STRATIGRAPHIC DISTRIBUTION OF MAJOR SPECIES BETWEEN 3 AND 0.8 MA

Since 3 Ma at Site 1018, diatom valves decrease during glacial periods, and the fluctuation of diatom abundance decreases during the interval between 3 and 0.8 Ma (Fig. 7). The cold-water species Neodenticula spp. (N. kamtschatica, N. koizumii, and N. seminae) are predominant. Four diatom datum levels are recognized at Site 1018 as follows: last occurrence (LO) of N. kamtschatica (2.6-2.7 Ma) is recognized between Samples 167-1018A-38X-4, 143 cm, and 38X-6, 140 cm (between 355.32 and 358.29 mbsf); LO of N. koizumii (2.0 Ma) between Samples 167-1018A-25X-6, 136 cm, and 26X-2, 142 cm (between 233.05 and 236.71 mbsf); first occurrence (FO) of Rhizosolenia matuyamai (1.18 Ma) between Samples 167-1018A-18X-7, 141 cm, and 19X-2, 141 cm (between 166.12 and 169.40 mbsf); and LO of R. matuyamai (1.02 Ma) between Samples 167-1018A-17X-6, 141 cm, and 18X-3, 141 cm (between 156.2 and 160.12 mbsf). The age assignments of Baldauf and Iwai (1995) and Yanagisawa and Akiba (1998) were recalibrated to the geomagnetic time scale of Cande and Kent (1995).

The floral succession of the last 3 m.y. is summarized in four stages.

Stage 1: Oceanic-environmental stage (387.5-260 mcd): Thalassionema nitzschioides, N. koizumii, and Stephanopyxis spp. are predominant, and Azpeitia nodulifera, Thalassiosira oestrupii, and Coscinodiscus marginatus are dominant (Table 4).

Stage 2: High-productivity stage (260-206 mcd): T. nitzschioides and N. seminae are predominant.

Stage 3: Low-productivity stage (206-138 mcd): diatoms are scarce, and Stephanopyxis turris and N. seminae are present.

Stage 4: High-productivity stage (138-0 mcd): T. nitzschioides and N. seminae are predominant.