Stratigraphic variations in the morphometry of the genus Gephyrocapsa (calcareous nannoplankton) were quantitatively investigated in the Quaternary sequence of Hole 709C recovered from the tropical Indian Ocean. Gephyrocapsa can be roughly divided into groups of small and large Gephyrocapsa, mainly on the basis of their significant differences in coccolith size. Four lineages were recognized among the large Gephyrocapsa, and members of these lineages were tentatively named Gephyrocapsa sp. A, B, C, and D. These species are approximately 3 /xm large at their incipient stages, and the latter three species become progressively larger thereafter. In the case of Gephyrocapsa sp. B and C, well-developed large forms disappeared abruptly, only to be succeeded by a new cycle of evolutionary size increase, respectively performed by Gephyrocapsa sp. C and D.The first cycle observed in Gephyrocapsa sp. B occurred at the 16.02-12.02 mbsf interval (around 1.6-1.1 Ma), and Gephyrocapsa sp. A occurred simultaneously within this cycle. The second and third cycles, performed respectively by Gephyrocapsa sp. C and D, occurred at 9.62-6.02 mbsf (around 0.9-0.5 Ma) and above 5.62 mbsf (0.5 Ma to the present). The percentage abundance of large Gephyrocapsa species that showed an evolutional size increase conversely decreased with an increase in overall size. An interval lacking the large species that roughly corresponds to the "small Gephyrocapsa zone" was found between the first and second cycles, but changes in abundance of the small and large forms seem to be independent.The appearance and disappearance of the large forms as well as their changes in overall size and abundance play major roles in the floral change throughout the Quaternary. The six nannoflora assemblages dividing the last 1.3 m.y. reported from the subtropical Pacific Ocean were also recognized in the tropical Indian Ocean. A new additional assemblage was identified in the basal part of the Pleistocene. INTRODUCTIONMatsuoka and Okada (1989) studied time-progressive changes in the floral compositions and morphometries of major calcareous nannofossil taxa during the last 1.3 m.y. in the subtropical northwestern Pacific Ocean. They semiquantitatively measured the morphology of several placolith groups and demonstrated the existence of remarkable time-progressive changes in morphology as well as in the overall size of the Gephyrocapsa species. Because these measurements were conducted in a semiquantitative mode, details of the morphologic changes that have strong potential for paleoceanography as well as for improved Quaternary biostratigraphy are yet to be clarified.A complete sequence of Quaternary sediment was retrieved from the tropical Indian Ocean at Hole 709C, and a set of highresolution samples taken at 10-cm intervals was available to us for a quantitative study of Quaternary nannofossils. The purposes of this study were to investigate quantitatively the timeprogressive changes in the morphometry of the genus Gephyrocapsa throughout the entire Quaterna...

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Living coccolithophore assemblages were studied off the Japanese coast in the boundary zone between the cold Oyashio and the warm Kuroshio Currents as well as in adjacent water masses. A total of 64 coccolithophore taxa were identified. Ten taxa that comprised more than 10% of at least one sample were designated major taxa. Morphological observations and morphometric measurements enabled us to sub-divide the Emiliania huxleyi into four morphotypes: Type A, Type B, Type B/C, and Type B/C-2. Based on the composition of major taxa and morphotypes of Emiliania huxleyi, four coccolithophore assemblages were recognized, corresponding to distinct water masses: Oyashio Current, Tsugaru Warm Current, Kuroshio Current, and Western North Pacific Central Water (WNPCW). In the Oyashio Current regime, the standing crop of coccolithophores was high, although diversity was low. The floral assemblage was dominated by E. huxleyi Type B and consistently contained Coccolithus pelagicus ssp. pelagicus HOL, Calciopappus caudatus, and Syracosphaera orbiculus. In the Tsugaru Warm Current regime, the flora was characterized by abundant E. huxleyi Type B/C-2 and Gephyrocapsa oceanica as well as by relatively common Braarudosphaera bigelowii. In the Kuroshio Current regime, E. huxleyi Type A, Gephyrocapsa ericsonii, or G. oceanica were abundant. The coccolithophore flora in the WNPCW was characterized by abundant E. huxleyi Type A, Discosphaera tubifera, and Umbellosphaera tenuis Type IV. Large Emiliania huxleyi specimens (≥4.2 μm in length) only occurred north of the Oyashio Front, while to the south, E. huxleyi were consistently ≤4.1 μm long, regardless of the morphotype. Large E. huxleyi can be a useful paleoenvironmental indicator for reconstructing past migrations of the Kuroshio and Oyashio Currents in the North Pacific Ocean

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