Six holes were drilled at Site 1244, and five holes were cored. Hole 1244C contains the most complete sequence, a 333-m-thick upper Pliocene-Quaternary sedimentary sequence. The biostratigraphy determined for Site 1244 was based on an examination of all core catcher samples and a few of the samples taken from cores at Hole 1244C.
Diatoms are common to abundant and moderately to well preserved in the interval from the seafloor to 120 mbsf, whereas diatom abundances below 120 m to the bottom of Hole 1244C vary significantly from barren to abundant. Diatom assemblages at Hole 1244C 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 (Table T3).
The interval between the seafloor and Sample 204-1244C-9H-CC (0-82.16 mbsf) contains Neodenticula seminae but lacks Proboscia curvirostris. This interval was assigned to North Pacific Diatom Zone (NPD) 12 (N. seminae Zone). The last occurrence (LO) of P. curvirostris was found in Sample 204-1244C-10H-CC (91.13 mbsf), marking NPD 12/11 (P. curvirostris Zone). A fragment of Actinocyclus oculatus was found in Sample 204-1244C-25X-CC (206.04 mbsf); thus, the LO of A. oculatus is tentatively placed between this sample and Sample 204-1244C-24X-CC (195.95-206.04 mbsf). Further study is needed to confirm this LO because, at present, no more than one fragment of A. oculatus has been found from the Hole 1244C samples. The interval between these two LOs (91.13-195.5 mbsf) was assigned to NPD 11 (P. curvirostris Zone). The LO of Neodenticula koizumii was found in Sample 204-1244C-34X-CC (292.05 mbsf). The interval between this LO and the LO of A. oculatus (206.64-283.17 mbsf) was assigned to NPD 10 (A. oculatus Zone). The first occurrence (FO) of P. curvirostris was placed in Sample 204-1244C-28X (235.24 mbsf) in NPD 10. The interval between the LO of N. koizumii and the bottom of Hole 1244C (292.05-333.64 mbsf) contains N. koizumii but lacks Neodenticula kamtschatica and was assigned to NPD 9 (N. koizumii Zone). The FO of N. seminae was placed in Sample 204-1244C-36X-CC (312.34 mbsf) from the middle part of this zone. The age of the bottom of Hole 1244C was estimated to be 2.4-2.7 Ma, based on the presence of N. koizumii and the absence of N. seminae and N. kamtschatica in Sample 204-1244C-39X-CC (333.64 mbsf).
Calcareous nannofossils from Hole 1244C were investigated in smear slides from core catcher samples and from selected samples to better constrain the biohorizons. Calcareous nannofossils are barren or of trace abundance for the majority of samples investigated (e.g., between Samples 204-1244C-2H-CC and 7H-CC [15.46-62.59 mbsf], 12H-CC and 25X-CC [110.38-206.04 mbsf], 30X-CC and 34X-CC [253.5-292.05 mbsf], and 38X and 39X [331.70-333.64 mbsf]). Rare to few and moderately preserved calcareous nannofossils were observed in samples in a few intervals (e.g., between Samples 204-1244C-8H-CC and 11H-CC [71.6-100.64 mbsf], 26X-CC and 29X-CC [214.31-244.54 mbsf], and 35X and 37X [302.70-314.89 mbsf]). The calcareous nannofossil biostratigraphy determined for Site 1244 was mainly based on the nannofossil record deduced from these samples (Table T3).
A Pliocene-Quaternary calcareous nannofossil assemblage was found in the 333-m sequence from Hole 1244C. This assemblage is characterized by dominance of cold-water species (e.g., Coccolithus pelagicus and Gephyrocapsa carribeanica) and by the common to rare presence of cosmopolitan species, such as several small-sized Gephyrocapsa species. Warm-water species (e.g., species of Discoaster) are absent in the assemblage. For the same reason, many Pliocene and Quaternary age-diagnostic species (e.g., various Discoaster species, Sphenolithus abies, Helicosphaera sellii, and Reticulofenestra asanoi) are absent too.
The poor preservation of calcareous nannofossils in sediments and the absence of a number of age-diagnostic species together made the biostratigraphic determination based on calcareous nannofossils very difficult. However, several Pliocene and Quaternary age diagnostic species, such as Emiliania huxleyi, Pseudoemiliania lacunosa, and Calcidiscus macintyrei, are rarely present in relatively good nannofossil samples from Hole 1244C, allowing us to recognize a few nannofossil events.
