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doi:10.2204/iodp.proc.346.108.2015

Biostratigraphy

In Hole U1427A, a ~548 m thick succession of Pleistocene sediment was recovered. Throughout the entire succession, nannofossils are generally common to abundant and exhibit moderate to good preservation. Six calcareous nannofossil datums were documented. Planktonic foraminifers are generally abundant, except for a few barren horizons (through 210.7–228.7 m CSF-A) and rare occurrences deeper than 511.8 m CSF-A. Planktonic foraminiferal assemblages are characteristic of temperate to subarctic environments with intermittent incursions of subtropical species. Benthic foraminifers are generally abundant and moderately to well preserved, except for a few samples which are either barren or contain impoverished assemblages. The overall composition of assemblages indicates shelf to upper slope paleodepths. However, downcore changes in assemblage composition reflect changing paleoenvironmental conditions, probably related to distinct phases of Pleistocene climate evolution. In particular, alternating peak abundances of Cassidulina and Uvigerina in the upper part of the succession (to 308.9 m CSF-A) suggest marked glacial–interglacial changes in surface productivity and bottom water ventilation following inception of high-amplitude 100 k.y. climatic cycles after ~0.9 Ma. Ostracods are present and well preserved throughout the succession and show highest abundance peaks deeper than ~346 m CSF-A in Hole U1427A. Radiolarians are generally rare throughout the section, with the exception of the base of Hole U1427A (547.9 m CSF-A) where they are abundant. Four biostratigraphic markers are documented in Hole U1427A from the Stylatractus universus Zone (Middle Pleistocene) and the Botryostrobus aquilonaris Zone (Late Pleistocene). Diatoms are generally abundant and exhibit excellent preservation throughout the succession. No biostratigraphically useful marker species were recorded. High abundances of Chaetoceros spores and Paralia sulcata throughout the succession indicate a productive coastal environment. Freshwater diatom species differing from those at previous sites were also documented. The presence of Fragilaria (benthic) and Cyclotella (planktonic) specimens indicates running waters and/or lake origin. Phytoliths were also observed in all samples. The radiolarian, calcareous nannofossil, and planktonic foraminifer datums generally agree with only minor inconsistencies. The integrated calcareous and siliceous microfossil biozonation is shown in Figure F14 with microfossil datums presented in Table T4. A biostratigraphic age-depth plot is shown in Figure F15. See “Stratigraphic correlation and sedimentation rates” for a discussion of sedimentation rates at Site U1427.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on the analysis of 87 core catcher and 26 split-core section samples from Hole U1427A. Nannofossils are present throughout the Pleistocene succession with only minor barren intervals (Table T5; Fig. F16). Nannofossils are generally common to abundant and preservation is mostly moderate to good (Table T5).

Nannofossil diversity at Site U1427 is higher than at IODP Sites U1422–U1426. The nannofossil assemblage consists of 28 taxa, including Braarudosphaera bigelowii, Calcidiscus leptoporus, Calcidiscus macintyrei, Coccolithus pelagicus, Dictyococcites spp., Emiliania huxleyi, Florisphaera profunda, Gephyrocapsa caribbeanica, Gephyrocapsa margerelii/muellerae, Gephyrocapsa oceanica, Gephyrocapsa omega, Gephyrocapsa spp. (>4 µm), Gephyrocapsa spp. large (>5.5 µm), Gephyrocapsa spp. (<4 µm), Helicosphaera carteri, Helicosphaera sellii, Helicosphaera spp., Pontosphaera japonica, Pontosphaera spp., Pseudoemiliania lacunosa, Reticulofenestra asanoi, Reticulofenestra “small,” Reticulofenestra spp., Rhabdosphaera clavigera, Syracosphaera spp., and Umbilicosphaera sibogae. Specimens of Reticulofenestra <5 µm are referred to as Reticulofenestra “small.” Medium-sized specimens of Reticulofenestra (5–7 µm), excluding R. asanoi, are referred to as Reticulofenestra spp. in Table T5. The only specimen of Coronocyclus nitescens found in Sample 346-U1427A-51H-5W, 75 cm (351.5 m CSF-A), is documented as a reworked species (last occurrence [LO] at 12.12 Ma). The documented occurrences of C. macintyrei are tentative, as individuals were rare, confined to only one sample, and generally <10 µm.

