An apparently continuous sequence of upper Miocene through Pleistocene sediments and sedimentary rocks was recovered from Site 1150. The 1181-m-thick section consists mainly of diatomaceous silty clay and diatomaceous clay. Calcareous nannofossils are generally barren to abundant throughout, with variable preservation. Except for distinct ash and/or sand/silt layers, the sequence contains common to abundant diatoms throughout. Diatoms are moderately well preserved. Because the first occurrence (FO) of the diatom Denticulopsis dimorpha was not recognized, we interpret the base of the section to be younger than 9.9 Ma.
Calcareous nannofossils at this site were dated primarily from core-catcher samples. The sequence ranges from Quaternary to late Miocene in age. They are barren to abundant and poorly to well preserved in Holes 1150A and 1150B (Table T6).
The Pleistocene assemblage is marked by Calcidiscus leptoporus, Coccolithus pelagicus, Emiliania huxleyi, Pseudoemiliania lacunosa, the genus Gephyrocapsa, Reticulofenestra, and the rare genus Helicosphaera. The FO of E. huxleyi, which defines the boundary of the Zones CN15 and CN14b, was estimated to be in Core 186-1150A-6H by using an optical microscope, but this needs to be reexamined postcruise with a scanning electron microscope (SEM). Without the detail provided by an SEM, it is difficult to tell E. huxleyi from the small-sized Gephyrocapsa. Trace reworked Discoaster brouweri and Calcidiscus macintyrei were found in Sample 186-1150A-1H-1, 30 cm. Samples 186-1150A-6H-CC and 7H-CC were assigned to Subzone CN14b because neither E. huxleyi nor P. lacunosa was observed. Calcareous nannofossils are few to common, moderately preserved, and dominated by the genus Gephyrocapsa in these two samples. Samples 186-1150A-8H-CC, 9H-CC, and 10H-CC were assigned to Subzone CN14a because they contain both P. lacunosa and Gephyrocapsa parallela. Calcareous nannofossils are abundant and well preserved in these samples. Samples 186-1150A-11H-CC to 14X-CC were assigned to Subzone CN13b based on the rare occurrence of Gephyrocapsa caribbeanica in Sample 186-1150A-14X-CC. Calcareous nannofossils are rare and poorly preserved in these four core-catcher samples. Sample 186-1150A-15X-CC was assigned to Subzone CN13a because neither Discoaster nor Gephyrocapsa caribbeanica was present even though calcareous nannofossils are common and well preserved in this sample. The lower boundary of the Pleistocene was assigned between Sample 186-1150A-15X-CC and 16X-CC. The rare and scattered presence of Helicosphaera inversa makes it impossible to delimit its FO and last occurrence (LO). The LO of C. macintyrei occurs before the FO of the Gephyrocapsa oceanica and the FO of G. caribbeanica occurs in the same sample as the FO of G. oceanica (Sample 186-1150A-14X-CC), so only the FO of G. oceanica was used to estimate the age.
In the Pliocene and upper Miocene sediment, the assemblage is dominated by the genus Reticulofenestra, C. leptopora, C. pelagicus, and C. macintyrei. However, the genus Discoaster, which prefers warm water, consistently is only rare to few in abundance in the nonbarren samples, which makes determining the zonation difficult. The preservation is poorer downhole. Samples 186-1150A-16X-CC to 21X-CC were assigned to Subzone CN12d, the base of which is marked by the LO of Discoaster pentaradiatus. Because nannofossils are rare and are poorly preserved in the lower two samples, this boundary may need to be moved up. Samples 186-1150A-22X-CC to 26X-CC were assigned to Subzone CN12b-12c, the base of which is marked by the FO of Discoaster tamalis. Discoaster surculus was not observed. Samples 186-1150A-27X-CC to 37X-CC were assigned to Subzone CN12a because no Reticulofenestra pseudoumbilicus was observed. Nannofossils in these samples are usually poorly preserved and rare to common. A single well-preserved Ceratolithus rugosus in Sample 186-1150A-64X-CC indicates that this sample is in Subzone CN10c. The first datum in Hole 1150B is the LO of Discoaster quinqueramus, which defines the CN10/CN9 zonal boundary. The base of CN10 was put between Samples 186-1150B-11R-1, 10 cm, and 11R-3, 10 cm. A single well-preserved Amaurolithus amplificus in Sample 186-1150B-19R-CC puts the age of this sample between 5.993 and 6.840 Ma. From Samples 186-1150A-38X-CC to 76X-CC at the bottom of Hole 1150A, about one-third of the core-catcher samples (13 out of 38 samples) are barren or contain rare nannofossils. Nannofossils are rare or barren in about half (9 out of 19 samples) of the core-catcher samples from Sample 186-1150B-1R-CC to 19R-CC. Samples 186-1150B-20R-CC to 38R-CC were assigned to Zone CN9 because the FO of Discoaster berggrenii was identified, though 10 out of 19 core-catcher samples have rare abundances of nannofossils or are barren in this interval. Below this, no nannofossil datums were identified. The base of the section is probably in Zone CN8 based on diatoms. In Cores 186-1150B-48R, 49R, and 50R, Catinaster coalitus abundances are rare to common. The LO of D. surculus, LO and FO of Amaurolithus spp., Ceratolithus acutus, Triquetrorhabdulus rugosus, and FO of Discoaster loeblichii were not identified owing to the sparsity of these nannofossils.
