Site 1250 was drilled near the carbonate chemoherm known as the Pinnacle, located near the summit of the southern Hydrate Ridge. At this site, six holes were drilled and four of these (Holes 1250C, 1250D, 1250E, and 1250F) were cored. Hole 1250C was drilled to 138.50 mbsf, with good recovery (~82%). Hole 1250F was cored from 100.00 to 181.59 mbsf, extending 43.09 m deeper than Hole 1250C. A combination of Holes 1250C and Hole 1250F yielded a complete 181-m-thick Pleistocene-Holocene sedimentary sequence at Site 1250. The biostratigraphy determined for Site 1250 was based on examination of diatoms and calcareous nannofossils in all core catcher samples from Holes 1250C and 1250F.
Holes 1250C and 1250E yield rare to common, poorly to moderately preserved diatoms, except for the interval from Samples 204-1250F-9H-CC to 11H-CC, where diatoms are trace or barren. Diatom assemblages in Holes 1250C and 1250F are dominated by such species as Stephanopyxis dimorpha, Stephanopyxis spp., Neodenticula seminae, and Thalassionema nitzschioides. Warm-water species, such as Fragilariopsis doliolus and Thalassiosira oestrupii, are frequently present throughout the sediment sequence recovered.
Proboscia curvirostris are continuously present in almost all the samples from Holes 1250C and 1250F, except for the interval from the seafloor to Sample 204-1250C-4H-CC (30.42 mbsf), where the presence of P. curvirostris is sporadic. Since the nannnofossil Emiliania huxleyi is first present at 41.30 mbsf (see "Calcareous Nannofossils") and the FO of E. huxleyi is younger than the last occurrence (LO) of P. curvirostris, the sporadic presence of P. curvirostris in the uppermost part of Hole 1250C was considered to represent reworked fossils. Therefore, the LO of P. curvirostris was placed at the top of the continuous presence of this species in Sample 204-1250C-5H-CC (41.3 mbsf). The interval above 30.42 mbsf was assigned to North Pacific Diatom Zone (NPD) 12 (N. seminae zone). P. curvirostris is continuously present from 41.30 mbsf to 181.54 mbsf. This means that the sequence at Site 1250 does not reach the level of the FO of P. curvirostris (1.6 Ma); therefore, the age of sediments at 181.54 mbsf is estimated to be younger than 1.6 Ma.
Core catcher samples from most intervals of the 181-m composite sequence at Site 1250 contain rare to common, moderately to well-preserved calcareous nannofossils. Calcareous nannofossils, however, are barren or poorly preserved in a few intervals (e.g., from 51.34 to 71.23 and from 109.89 to 127.14 mbsf in Hole 1250C and from 125.49 to 149.51 mbsf in Hole 1250F).
Common to rare E. huxleyi is continuously present from the seafloor to 41.30 mbsf, assigning this interval to the Zone NN21. Pseudoemiliania lacunosa is frequently present from 51.34 to 181.54 mbsf, allowing us to determine the LO of P. lacunosa and place the NN19b/NN20 zonal boundary at 51.34 mbsf. Samples from 51.34 to 71.23 mbsf contain a few poorly preserved small-sized Gephyrocapsa species. Therefore, we could not see the small Gephyrocapsa spp. Acme Zone in Hole 1250C from core catcher samples analyzed. Calcidiscus macintyrei is present frequently, although not abundantly (from 81.89 to 181.54 mbsf), suggesting the LO of C. macintyrei (1.59 Ma) is at 81.89 mbsf. The sediments at 181.54 mbsf (Sample 204-1250F-13X-CC) contain common Gephyrocapsa lumina and were assigned to the lower part of early Pleistocene Zone NN19 with an age of younger than 1.67 Ma.
Based on analysis of diatoms and calcareous nannofossils from the composite 181-m sequence at Site 1250, four microfossil events (one diatom and three nannofossil events) were recognized (Table T2). The age at the base of Hole 1250F (181.54 mbsf) was estimated to be younger than 1.6 Ma, based on the diatom assemblage, or to be younger than 1.67 Ma, based on the nannofossil assemblage. This means that the age of the 181-m-thick composite sequence at Site 1250 is early Pleistocene-Holocene.
We estimated linear sedimentation rates using these events (Fig. F15). The sedimentation rates are 15 cm/k.y. for the interval from the top of Hole 1250C to 42 mbsf and 2.4 cm/k.y. for the interval from 42 to 76 mbsf.