Only one hole was drilled at Site 1252. Hole 1252A contains a 259-m-thick upper Pliocene-Quaternary sedimentary sequence. Biostratigraphy determined for Site 1252 was based on examination of all core catcher samples from Hole 1252A.
Diatoms are few to common in Hole 1252A, except for one sample (Sample 204-1252A-13H-CC [116.65 mbsf]) where diatoms are absent. Preservation of diatoms is generally poor, except in the interval between Samples 1252A-25X-CC and 28X-CC, where diatoms are moderately to well preserved. Diatom assemblages from sediments in Hole 1252A are dominated by species such as Stephanopyxis spp., Neodenticula spp., and Thalassionema nitzschioides. A few warm-water taxa, such as Hemidiscus cuneiformis, Fragilariopsis doliolus, and Thalassiosira oestrupii, are frequently present.
The interval from the seafloor down to 81.28 mbsf contains Neodenticula seminae but lacks Proboscia curvirostris; this interval was assigned to North Pacific Diatom Zone (NPD) 12 (N. seminae Zone). The LO of P. curvirostris was found in Sample 204-1252A-10H-CC (90.8 mbsf), and the next event, the LO of Actinocyclus oculatus, was found in Sample 14H-CC (128.72 mbsf). Therefore, the interval between these two levels (90.8-128.72 mbsf) was assigned to the Zone NPD 11 (P. curvirostris Zone). The interval between Samples 204-1252A-14H-CC and 27X-CC (128.72-249.34 mbsf) contains A. oculatus but lacks Neodenticula koizumii and was assigned to the Zone NPD 10 (A. oculatus Zone). The FO of P. curvirostris was placed in Sample 204-1252A-19X-CC (173.39 mbsf). Sample 204-1252A-28X-CC (258.96 mbsf) contains N. koizumii and N. seminae but lacks Neodenticula kamtschatica, and thus, the level of 258.96 mbsf was determined as the LO of N. koizumii and assigned to the upper part of the Zone NPD 9 (N. koizumii zone). This indicates an age of ~ 2.0 Ma for the bottom sediments from Hole 1252A.
Calcareous nannofossils from Hole 1252A were investigated in smear slides from core catcher samples. About 40% of these samples contain rare to common, moderately preserved calcareous nannofossils; these samples are located predominantly in the upper part of the sequence in Hole 1252A, from the seafloor to 116 mbsf. Samples below 116 mbsf do not contain calcareous nannofossils or else yield trace, poorly preserved nannofossils, except for three samples (Samples 204-1252A-17X-CC, 20X-CC, and 23X-CC) that contain rare nannofossils. Dissolution of calcareous nannofossils was observed in sediments at Site 1252, whereby the edge of a coccolith shield was partly dissolved or the major part of the shield was lost by dissolution. Such dissolution of calcareous nannofossils was not observed in sediments at sites located nearby, such as at Sites 1244 and 1251 or at other Leg 204 sites. Therefore, we ruled out the possibility that carbonate dissolution occurred below the calcite compensation depth in this region. The observed dissolution of calcareous nannofossils in the sediments at Site 1252, on the other hand, probably implies the influence of fluid affecting the sediments at this site. However, this assumption needs further study postcruise and comparison with the results from geochemistry studies of pore water samples from this site.
The calcareous nannofossil biostratigraphy determined for Site 1252 during the last phase of the cruise was based on the nannofossil record deduced from the 40% of samples that were relatively good.
A Quaternary calcareous nannofossil assemblage was discovered in the sequence from Hole 1252A. Emiliania huxleyi is frequently present from the seafloor down to 71.42 mbsf, assigning this interval to Zone NN21. Abundant E. huxleyi was observed in the upper part of Zone NN21, the interval from seafloor to 14.88 mbsf, allowing us to assign the top part of the interval to the E. huxleyi Acme Zone. The interval of 81.28-90.80 mbsf lacks E. huxleyi and Pseudoemiliania lacunosa; this interval was assigned to Zone NN20. The next event, the LO of P. lacunosa found in Sample 204-1252A-11H-CC (100.59 mbsf), marks the NN19b/NN20 zonal boundary. Below 116.65 mbsf, samples are barren of calcareous nannofossils poorly preserved in the majority of samples examined and very few small Gephyrocapsa species are present infrequently in a few of these samples. Consequently, the mid-Pleistocene small Gephyrocapsa spp. Acme Zone (1.0-1.2 Ma) was not determined for Site 1251. Three samples (Samples 204-1252A-17X-CC, 20X-CC, and 23X-CC) in the interval from 153.41 to 179.54 mbsf contain rare, moderately preserved calcareous nannofossils. Rare Calcidiscus macintyrei is present in three of these relatively good samples; the LO of C. macintyrei (1.59 Ma; the lower part of the Zone NN19) was placed at 153.41 mbsf. Rare Gephyrocapsa lumina is also present in these three samples, suggesting an age younger than 1.67 Ma (younger than the FO of G. lumina) for sediments at 179.54 mbsf. Samples below 179.54 mbsf (Samples 204-1252A-20X-CC through 28X-CC) do not contain any calcareous nannofossils. As a result, no calcareous nannofossil data were obtained from the lowest part of the sequence in Hole 1252A.
Nine microfossil events (four diatom and five nannofossil events) were recognized for Site 1252 (Table T2). An age-depth plot shows that biohorizons from the two fossil groups agree well with each other (Fig. F12).
The lack of more detailed events, especially nannofossil events from the lowest part of the sequence from Hole 1252A, makes it impossible to accurately determine the Pleistocene/Pliocene boundary for Site 1252. However, this boundary could be roughly placed between 179.54 and 258.96 mbsf, based on an estimated nannofossil age younger than 1.67 Ma at 179.54 mbsf and a diatom-based estimate of ~2 Ma at 258.96 mbsf (Table T2).
Linear sedimentation rates at Site 1252 have been estimated based on biostratigraphic data (Table T2). The sedimentation rates are estimated to be 28 cm/k.y. for the interval from the top of Hole 1252A to 90 mbsf, 9 cm/k.y. for the interval from 90 to 150 mbsf, 70 cm/k.y. for the interval from 150 to 210 mbsf, and 14 cm/k.y. for the interval from 210 mbsf to the bottom of Hole 1252A (Fig. F12). According to seismic data, an unconformity is located at ~120 mbsf at Site 1252. Considering this unconformity, an alternative sedimentation rate of 6 cm/k.y. is estimated for the interval from 95 to 130 mbsf (Fig. F12), extrapolating the sedimentation rate above and below the unconformity.