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Cyclic changes in the morphometry of large specimens of the calcareous nannofossil genus Gephyrocapsa, first discovered in the Indian Ocean, are recognized in the middle Quaternary sedimentary sequence of Site 790 drilled in the Izu-Bonin forearc. The second and a part of the third cycle of morphometric development of large Gephyrocapsa were identified at this site. Small Gephyrocapsa (in particular, Gephyrocapsa protohuxleyi var. A and Gephyrocapsa ornata var. A) generally compose approximately 50% of the middle Quaternary nannoflora. Four abundance peaks of the small Gephyrocapsa complex were seen in this middle Quaternary sequence, three of which were not seen in previously studied Indian Ocean or Mariana Arc cores. INTRODUCTION Matsuoka and Okada (1989) studied morphometric changes of the calcareous nannofossil genus Gephyrocapsa in subtropical Pacific Ocean Core KH84-1 st. 21 (Fig. 1). They recognized significant variations in the morphometry of Gephyrocapsa during the last 1.3 m.y. and a prominent acme of Gephyrocapsa protohuxleyi var. A near the Brunhes/Matuyama boundary. Matsuoka and Okada (1990) also reported morphometric changes of the genus Gephyrocapsa in the Quaternary sequence cored at Site 709 in the equatorial Indian Ocean. They identified three development cycles of the large morphotype of Gephyrocapsa (1.6-1.1,0.9-0.5, and 0.5 Ma to the present). The large morphotype of Gephyrocapsa was approximately 3 μm in diameter during the early stage of each cycle and increased in size through the latter part of each cycle. The morphotypes disappeared rapidly at the end of the earlier two cycles. The prominent acme of G. protohuxleyi var. A was also observed at Site 709. The coccoliths examined in the Quaternary sedimentary sequence recovered at Site 790 in the Izu-Bonin Arc region display good preservation characteristics. We studied this Quaternary sequence to determine the temporal and spatial differences of morphometric changes of Gephyrocapsa between the tropical Indian Ocean and the Izu-Bonin Arc. MATERIALS AND METHODS Site 790 is located at the center of Sumisu Rift (30°54.95'N, 139°50.69Ti, water depth of 2223.5 m below sea level [mbsl]) in the Izu-Bonin Arc (Fig 1). The upper Quaternary portion of the section consists of volcaniclastic sediments and thus was not suitable for this morphometric study. The middle Quaternary sedimentary sequence, however, consists of nannofossil-rich clays. Two datum events were identified in this middle Quaternary sequence (Fig. 2): (1) the last occurrence (LO) of Pseudoemiliania lacunosa, located between Sections 126-790C-15X-CC and -16X-1, 4 0 ^ 1 cm; and (2) the LO of Reticulofenestra sp. A, located between Cores 126-790C-18X and -19X (Shipboard Scientific Party, 1990). These events also are present at Site 709 in the Indian Ocean and at Site KH84-1 st. 21 in the Mariana Arc, providing correlatable stratigraphic horizons for spatial comparisons with samples from this study. We selected a set of 20 samples from Cores 126-790C-16X through -19X and observed them Taylor, B., Fujioka, K., et al., 1992. Proc. ODP, Sci. Results, 126: College Station, TX (Ocean Drilling Program). 2 Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164, Japan. 3 Present address: Japan Marine Science and Technology Center, 2-14 Natsushima, Yokosuka, Kanagowa 238, Japan. under a transmission electron microscope to investigate the morphometric variations of Gephyrocapsa (Table 1). The first 200 specimens of Gephyrocapsa identified in each sample were measured for overall size, diameter of the central opening, and angle of the central bridge relative to the long axis of the coccolith. The length and width axes were measured to within 0.1 μm. The bridge angle is difficult to measure precisely under a transmission microscope; thus, it was classified into one of three broad categories: 0°-30°, 30°-60°, and 60°-90°. For samples in which the small forms were dominant, measurements were made on 100 additional larger specimens to obtain a sufficiently large Gephyrocapsa data set. We also determined the composition of the nannoflora, excluding Florisphaera profunda, by identifying the first 350 coccoliths encountered. RESULTS AND DISCUSSION Classification of Gephyrocapsa The classification of Gephyrocapsa in this study closely follows the one proposed by Matsuoka and Okada (1990). They identified two groups of Gephyrocapsa: small Gephyrocapsa, which consist of small specimens with relatively large central openings; and large Gephyrocapsa, which consist of large specimens with relatively small central openings. In this study, plots of the overall coccolith size vs. the diameter of the central opening were used to identify each group (Fig. 3). Matsuoka and Okada (1990) further divided the large Gephyrocapsa into four subspecies {Gephyrocapsa sp. A, Gephyrocapsa sp. B, Gephyrocapsa sp. C, and Gephyrocapsa sp. D). The two latter species were identified in samples examined in this study. Large Gephyrocapsa specimens identified in the interval from 228.1 to 255.5 mbsf (Samples 126-790C-16X-4, 80-81 cm, through -19X-3, 80-81 cm) are characterized by high-angle (>60°) bridges and were classified as Gephyrocapsa sp. C. Gephyrocapsa sp. D, characterized by a low-angle bridge, were identified in the interval from 223.6 to 229.6 mbsf (Samples 126-790C-16X-1, 80-81 cm, through -16X-5, 80-81 cm). Only two taxa, Gephyrocapsa protohuxleyi var. A with slits on the distal shield and Gephyrocapsa spp. (small) without slits on the distal shield, were recognized in the small Gephyrocapsa complex by Matsuoka and Okada (1990). In addition to these two taxa, Gephyrocapsa ornata var. A was abundant in samples examined in this study. G. ornata var. A is larger than the type species (an average length of 2.9 μm); it has moderate protrusions, a high archlike bridge, and tooth-shaped protrusions around the central opening. This variety is restricted temporally to the middle Quaternary. Gephyrocapsa ornata var. A was further subdivided into two morphotypes: one with slits on its distal shield and the other without slits. Heimdal's (1973) classification noted the presence of specimens similar to the type of G. ornata var. A that do not have a bridge.

Morphometric Changes of the Genus Gephyrocapsa at Site 790, Subtropical Pacific Ocean