The tabulated occurrences of stratigraphically important taxa are shown in Tables T2 and T3, including the sample depth in meters below seafloor. Table T4 provides the sample interval and meters below seafloor depth, which are constrained to stratigraphic events identified for Sites 1150 and 1151.

Site 1150 is located at a water depth of 2681 m in the deep-sea terrace on the landward side of the Japan Trench. An apparently continuous sequence of upper Miocene through Pleistocene sediments and sedimentary rocks was recovered from Site 1150 (Shipboard Scientific Party, 2000b). The 1181-m-thick section consists mainly of diatomaceous silty clay and diatomaceous clay. Diatoms are generally common to abundant and moderately well preserved throughout. In the upper Miocene section, however, abundance of diatoms slightly decreases because of the increase in clay content and probably siliceous diagenetic alteration.

Diatom assemblages from all samples consist almost entirely of oceanic species, which are typical of the subarctic North Pacific Ocean. Diatoms are represented by open-ocean forms not characteristic of coastal upwelling regions. Sparse occurrences of temperate taxa, such as Fragilariopsis doliola, Hemidiscus cuneiformis, Nitzschia reinholdii, and Nitzschia marina, indicate the influence of temperate water masses from the south during the Neogene through Quaternary.

The Shipboard Scientific Party (2000b) reported that a significant difference in the sedimentation rate is observed between Site 1150 and nearby areas such as Holes 438A and 584. The Neogene basin around Site 1150 is called the Kitakami Basin and is situated at ~38°-40°N; on the other hand, the Ishikari-Hidaka Basin around Sites 438 and 584 extends northward. Oceanic diatoms have been deposited continuously in the vicinity of Site 1150 since at least the late Miocene, whereas in Holes 438A and 584 some hiatuses are recorded as the result of a slightly shallower condition during the paleoceanographic history.

The boundary between the late Quaternary Neodenticula seminae and Proboscia curvirostris Zones (NPD12/NPD11) is indicated by the LO of P. curvirostris (0.30 Ma) between Samples 186-1150A-5H-CC and 6H-CC. The biostratigraphic interval through Zone NPD12 and the uppermost part of Zone NPD11 corresponds well to positive magnetic inclinations above Section 186-1150A-10H-1 (84.18 mbsf), representing Chron C1n (Brunhes).

The LO of A. oculatus immediately above Sample 186-1150A-12H-CC marks the boundary between the P. curvirostris and A. oculatus Zones (NPD11/NPD10). This boundary is estimated to be between 1.01 and 1.46 Ma in the subarctic region by Barron and Gladenkov (1995), based on its LCO. The FO of P. curvirostris, with an age of 1.58 Ma, is recognized between Samples 186-1150A-14H-CC and 15H-CC.

Koizumi (1992) proposed that the LO of N. koizumii clearly defines the base of the A. oculatus Zone (NPD10) and the top of the underlying N. koizumii Zone (NPD9). This latest Pliocene event, at ~2.0 Ma, falls between Samples 186-1150A-15X-CC and 16X-CC. The occurrence of F. doliola (= Pseudoeunotia doliolum) in Sample 186-1150A-13X-CC supports the recognition of the NPD10/NPD9 zonal boundary because the FO of that warm-water species has an estimated age of 1.9 Ma in the eastern equatorial Pacific Ocean (Baldauf and Iwai, 1995).

The LCsO of N. kamtschatica (2.61-2.68 Ma) between Samples 186-1150A-21X-CC and 22X-CC marks the top of the N. koizumii-N. kamtschatica Zone (NPD8). The FO of N. koizumii (3.53-3.95 Ma), which defines the boundary between the N. koizumii-N. kamtschatica Zone and the underlying N. kamtschatica Zone (NPD8/NPD7B), is clearly identified between Samples 186-1150A-27X-CC and 28X-CC.

