Five holes were drilled and four holes were cored at Site 1245 on the northwestern flank of the summit of south Hydrate Ridge. Hole 1245B was cored from the seafloor down to 473.7 mbsf, and Hole 1245E was cored from 473.7 to 540.3 mbsf, thereby obtaining a composite 540-m Quaternary sequence. The biostratigraphy determined for Site 1245 was based on examination of diatoms and calcareous nannofossils from all core catcher samples and a few discrete samples from cores in Holes 1245B and 1245E.
Diatoms are common to few and moderately preserved in the interval from 0 to 66 mbsf. In the interval from 66 to 470 mbsf in Hole 1245B they are rare to absent and poorly preserved, except in the interval from 128 to 163 mbsf, where common to few, poorly to moderately preserved diatoms are present. Samples from Hole 1245E do not contain diatoms, probably as a result of diagenetic dissolution of diatom valves. Diatom assemblages in Hole 1245B are dominated by species such as Stephanopyxis dimorpha, Stephanopyxis spp., Neodenticula seminae, and Thalassionema nitzschioides. A warm-water taxa Fragilariopsis doliolus is present sporadically.
The interval from the seafloor to 38.7 mbsf in Hole 1245B contains N. seminae but lacks Proboscia curvirostris. This interval was assigned to North Pacific Diatom (NPD) Zone 12 (N. seminae Zone). The last occurrence (LO) of P. curvirostris was found in Sample 204-1245B-5H-CC (46.66 mbsf). This species is present sporadically down to Sample 204-1245B-53X-5, 68-70 cm (470.44-470.46 mbsf), indicating the age of this sample to be younger than 1.6 Ma. The FO of Actinocyclus oculatus (1.0 Ma) was not found in Hole 1245B; this is likely a result of the trace to rare presence and poor preservation of diatoms in the middle part of the hole.
The calcareous nannofossil biostratigraphy for Site 1245 was based on an investigation of core catcher samples from the 540-m composite sequence of cores recovered from Holes 1245B and 1245E. Common to rare and well- to moderately preserved calcareous nannofossils were observed in three intervals at Site 1245, from 127.79 to 144.82, from 174.47 to 319.72, and from 412.87 to 540.20 mbsf. They are poorly preserved or are infrequently present, with low abundance in the rest of the intervals. This agrees with sedimentological core description data from this site (see Fig. F4). An early Pleistocene-Holocene calcareous nannofossil assemblage was found in the composite 540-m sequence.
Emiliania huxleyi is present infrequently from the seafloor down to 55.58 mbsf (Sample 204-1245B-5H-CC). Common and well-preserved, large-sized (~4 µm) E. huxleyi were observed in Samples 204-1245B-5H-CC and 6H-CC (46.66-55.58 mbsf). As large-sized forms of E. huxleyi are present in the early evolutionary stage of this species (Pujos-Lamy, 1977), the interval from the seafloor to 55.58 mbsf was assigned to Zone NN21. Two samples below 55.58 mbsf (Samples 204-1245B-7H-CC and 8H-CC) contain a few poorly preserved small Gephyrocapsa species but lack either E. huxleyi or Pseudoemiliania lacunosa. Therefore, these samples were assigned to Zone NN20. P. lacunosa is continuously present from 84.79 to 540 mbsf. The LO of P. lacunosa was placed at 84.79 mbsf, which marks the NN19b/NN20 zonal boundary. Various small Gephyrocapsa species are abundant in the interval from 156.72 to 271.93 mbsf (Samples 204-1245B-18X-CC to 30X-CC), assigning this interval to the mid-Pleistocene small Gephyrocapsa spp. Acme Zone (1.0-1.2 Ma). Therefore, the LO of the small Gephyrocapsa spp. Acme was placed at 156.72 mbsf and the FO of the small Gephyrocapsa spp. Acme at 271.93 mbsf. The LO of Calcidiscus macintyrei, which was found in Sample 204-1245E-6R-CC (522.33 mbsf), suggests an age of 1.59 Ma (the lower part of Pleistocene Zone NN19) for this sample. Rare to common Gephyrocapsa lumina is present in the base sediments (540.20 mbsf) of Site 1245, indicating an age younger than 1.67 Ma.
The presence of age-diagnostic fossils is summarized in Figure F21. One diatom event and five nannofossil events were recognized at Site 1245 (Table T4). The diatom assemblage from 470.46 mbsf indicates the age at this depth to be younger than 1.6 Ma. This corresponds well to the nannofossil record, which suggests that the age of the bottom of Hole 1245E is younger than 1.67 Ma. Based on evidence from diatoms and calcareous nannofossils, the 540-m sequence recovered from Site 1245 is of Quaternary age (early Pleistocene-Holocene).
Linear sedimentation rates were estimated based on diatom and calcareous nannofossil events (Table T4; Fig. F22). Sedimentation rates at this site are generally higher than those at the other sites drilled during Leg 204, except for Site 1251. The sedimentation rate is estimated to be 23 cm/k.y. for the interval from 0 to 60 mbsf, 10 cm/k.y. for the interval from 60 to 80 mbsf, 13 cm/k.y. for the interval from 80 to 150.77 mbsf, and 62 cm/k.y. for the interval from 150.77 to 540.20 mbsf. The higher sedimentation rate below 150 mbsf is well correlated with the record of frequent turbidites observed within this interval (see "Lithostratigraphy").