and Akiba (1998). As a result, our material contains eight samples belonging to the lower Pliocene, thirteen to the upper Miocene, two to the upper middle Miocene, and eleven to the lower middle Miocene.
The samples were treated with HCl and HF at room conditions to eliminate carbonate and siliceous minerals. The organic residues were then concentrated using zinc bromide heavy liquid (specific gravity = 2.0), screened on a 20-µm sieve, and mounted on slide with polyvinyl alcohol and polyester resin. No oxidation was carried out in the sample processing.
A Carl Zeiss Axioplan microscope was used for microscopic analysis. Each microscope slide was scanned at 160x along the shorter side of the coverslip. Cysts were counted until ~200 specimens had been enumerated or, if less, until five traverses had been counted. Identification was conducted at 600x using differential interference contrast light. Each raw count of a given taxon in a sample was converted to a percentage against the total count in the sample as shown in Table T1. A "+" in Table T1 indicates the presence of reworked cysts and other palynomorphs. In Figure F2, the percentages are converted as follows:
In addition, the abundance of dinoflagellate cysts relative to the total mounted residue was evaluated by means of the average number of cyst specimens per traverse. This evaluation assumes that the density of residue strewn on the slide is constant for all the samples, which is only approximately true in this case. Thus, the relative abundance indicated here should be regarded as no more than a broad estimate. In Figures F2 and F3, the relative abundance is expressed as follows:
The cyst nomenclature used in this study generally follows Williams et al. (1998).
Terms are as follows:
All the material discussed in this paper is housed in the palynological collection at the Department of Geology, Faculty of Science, Niigata University.