Shipboard Scientific Party (2000b) reported that the FO of Thalassiosira "oestrupii" sensu amplificato (that notation has been generally written as sensu lato or s.l.), denoting the Miocene/Pliocene boundary from a standpoint of diatom biostratigraphy, is detected between Sample 186-1150A-56X-CC and Section 57X-1 and is useful to subdivide the NPD7B Zone. Based on recent progress in taxonomy of the Thalassiosira group (Shiono and Koizumi, 2000, 2001), the FO of T. "oestrupii" s. ampl. (5.49 Ma) is not suitable for a datum plane in the Neogene diatom zonation. We cannot help discarding the name of the T. oestrupii Subzone from the North Pacific diatom zones and reconstructing the subzonal framework during the uppermost Miocene through lower Pliocene series.

Drilling in Hole 1150A stopped in Core 186-1150A-76X, and the lowest stratigraphic sample (186-1150A-76X-CC) at 722.85 mbsf remained in Zone NPD7B. Drilling continued in Hole 1150B with the topmost core catcher (Sample 186-1150B-1R-CC) still remaining in the same NPD7B Zone at 708.22 mbsf. The top of the upper Miocene R. californica Zone (NPD7A) is defined by the FCsO of N. kamtschatica (6.4 Ma) between Samples 186-1150B-21R-CC and 22R-CC.

A complete sequence of diatom zones from the middle upper Miocene Thalassionema schraderi Zone (NPD6B) to the lower upper Miocene Thalassiosira yabei Zone (NPD5C) was penetrated in the lower part (>1044.73 mbsf) of Hole 1050B. The LCO of T. schraderi (7.6 Ma), just above Sample 186-1050B-36R-CC, marks the boundary between the NPD7A Zone and the underlying the T. schraderi Zone (NPD6B). From Zones NPD7A through NPD6B, both characteristics of R. californica and T. schraderi occur consistently but not abundantly. The LCsO of D. simonsenii (8.6 Ma) is assigned between Samples 186-1050B-40R-CC and 41R-CC, where the NPD6B/NPD6A boundary is obviously placed. The LO of D. katayamae was not determined clearly because of the presence of a poorly preserved horizon at ~1070 mbsf in Hole 1150B.

Assemblages in Samples 186-1150B-43R-CC through 48R-CC (1111.28 to 1157.71 mbsf) containing D. dimorpha are assigned to the lower upper Miocene D. dimorpha Zone (NPD5D). Within Zone NPD5D, Sample 186-1150B-45R-CC records the FO of D. katayamae and Sample 186-1150B-46R-CC contains the FO of T. schraderi.

At the bottom of Hole 1150B, the T. yabei Zone (NPD5C) lies in Samples 186-1150B-49R-CC through 50R-CC. Neither D. praedimorpha nor D. dimorpha persist through Zone NPD5C, so the evolutionary lineage from D. praedimorpha to D. dimorpha is disrupted in the midlatitude northwestern Pacific Ocean. It is surely indicated that the base of Hole 1150B lies between 9.9 and 11.5 Ma, although on board the ship a much older age of 11.6 Ma was given for the bottom sediments from an extrapolation of the sedimentation rate.

The sedimentary sequence recovered from Site 1151 consists of an apparently continuous 1113-m-thick interval of lower middle Miocene (<16.3 Ma) to Pleistocene sediment. Site 1151 is located on the deep-sea terrace at 2681 m water depth, ~100 km west of the Japan Trench axis. The geologic setting at Site 1151 is similar to that at Site 1150, which is 48 km north from Site 1151 on the eastern edge of the forearc basin.

Four holes were completed at Site 1151 to a maximum depth of 1113.6 mbsf. Dominant lithologies are diatomaceous silty clay. Diatoms are generally common to abundant and moderately to well preserved throughout the section cored with the exception that some ash and dolomite layers contain few to rare diatoms. Diatom assemblages are typical of hemipelagic sedimentary sequences of temperate middle latitudes, and the standard North Pacific diatom zonation has been adapted for use in Hole 1151A (Table T3). Reworked forms and neritic assemblages are not significant at this site. Sparse occurrences of subtropical taxa including H. cuneiformis, Nitzschia fossilis, and N. reinholdii suggest the influence of relatively warmer waters.

