Seven holes were drilled at Site 1251. Of these holes, Hole 1251B contains the most complete sequence, a 445-m-thick upper Pliocene-Quaternary sedimentary sequence. Biostratigraphy for Site 1251 was based on examination of all core catcher samples and a few of the samples within cores from Hole 1251B.
The abundance and preservation of diatoms in sediments from Hole 1251B varies significantly, indicating possible variations in ocean productivity and/or dilution by nonsiliceous biogenic and clastic components. Diatoms are few to abundant and poorly to well preserved in the interval from 0 to 120 mbsf and from 320 mbsf to the bottom of the hole. Diatoms are rare or absent in the interval from 120 to 320 mbsf, except for Samples 204-1251B-19H-CC (163.6 mbsf), 27X-CC (222.8 mbsf), and 31X-CC (258.3 mbsf), where diatoms are common. Diatom assemblages from Hole 1251B are dominated by species of the subarctic North Pacific Ocean. A few warm-water taxa, such as Hemidiscus cuneiformis and Fragilariopsis doliolus, are sporadically present. Neritic species, such as Actinoptychus senarius and Cocconeis spp., are present in most samples, whereas freshwater species, such as Aulacoseira spp., are present sporadically.
The interval between the top of Hole 1251B and Sample 204-1251B-19H-CC (0-163.6 mbsf) contains Neodenticula seminae but lacks Proboscia curvirostris. This interval is assigned to North Pacific Diatom Zone (NPD) 12 (N. seminae Zone). The last occurrence (LO) of P. curvirostris was found in Sample 204-1251B-20H-CC (171.7 mbsf), whereas the LO of Actinocyclus oculatus was found in Sample 39X-6, 57-59 cm (328.0 mbsf). The interval between these two LOs (171.7-28.0 mbsf) is assigned to NPD 11 (P. curvirostris Zone). The interval between Samples 204-1251B-39X-6, 57-59 cm, and 52X-CC (328.0-434.7 mbsf) contains A. oculatus but lacks Neodenticula koizumii and is assigned to NPD 10 (A. oculatus zone). The first occurrence (FO) of P. curvirostris was found in Sample 204-1251B-42X-CC (348.4 mbsf). The LO of N. koizumii was found in Sample 204-1251B-53X-2, 64-66 (437.6 mbsf), indicating an age of ~2.0 Ma for the bottom of Hole 1251B 5 m below this event. The interval between the LO of N. koizumii and the bottom of Hole 1251B contains N. koizumii and N. seminae but lacks Neodenticula kamtschatica. It was assigned to the upper part of NPD 9 (N. koizumii Zone).
Calcareous nannofossils from Hole 1251B were investigated in smear slides from core catcher samples and from selected within-core samples to better constrain the biohorizons. Hole 1251B yielded common to rare, well- to moderately preserved nannofossils in Holocene-Pliocene sections.
Emiliania huxleyi is continuously present from the top of Hole 1251B down to Sample 204-1251B-19H-1, 74-76 cm (156.3 mbsf), assigning this interval to Zone NN21. Abundant E. huxleyi was observed in Sample 204-1251B-1H-CC (9.0 mbsf). The abundance decreases distinctly in the next Sample 204-1251B-3H-CC, allowing us to assign Sample 1H-CC to the E. huxleyi Acme Zone. The interval between Samples 204-1251B-19H-CC and 24X-CC (between 163.6 and 189.0 mbsf) contains abundant Gephyrocapsa spp. but lacks E. huxleyi and Pseudoemiliania lacunosa; this interval was assigned to Zone NN20. The presence of P. lacunosa in Sample 204-1251B-25X-CC (204.1 mbsf) marks the NN19b/NN20 zonal boundary. Various small Gephyrocapsa spp. are abundant in Samples 204-1251B-33X-CC and 34X-CC (280.5-289.1 mbsf). This interval was assigned to the mid-Pleistocene small Gephyrocapsa spp. Acme Zone (1.0-1.2 Ma). The LO of Calcidiscus macintyrei was found in Sample 204-1251B-34X-CC (289.1 mbsf), indicating an age of 1.59 Ma (NN19) for this stratigraphic level. Gephyrocapsa lumina is present in Sample 204-1251B-38X-4, 66-68 cm (315.5 mbsf), suggesting an age of 1.67 Ma.
Based on calcareous nannofossils, the Pliocene/Pleistocene boundary (1.77 Ma) (Berggren et al., 1995) is located below the FO of medium-sized Gephyrocapsa spp. (>4 µm). The FO of a few medium-sized Gephyrocapsa spp. was observed in Sample 204-1251B-45X-3, 66-68 cm (371.7 mbsf), indicating this sample is of Quaternary age. The interval from Samples 204-1251B-45X-CC to 53X-CC (378-443 mbsf) does not contain any Pliocene bioevent marker species; for example, there are no species of genus Discoaster, which are warm-water species and absent in high latitudes and cold-current regions. However, this interval contains a calcareous nannofossil assemblage with abundant Reticulofenestra productus and P. lacunosa, whereas the early Pliocene event marker Reticulofenestra pseudoumbilicus is absent. This evidence suggests that the lowest sequence of 378-443 mbsf in Hole 1251B is of late Pliocene age.
Sediments recovered at Site 1251 yielded few to abundant diatoms and rare to common calcareous nannofossils in most samples examined. A total of eleven microfossil events (four diatom and seven nannofossil events) was recognized at Site 1251 (Table T3). An age-depth plot shows that biohorizons from the two fossil groups generally agree (Fig. F11). The Pleistocene/Pliocene boundary is estimated by the FO of medium Gephyrocapsa spp. and is located between 371 and 377 mbsf.
The linear sedimentation rates at Site 1251 have been calculated based on biostratigraphic data (Table T3) and are estimated to be 60 cm/k.y. for the interval from the top of Hole 1251B to 170 mbsf, 17 cm/k.y. for the interval from 170 to 290 mbsf, 3 cm/k.y. for the interval from 290 to 310 mbsf, and 33 cm/k.y. for the interval from 310 mbsf to the bottom of Hole 1251B (Fig. F11). According to seismic data, a regional unconformity is located at about 130 mbsf at Site 1251. An alternative sedimentation rate is estimated for the interval from the top of Hole 1251B to 130 mbsf by putting an inflexion point at 130 mbsf, the depth of the unconformity. In this case, the sedimentation rate above 130 mbsf is 160 cm/k.y. (Fig. F11). In conclusion, sedimentation rates are very high in the two intervals from the top of Hole 1251B to 170 (or 130) mbsf and from 310 mbsf to the bottom of Hole 1251B.