Nannofossil Zones CN15/NN21 through CN14a/NN19 are recognized (Fig. F14) based on the first occurrence (FO) of E. huxleyi, the LO of P. lacunosa, and the presence of G. oceanica and G. caribbeanica. Additionally, the LO of R. asanoi at 347.0 m CSF-A (Sample 346-U1427A-51H-1W, 75 cm), the base common occurrence (Bc) of R. asanoi at 426.6 m CSF-A (Sample 67H-CC), the LO of Gephyrocapsa spp. large (>5.5 µm) at 482.0 m CSF-A (Sample 80H-1W, 23 cm), and the LO of H. sellii at 484.6 m CSF-A (Sample 80H-3W, 80 cm) (Fig. F17) provide good age control for the Pleistocene sequence.

Radiolarians

A total of 59 core catcher samples from Hole U1427A were prepared for radiolarian analyses. Radiolarians are generally rare throughout the section (Table T6; Fig. F16), with the exception of the base of the hole, where radiolarians are abundant (Sample 346-U1427A-87X-CC; 547.9 m CSF-A).

Four biostratigraphic markers were found in Hole U1427A (Table T4), including the S. universus (Middle Pleistocene) and B. aquilonaris (Late Pleistocene) Zones (Fig. F14). Sample 346-U1427A-1H-CC (1.7 m CSF-A) is dominated by the Larcopyle polyacantha group (including Larcopyle buetschlii), belonging to the L. buetschlii Interval Biozone (0.01 Ma to recent) of Itaki and Ikehara (2003) of Holocene age. Pleistocene datums include the LOs of Lychnocanoma sakaii (0.05 Ma) at 21.4 m CSF-A (Sample 3H-CC), Amphimelissa setosa (0.08 Ma) at 50.0 m CSF-A (Sample 6H-CC), and Spongodiscus sp. (0.29 Ma) at 97.0 m CSF-A (Sample 11H-CC). According to Sakai (1980), the FO of Spongodiscus sp. is close to the top of the Eucyrtidium matuyamai Zone in the North Pacific. Based on the absence of E. matuyamai and presence of Spongodiscus sp. at 528.1 m CSF-A (Sample 85X-CC), the lower part of Hole U1427A can be placed in the S. universus Zone.

In Hole U1427A, the abundance of warm-water taxa such as Dictyocoryne profunda, Dictyocoryne truncatum, Didymocyrtis tetrathalamus, Euchitonia furcata, Hymeniastrum euclidis, and the Octopyle/Tetrapyle group are much higher compared to Sites U1422–U1426. Because these warm-water radiolarians migrate from the East China Sea to the marginal sea with the TWC, their higher abundances are likely associated with interglacial highstands. In contrast, Stylochlamydium venustum, Ceratospyris borealis, A. setosa, and Spongodiscus sp. are characteristic cold-water species typical of glacial periods (Itaki et al., 2007). These taxa frequently occur in this hole.

The “Tr” value, which is a simple temperature index based on radiolarians present in the North Pacific Ocean by Nigrini (1970), was calculated in Hole U1427A. In this report, it is defined by

where W and C are cumulative ranks of abundance for warm- and cold-water species, respectively. Ranks of each species are

• 5 = abundant (A).
• 4 = common (C).
• 3 = few (F).
• 2 = rare (R).
• 1 = present (P).
• 0 = absent (B).

In Hole U1427A, the Tr value fluctuates significantly through the section, with a similar trend to the sediment color index b* (e.g., high Tr values coincide with high b* values) (Fig. F18). As Tr is closely related to the interglacial inflow of the TWC, higher b* values most likely correlate with interglacial intervals of the marine isotope curve (Lisiecki and Raymo, 2005). This probable correlation is broadly consistent with biostratigraphic datums in Hole U1427A (Fig. F15).