The top of Hole 1151A contains P. curvirostris, indicating that the age of Sample 186-1151A-2R-CC is older than 0.3 Ma. Because the late Pleistocene diatom N. seminae is also continuously present, the top of Hole 1151A apparently corresponds to the P. curvirostris Zone (NPD11). The LO of A. oculatus (1.01-1.46 Ma), which defines the boundary between the middle Pleistocene P. curvirostris Zone and the lower Pleistocene A. oculatus Zone (NPD11/NPD10), is identified between Samples 186-1151A-3R-CC and 4R-CC. The beginning of a consistent occurrence of the Pleistocene diatom P. curvirostris lies between Samples 186-1151A-4R-CC and 5R-CC, and its ancestral form already appeared in the upper Pliocene NPD9 Zone.

The base of the A. oculatus Zone (NPD10) and the top of the underlying N. koizumii Zone (NPD9) is defined by the LO of N. koizumii. This latest Pliocene event occurs between Sample 186-1151A-5R-CC and 6R-CC. The LCsO of N. kamtschatica (2.61-2.68 Ma) between Samples 186-1151A-11R-CC and 12R-CC marks the top of the N. koizumii-N. kamtschatica Zone (NPD8). The FO of N. seminae is detected in the upper part of this zone from Sample 186-1151A-12R-CC. 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 through upper Miocene N. kamtschatica Zone (NPD8/NPD7B boundary), is clearly identified between Samples 186-1151A-14R-CC and 15R-CC.

The FO of T. oestrupii s. ampl. (5.49 Ma) is accepted as the top of the upper Miocene N. kamtschatica Zone of Koizumi (1992). Also Yanagisawa and Akiba (1998) used this event as the zonal marker at the Subzone NPD7Bb/NPD7Ba boundary. The Leg 186 Initial Reports volume reported that this datum exists between Sections 186-1151A-38R-1 and 38R-CC (Sacks, Suyehiro, Acton, et al., 2000). Recent rapid progress of taxonomic study on the genus Thalassiosira (Shiono and Koizumi, 2000, 2001) prohibits the FO of T. oestrupii s. ampl. from defining the subzonal boundary between NPD7Bb and NPD7Ba and indicates the reexamination of diatom biostratigraphy covering the Miocene/Pliocene boundary in the North Pacific Ocean.

The top of the upper Miocene R. californica Zone (NPD7A) was originally defined by the LCO of R. californica (Akiba, 1986; Maruyama, 2000), but its stratigraphic occurrence is not continuous and its abundance is rare in Hole 1151A. Thus, the top of the NPD7A Zone is recognized as the FCsO of N. kamtschatica (6.4 Ma) between Samples 186-1151A-77R-CC and 79R-CC. Within the NPD7A Zone, the FO of N. kamtschatica (7.3-7.4 Ma) is also an important event as a zonal marker, but the actual distinction of N. kamtschatica from its ancestor Nitzschia rolandii (based on broader spacing of pseudosepta or costae) is difficult because of their gradual evolutionary change (Yanagisawa and Akiba, 1990, 1998). We should stress the biostratigraphic importance of the FCsO of N. kamtschatica rather than its FO datum in the upper Miocene.

The LCO of T. schraderi (7.6 Ma) is detected sharply between Samples 186-1151A-79R-CC and 80R-CC and marks the top of the upper Miocene T. schraderi Zone (NPD6B). In the Leg 186 Initial Reports volume, the D. katayamae Zone (NPD6A) is observed from the short interval between Sections 186-1151A-91R-5 and 91R-CC and, moreover, is conformably overlain by the T. schraderi Zone (NPD6B) from beyond Section 91R-3 (Sacks, Suyehiro, Acton, et al., 2000). Both datums of the LCsO of D. simonsenii (8.6 Ma) and the LO of D. dimorpha (9.16 Ma), however, simultaneously appear between Samples 186-1151A-90R-CC and 91R-CC from our shore-based analysis. These events without the observation of the LO of D. katayamae suggest that the NPD6A Zone is missing or compressed in the interval between Sections 186-1151A-91R-4 and 91R-5.