Most of our observations come from core-catcher samples. For determination of zonal boundaries, additional smear slides were prepared from other core intervals. Diatoms are generally common to abundant and moderately well preserved throughout the Pleistocene through upper Miocene section recovered at Site 1150. In the upper Miocene section, however, abundance of diatoms slightly decreases because of the increase of clay content and probably because diagenetic alteration destroys diatoms as opal-A is converted to opal-CT. A range chart showing the distribution of the index species and other selected species is shown in Table T7 (also available in ASCII format). From the distribution of these species, 17 datum levels were identified (Table T8, also available in ASCII format).
The boundary between the upper Pleistocene Neodenticula seminae and Proboscia curvirostris Zones (North Pacific diatom [NPD] Zones 12/11) is indicated by the LO of Proboscia curvirostris (0.30 Ma) between Section 186-1150A-6H-CC and 7H-4. The continuous occurrence of Thalassiosira jouseae, the LO of which corresponds to 0.30-0.41 Ma, is observed near this depth. The warm-water species Nitzschia reinholdii, the LO of which corresponds to 0.62 Ma in the equatorial Pacific (Shackleton et al., 1995), is found in the lower part of this zone.
The last common occurrence (LCO) of Actinocyclus oculatus (1.01-1.46 Ma), which defines the boundary between the upper Pleistocene P. curvirostris Zone and the lower Pleistocene A. oculatus Zone (NPD 11/10), is clearly identified between Sections 186-1150A-11H-CC and 12H-1. Thalassiosira antiqua, which has its LO from 1.52 to 1.8 Ma, is scarce in the lower part of this zone (Section 186-1150A-13X-3). The FO of P. curvirostris lies between Sections 186-1150A-14X-3 and 14X-5. According to Koizumi and Tanimura (1985), this datum is within the geomagnetic Chron C1r.2r (1.07-1.77 Ma). Furthermore, the warm-water species Fragilariopsis (= Pseudoeunotia) doliolus, the FO of which corresponds to 2.0 Ma, is observed in Section 186-1150A-13X-CC. The rare occurrence of Neodenticula koizumii (LO at 2.0 Ma) in Core 186-1150A-13X is, therefore, thought to be reworked specimens.
The base of the A. oculatus Zone (NPD 10) and the top of the underlying N. koizumii Zone (NPD 9) is defined by the LO of N. koizumii. This uppermost Pliocene event occurs between Sections 186-1150A-14X-3 and 14X-5.
The LCO of Neodenticula kamtschatica (2.61-2.68 Ma) between Sections 186-1150A-21X-CC and 22X-3 marks the top of the N. koizumii-N. kamtschatica Zone (NPD 8). The FO of N. seminae is observed in Section 186-1150A-22X-3, which is concordant to the previous studies (e.g., Koizumi, 1992), suggesting that the FO of N. seminae falls immediately below the LCO of N. kamtschatica. Yanagisawa and Akiba (1998) reported, however, that this datum lies at the middle of NPD 9 and that this diachroneity was caused by a taxonomic problem because N. seminae and its ancestor N. koizumii are very similar in valve morphology. They also propose that these two species can be distinguished by the type of copula: N. seminae with a closed copula and N. koizumii with an open copula. In Section 186-1150A-22X-3, closed copula are rare but do exist; therefore, the discrepancy of the FO of N. seminae does not seem to be caused by the taxonomic problem mentioned above.
The FO of N. koizumii (3.53-3.95 Ma), which defines the boundary between the upper Pliocene N. koizumii-N. kamtschatica Zone and the lower Pliocene Thalassiosira oestrupii Zone (NPD 8/7B), is clearly identified between Sections 186-1150A-28X-CC and 29X-3. Within NPD 7B, the FO of Thalassiosira latimarginata (5.07 Ma) lies between Sections 186-1150A-45X-CC and 46X-CC.
