Small and very small placoliths dominate the upper photic-zone flora in most intervals (Fig. 6). Following the gradual decrease in the abundance of medium to large specimens of Reticulofenestra and Sphenolithus, which is possibly due to a global cooling, Reticulofenestra minuta and small specimens of Gephyrocapsa dominated the flora. Their dominance was temporarily reduced in the early late and middle late Pliocene because of the increased abundance of medium placoliths such as Pseudoemiliania lacunosa, Reticulofenestra minutula, and Umbilicosphaera aequiscutum. From the uppermost Pliocene upward, the small and very small placoliths dominate the flora except in two short intervals in the upper Pleistocene, where the flora is mostly composed of medium Gephyrocapsa and Emiliania huxleyi (Fig. 6).
During the late Pleistocene, rapid evolution and abrupt alteration of the flora occurred. The taxa that dominate this interval include small and medium forms of Gephyrocapsa and Emiliania such as, Gephyrocapsa protohuxleyi, Gephyrocapsa muellerae, and Emiliania huxleyi. Some of these new taxa possibly were opportunists that could successively compete with the very small placoliths in a condition of moderately strong upwelling.
The abrupt dominance of Gephyrocapsa caribbeanica and Gephyrocapsa oceanica (medium) at ~0.55-0.25 Ma is well documented in the other oceans, including the central Pacific (Erba, 1995), Philippine Sea (Matsuoka and Okada, 1989), and the eastern Indian Ocean (Okada and Wells, 1997). Therefore, the nannofossil indicators of upwelling, an increased abundance of the very small placoliths, will not be detected in upper Pleistocene sequence unless the studied core came from particular hemipelagic settings, where a fluctuation in upwelling strength or the influence of coastal waters has occurred. In such areas the very small placoliths may be able to compete with the newcomers for the ecological superiority.
Emiliania huxleyi evolved only at 0.26 Ma (Berggren et al., 1995b) and has been the most dominant component of the flora in a large part of the world oceans for the last 85 to 73 k.y. (Thierstein et al., 1977). The fact that E. huxleyi produces big spring blooms in nutrient-rich, high-latitude waters is proof of its high potential as an opportunist. In Hole 994C, E. huxleyi dominated the upper photic-zone flora in Core 1H, where the abundance of F. profunda is also high (Table 2, PDF format only, oversized material, this volume; Fig. 6, Fig. 7). This observation illustrates E. huxleyi's ecological success even in normally mesotrophic to oligotrophic stratified waters. Perhaps temporal eutrophication is responsible for the co-abundance of F. profunda and E. huxleyi.