Diatoms

Diatom biostratigraphy was based on smear slides from core catcher samples. A total of 87 core catcher samples were examined. Because no biostratigraphically useful marker species were found, age assignments were not possible. Although rare abundances of Neodenticula koizumii (one of the biostratigraphic markers) were found (Table T7), these are reworked and were not considered. Rare, reworked specimens from other species were also found throughout the entire succession. However, they represent <0.5% of the assemblage, indicating that the ecological significance of the diatom associations can still be used. Overall, the preservation of the diatoms is excellent, with a considerable amount of specimens still preserving their organic matter (Fig. F19).

Overall, diatom abundance is commonly high (20%–60%) through the succession. Abundance >60% is also common (Figs. F16, F20; Table T7). The high abundances (20%–60%) of Chaetoceros spores and P. sulcata (Fig. F20; Table T7) indicate a productive coastal environment. Diatom freshwater species are present in the succession. However, the species found here, benthic specimens from the genus Fragilaria and planktonic specimens from the genus Cyclotella, were not found at the previous sites. Phytoliths were found, with no particular pattern of distribution, in all observed samples.

Planktonic foraminifers

Planktonic foraminifers were examined in core catcher samples from Hole U1427A (87 samples). Relative abundance of taxa and visual estimates of assemblage preservation are presented in Table T8. Planktonic foraminifers are generally abundant, except for a few barren horizons (Samples 346-U1427A-23H-CC through 25H-CC) and rare occurrences deeper than Sample 84X-CC (511.8 m CSF-A) (Fig. F16). Planktonic foraminifers are found only in the finer size fraction (63–150 µm) in Sample 19H-CC (172.9 m CSF-A). To assess assemblage composition and variability, all specimens from the >150 µm fraction were picked and transferred to slides for identification and counting. Overall, preservation is moderate to good throughout the succession, becoming poor in samples where planktonic foraminifers are rare.

Planktonic foraminiferal assemblages are characteristic of temperate to subarctic environments with intermittent incursions of subtropical species. The assemblages mainly consist of Globigerina bulloides, Neogloboquadrina pachyderma (sinistral and dextral), and the Neogloboquadrina kagaensis group (N. kagaensis and Neogloboquadrina inglei) with rare occurrences of Globigerina umbilicata, Globigerina quinqueloba, Globigerinita glutinata, Globigerinoides ruber, Globorotalia inflata, Globorotalia praeinflata, Globorotalia menardii, Globoturborotalita woodi, Neogloboquadrina dutertrei (= Neogloboquadrina himiensis), Neogloboquadrina humerosa, and Pulleniatina obliquiloculata. The subtropical species G. ruber and P. obliquiloculata are rare (Table T8). G. ruber co-occurs with abundant dextral N. pachyderma in Sample 346-U1427A-43H-CC (313.1 m CSF-A). The presence of these subtropical species suggests intermittent warm ocean conditions during sea level highstands, when the Tsushima Strait remains open (Fig. F21).

G. bulloides shows a marked increase in abundance from Sample 346-U1427A-51H-CC to the top of the section (Fig. F21) associated with sporadic peaks in G. quinqueloba (in Samples 45H-CC, 8H-CC, and 7H-CC) (Fig. F21; Table T8). This pattern may be related to increasing (yet variable) upwelling and cooling conditions from the Middle to Late Pleistocene.

The top of the high abundance interval of G. inflata in Sample 346-U1427A-82X-CC (491.9 m CSF-A) indicates an age older than 1.24–1.34 Ma. The coiling direction of N. pachyderma changes from dextral to sinistral between Samples 78H-CC (477.4 m CSF-A) and 65H-CC (417.0 m CSF-A). This event marks the Zone PF7/PF8 boundary in the regional zonation for the marginal sea (Maiya, 1978). The LO of the N. kagaensis group (0.7 Ma) (Kucera and Kennett, 2000) is in Sample 29H-CC (246.3 m CSF-A). The species G. ruber (pink), which ranges from 0.12 to 0.4 Ma in the Pacific and Indian Oceans, is only present in Sample 5H-CC (39.5 m CSF-A).