The FO of T. oestrupii sensu lato defines the top of the upper Miocene N. kamtschatica Zone of Koizumi (1992). This datum exists between Sections 186-1150A-56X-CC and 57X-1. Though the abundance is very rare, Rouxia californica appears continuously in the lower part of this zone. It suggests that the boundary of NPD 7Ba and 7A (6.65 Ma), which divides the N. kamtschatica Zone, exists between Sections 186-1150B-20R-CC and 21R-3. The existence of the LO of Cavitatus jouseanus (6.7-6.8 Ma) between Sections 186-1150B-23R-CC and 24R-CC supports this zonal boundary. The FO of Pyxidicula (= Thalassiosira) zabelinae falls within NPD 7A of this zone, but its age has not been determined.
The upper Miocene R. californica Zone of Koizumi (1992) lies in the lower part of NPD 7A. The top of this zone is defined by the FO of N. kamtschatica (7.3-7.4 Ma). This datum is documented between Sections 186-1150B-28R-CC and 29R-3.
The LO of Thalassionema schraderi (7.6 Ma) between Sections 186-1150B-31R-CC and 32R-3 marks the top of the upper Miocene T. schraderi Zone (NPD 6B). Actinocyclus ingens appears continuously from this zone to the lowermost, but the age of its FO is uncertain.
Barron and Gladenkov (1995) defined the top of the upper Miocene Denticulopsis katayamae Zone (NPD 6A) by the LCO of Denticulopsis simonsenii (8.6 Ma), whereas Yanagisawa and Akiba (1998) defined it by the LO of Denticulopsis katayamae (8.5 Ma). Since the former zonation was adopted in this volume (see "Biostratigraphy" in the "Explanatory Notes" chapter), the zonal boundary is determined by D. simonsenii as between Sections 186-1150B-40R-CC and 41R-1. On the other hand, the LO of D. katayamae occurs between Sections 186-1150B-39R-CC and 40R-CC. The bottom of NPD 6A is defined by the LO of D. dimorpha (9.16 Ma). This datum is recognized between Sections 186-1150B-43R-1 and 43R-3.
The underlying D. dimorpha Zone (NPD 5D) is the oldest zone in Site 1150. Within NPD 5D, the FO of D. katayamae (9.16 Ma) and the FO of T. schraderi (9.26 Ma) are between Sections 186-1150A-45R-CC and 46R-CC and between 186-1150A-46R-CC and 47R-CC, respectively. The base of this zone is defined by the FO of D. dimorpha (9.9 Ma). This datum is not identified at this site, so the age of the lowermost sediment is slightly younger than 9.9 Ma. Extrapolation of the 43 m/m.y. sedimentation rate (see "Sedimentation Rates") between the LCO of D. simonsensii (8.60 Ma) and LO of D. dimorpha (9.16 Ma) would give a much older age of 11.6 Ma for the sediments at the bottom of the hole and is, therefore, unacceptable.
Diatom assemblages from all samples consist almost entirely of oceanic species. They are mainly of the subarctic North Pacific Ocean, although such warm-water taxa as N. reinholdii, Hemidiscus cuneiformis, and F. doliolus are typically present throughout the section. Neritic species such as Actinoptychus senarius, Paralia sulcata, and Cocconeis spp. vary in a small amount (from few to trace), but their presence is recognized in most samples. Freshwater species such as Aulacoseira spp. occur sporadically.
Resting spores of Chaetoceros spp. are mostly few to abundant between NPD 10 and 12 and rare to few in lower zones. At nearby Holes 438A and 584, however, the amount of these resting spores in the upper Miocene NPD 6B and 7A is one to three times as much as the total valves of usual diatoms (Akiba, 1986). Because resting spores frequently are present in nearshore sediments, Akiba (1986) suggests that this event is caused by the uplift of northeastern Honshu. The sedimentation rate also shows a significant difference between Site 1150 and the previously drilled sites. Two major episodes of low sedimentation rate, based mainly on diatom datums, were found at 9.16-8.6 and 2.0-0.41 Ma at Site 1150 (see "Sedimentation Rates"), whereas in Holes 438A and 584 there are hiatuses at these ages. According to the distribution of Neogene basins off northern Honshu based on seismic data (Ishiwada and Ogawa, 1976), the basin around Site 1150 is distinguished from the basin around Sites 438 and 584: the Kitakami basin situated on ~38°-40°N (this site) and the Ishikari-Hidaka basin extends northward (Sites 438 and 584). Therefore, it appears that the depositional environment and history of this site is different from that of the previous sites. The basin including Site 1150 was probably deeper even in the uplift period; thus, valves of in situ oceanic diatoms have been deposited continuously in the basin since at least the late Miocene through the Pleistocene.