The following eight biohorizons are recognized of both Sites 1150 and 1151 (Table T3; Fig. F4): (1) the LO of N. asanoi; (2) the LO of G. puncticulata; (3) the FO of G. inflata (modern form); (4) the FO of N. asanoi; (5) the LO of G. ikebei; (6) the FO of G. ikebei; (7) the LO of G. nepenthes; and (8) the LO of G. dehiscens. The stratigraphic order of the eight biohorizons is conformable in both present sites.
With respect to the diatom biostratigraphic data (Maruyama and Shiono, this volume) (Fig. F4; Table T3), the eight planktonic foraminiferal biohorizons have no contradiction with diatom zonations at both sites: the LO of N. asanoi, the LO of G. puncticulata, and the FO of G. inflata (modern form) are located in the diatom Zone NPD 9. The FO of N. asanoi, the LO and FO of G. ikebei, and the LO of G. nepenthes are placed in the diatom Zone NPD 7B. The LO of G. dehiscens is located within the diatom Zone NPD 5D. We consider that these eight biohorizons are possibly synchronous in the Japan Trench region.
Keller (1980) reported planktonic foraminiferal data from DSDP Sites 438 and 440, ~60 km northward of Site 1150. We correlated foraminiferal data from both Sites 1150 and 1151 with those of Hole 438A (40°38´N, 143°14´E, water depth = 1558 m) (Fig. F1), which have been stratigraphically well determined by the diatom biostratigraphy (Maruyama, 1984; Yanagisawa and Akiba, 1998). Stratigraphic distributions of planktonic foraminiferal species at Hole 438A are strongly discontinuous because of dissolution of foraminiferal tests and discontinuous coring. Despite these discontinuities, four of the eight biohorizons, namely, the FO of N. asanoi, the FO of G. ikebei, the LO of G. nepenthes, and the LO of G. dehiscens, show no contradiction in their stratigraphic relationships between the present sites and DSDP Hole 438A with respect to the diatom zonation.
According to stratigraphic relationship between the foraminiferal and diatom data (Fig. F3; Table T3), it can be noticed that the LO of G. dehiscens in the Japan Trench region is significantly older than tropical to subtropical age compared to data of Berggren et al (1995a). At the present sites, the LO of G. dehiscens is placed within diatom Zone NPD 5D (9.2 to 10.0 Ma). At DSDP Hole 438A, G. dehiscens disappeared just below the boundary top of the Denticulopsis dimorpha Zone (NPD 5D), ~9.2 Ma (Maruyama, 1984; Yanagisawa and Akiba, 1998). This event is assigned to 5.80 Ma in tropical to subtropical regions (Berggren et al., 1995a). We consider that this biohorizon is a diachronous event related to a latitudinal climatic change. Oda et al. (1984) reported that the LO of G. dehiscens became progressively older from the equator to the mid-latitude region along the western Pacific margin. The LO of G. dehiscens is placed just below the Am-40 tuff dated at 8.5 ± 0.5 Ma by fission-track dating in the Boso Peninsula located at ~35°N (Oda, 1977; Tokuhashi et al., 2000). The similar trend of the LO of G. dehiscens is also observed on the eastern Pacific margin (Keller and Barron, 1981). The progressive equatorward restriction of the biogeographic province of G. dehiscens occurred during the late Miocene, and the LO of G. dehiscens ranges from 9.5 Ma at 40°S to 5.0 Ma at the equator (Spencer-Cervato et al., 1994).