Since calcareous nannofossils are barren or found in trace amounts in sediments from the interval from 15.46 to 62.59 mbsf, the presence of E. huxleyi was only observed in Samples 204-1244C-1H-CC and 8H-CC, and this interval was assigned to the Zone NN21. Sample 204-1244C-9H-CC contains a few Gephyrocapsa species but lacks E. huxleyi and P. lacunosa; this sample was assigned to Zone NN20. The LO of P. lacunosa was found in Sample 204-1244C-10H-CC (91.13 mbsf), which marks the NN19b/NN20 zonal boundary. Sediments are barren or have trace abundances of calcareous nannofossils below 91.13 down to 187.26 mbsf. The dominance of various small Gephyrocapsa spp. in the calcareous nannofossil assemblage was observed only in one sample (Sample 204-1244C-24X-CC [195.95 mbsf]) that contains rare calcareous nannofossils. This sample can be definitely assigned to a part of the mid-Pleistocene small Gephyrocapsa spp. Acme Zone (1.0-1.2 Ma). However, considering that a ~100-m interval above 195.95 mbsf lacks or contains poor nannofossils, it was not possible to accurately assign this sample to the top or to the bottom of the small Gephyrocapsa spp. Acme Zone, so an age interval of 1.0 to ~1.2 Ma was estimated for this sample (Table T3). C. macintyrei is frequently present in samples that contain rare and well-preserved calcareous nannofossils from 214.31 to 312.32 mbsf. The LO of C. macintyrei in Sample 204-1244C-26X-CC (214.31 mbsf) indicates an age of 1.59 Ma (NN19) for this stratigraphic level. Common Gephyrocapsa lumina is present continuously from 214.31 down to 235.25 mbsf; the FO of G. lumina (1.67 Ma) was placed at the level of 235.25 mbsf.
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 204-1244C-29X-CC (244.54 mbsf) in the 333-m sequence of Hole 1244C, indicating a Quaternary age for sediments at 244.54 mbsf. Based on this estimate, the Pliocene/Pleistocene boundary (1.77 Ma) was placed just below 244.54 mbsf.
Samples below 244.54 mbsf are generally barren of calcareous nannofossils, except for three samples (Samples 204-1244C-35X-CC, 36X-CC, and 37X-CC), which yielded rare and moderately preserved calcareous nannofossils. The calcareous nannofossil assemblage found in these samples is composed of C. pelagicus, P. lacunosa, Reticulofenestra productus, Reticulofenestra minuta, and Reticulofenestra minutula, indicating these samples are Pliocene in age. This assemblage lacks diagnostic species of Pliocene (e.g., a number of Discoaster and Sphenolithus species), which are warm-water species and absent in high latitudes and cold-current regions. The absence of these age-diagnostic species leads to difficulty in subdividing the lower part of the sequence in Hole 1244 by means of calcareous nannofossil events.
Reticulofenestra ampla was found in Samples 204-1244C-36X-CC and 37X-CC (314.89-312.34 mbsf); thus, the LO of R. ampla (2.78 Ma) was placed at 314.89 mbsf.
Discoaster spp. are absent in almost the whole sequence at Hole 1244C. However, a few Discoaster variabilis are present in Sample 204-1244C-37X-CC (314.89 mbsf). According to Perch-Nielsen's study (1985), D. variabilis disappears in the lower upper Pliocene Zone NN16; however, discoasters disappear before the LO of Reticulofenestra pseudoumbilicus in high latitudes. Furthermore, a few specimens of an the lower Pliocene event marker, R. pseudoumbilicus, are also present in Sample 204-1244C-37X-CC; the LO of R. pseudoumbilicus (3.82 Ma) marks the top of the lower Pliocene Zone NN15 (Martini and Müller, 1986). Unfortunately, the two samples below 314.89 mbsf are barren of calcareous nannofossils, and we were unable to track down the presence of this species in these deeper sediments. There are two possibilities for interpreting the presence of D. variabilis and R. pseudoumbilicus at 314.89 mbsf—either assigning sediments at 314.89 to the lower Pliocene Zone NN15 or considering them as reworked fossils in upper Pliocene sediments. There is a lack of evidence to support either of these possibilities based on the nannofossil data obtained during the cruise.
However, based on the evidence derived from both the calcareous nannofossil assemblage and these age-diagnostic species, sediment at the bottom of Hole 1244C was roughly assigned to the lower Pliocene Zone NN15 and the early Pliocene Zone NN16.
Twelve microfossil (five diatom and seven nannofossil) events were recognized at Site 1244 (Table T3). The age-depth plot shows that biohorizons from these two fossil groups generally agree well with each other (Fig. F13). The Pleistocene/Pliocene boundary is located between 244.54 and 253.50 mbsf based on the FO of medium Gephyrocapsa spp. (Table T3). The age of the bottom of Hole 1244C is estimated to be 2.4-2.7 Ma (based on diatoms) and is estimated to be older than 2.8 Ma (based on nannofossils). Both estimations indicate that the bottom of Hole 1244C is of late Pliocene age, although some small discrepancies regarding the age remain.
The linear sedimentation rates at Site 1244 were calculated based on biostratigraphic data (Table T3). The sedimentation rates are estimated to be 27 cm/k.y. for the interval from the top of Hole 1244C to 80 mbsf, 6 cm/k.y. for the interval from 80 to 90 mbsf, 19 cm/k.y. for the interval from 90 to 200 mbsf, 7 cm/k.y. for the interval from 200 to 240 mbsf, 12 cm/k.y. for the interval from 240 to 290 mbsf, and 7 cm k.y. for the interval 290 mbsf to the bottom of Hole 1244C (Fig. F13). The sedimentation rate is generally higher above than below 200 mbsf.