The D. dimorpha Zone (NPD5D) extends consistently from Sample 186-1151A-91R-CC down through 95R-CC, with the FO of T. schraderi encountered between Samples 186-1151A-92R-CC and 93R-CC. The FO of D. dimorpha (9.9 Ma), which defines the top of T. yabei Zone (NPD5C), is clearly identified between Samples 186-1151A-95R-CC and 99R-CC. The stratigraphic horizon of this FO event agrees with the result in the Leg 186 Initial Reports volume between Sections 186-1151A-95R-CC and 96R-1, and also with the extent of the T. yabei Zone from Sections 186-1151A-96R-1 down to 99R-CC (979.32 to 1009.53 mbsf). On the other hand, only one sample (186-1151A-99R-CC) represents the NPD5C Zone based on the intermittent sample interval in our analysis.

The base of the T. yabei Zone (NPD5C) is determined by the LCO of D. praedimorpha (11.5 Ma), but that event is not found in the course of our shore-based analysis. Moreover, the D. praedimorpha Zone (NPD5B) starts with the FO of D. praedimorpha (12.9 Ma) and is characterized by the consistent occurrences of that species, but the NPD5B Zone is not detected in our studies of Hole 1151A. On board the JOIDES Resolution, however, a 10-m-thick sequence (1017.66 to 1027.31 mbsf) containing middle Miocene Zone NPD5B was recorded in Samples 186-1151A-100R-1, 66-69 cm, through 101R-1, 71-73 cm. The Shipboard Scientific Party (2000c) suggested some probabilities of coring gap or a hiatus of <1 m.y. between Sections 186-1151A-99R-CC and 100R-1.

The authors put great emphasis upon the coincidence among the following events; the FCO of D. simonsenii (13.1 Ma), the FO of D. simonsenii (14.4-14.6 Ma), and the LCO of D. hyalina (13.1 Ma) between Samples 186-1151A-99R-CC and 101R-CC (1009.53 to 1028.78 mbsf). The absence of the C. nicobarica Zone (NPD5A) and the deficiency of the acme of D. hyalina being assignable to the NPD4Bb Subzone (the upper part of the D. hyalina Zone) fairly characterized the diatom assemblages from ~1027 mbsf in Hole 1151A. Although the NPD4Bb Subzone was established for the 9-m-thick interval from Sample 186-1151A-101R-1, 90-91 cm, to 102R-1, 69-72 cm (1027.5 to 1036.89 mbsf), on board the JOIDES Resolution, the placement of Zone NPD5A is not at all as certain; neither is that in our onshore analyses. It is likely that the NPD5A interval, exhibiting the whole span of 0.2 Ma, is compressed or absent within Section 186-1151A-101R-1, close to 1027 mbsf.

The underlying D. hyalina Zone (NPD4Ba) is characterized by the continuous occurrence of D. hyalina and rare D. lauta, and its base is marked by the FO of D. hyalina (14.9 Ma) between Samples 186-1151A-103R-CC and 104R-CC. Within the D. lauta Zone (NPD4A), the group abundance of diatom assemblages decline from common to few, with sporadic intervals of poor preservation. Regardless of group abundance and preservation, the lowest occurrence of the index D. lauta in Sample 186-1151A-107R-CC defines the bottom of Zone NPD4A. Consistent distribution of D. praelauta without the presence of D. lauta in the bottom interval from Samples 186-1151A-108R-CC through 109R-CC designates the D. praelauta Zone (NPD3B) as the oldest sedimentary sequence at Site 1151, exhibiting an age between 15.9 and 16.3 Ma.