Benthic foraminifers

Benthic foraminifers were examined in core catcher samples from Hole U1427A (87 samples). Samples with an average volume of ~20 cm³ were processed from all core catchers to obtain quantitative estimates of benthic foraminiferal distribution patterns downhole. The mudline sample recovered in Hole U1427A was also investigated. To assess assemblage composition and variability, ~100 specimens from the >150 µm fraction were picked and transferred to slides for identification and counting. The presence and distribution of benthic foraminifers was additionally checked in the 63–150 µm fraction to ensure that assemblages in the >150 µm fraction were representative and that small species such as phytodetritus feeders or small infaunal taxa were not overlooked.

Benthic foraminifers are generally abundant and moderately to well preserved throughout the ~548 m thick Pleistocene succession recovered in Hole U1427A (Figs. F16, F22; Table T9). However, Samples 346-U1427A-23H-CC through 25H-CC (210.7–228.7 m CSF-A), which probably correspond to marine isotope Stage (MIS) 16 following a tentative correlation of color reflectance b* data to the benthic δ¹⁸O stack of Lisiecki and Raymo (2005), are barren. Samples 15H-CC (135.2 m CSF-A), 17H-CC (154.1 m CSF-A), and 45H-CC (323.0 m CSF-A) contain only impoverished assemblages (Figs. F16, F22).

A total of 64 benthic foraminiferal taxa were identified. Table T9 summarizes the downcore distribution of benthic foraminifers in core catcher samples from Hole U1427A. Figure F23 illustrates characteristic taxa. The overall composition of assemblages indicates shelf to upper slope paleodepths throughout the Pleistocene. Species commonly recorded through the succession include the calcareous species Bolivina pacifica, Cassidulina japonica, Cassidulina norcrossi, Cibicidoides lobatulus, Epistominella pulchella, Nonionellina labradoricum, Trifarina angulosa (= Angulogerina kokozuraensis of Kato, 1992), Uvigerina peregrina, and Uvigerina yabei, which typically indicate enhanced organic flux and/or dysoxic conditions at the seafloor and within the uppermost few centimeters of the sediment (Gooday, 1993; Jorissen et al., 1995, 2007; Jorissen, 1999). High abundance of Globobulimina pacifica is also recorded in Samples 346-U1427A-7H-CC through 11H-CC (59.12–97.04 m CSF-A), and Elphidium sp. occurs frequently in core catcher samples deeper than ~480 m CSF-A.

Downcore changes in assemblage composition at Site U1427 reflect changes in paleoenvironmental conditions that are probably linked to distinct phases of Pleistocene climate evolution. Alternating peak abundances of Cassidulina and Uvigerina in the upper part of the succession (Samples 346-U1427A-1H-CC through 42H-CC; 1.74–308.9 m CSF-A) suggests elevated but highly fluctuating food fluxes throughout the Late to Middle Pleistocene (~0.9–0 Ma). Cassidulina thrives on pulsed phytodetritus fluxes (Gooday, 1993), whereas Uvigerina is a high-productivity indicator tolerant of slightly dysoxic conditions (Jorissen et al., 1995, 2007; Jorissen, 1999). These fluctuations probably relate to major glacial–interglacial changes in surface productivity and bottom water ventilation following inception of high-amplitude climatic cycles after ~0.9 Ma. Higher frequency variability associated with millennial-scale climate events may be identified in higher resolution studies but is not detected in core catcher samples because of the coarse shipboard sampling resolution. In contrast, within the interval ~313–490 m CSF-A (Samples 43H-CC through 81H-CC, early Pleistocene) assemblages are generally dominated by Uvigerina and contain fewer Cassidulina, suggesting more consistent high-productivity conditions. Finally, samples deeper than ~490 m CSF-A (Samples 82H-CC through 87H-CC) contain higher abundances of Elphidium and low numbers of planktonic foraminifers, indicating a more marginal shelf environment.

Ostracods

Ostracods were also examined during shipboard preparation of benthic foraminifer samples. The mudline sample recovered in Hole U1427A was also investigated. All specimens present in the >150 µm fraction were picked and transferred to slides for identification and counting. Ostracod abundance varies markedly downhole, ranging from absent to 104 specimens per sample. The highest abundance peaks are observed deeper than ~346 m CSF-A in Hole U1427A. Abundance maxima are observed at 59.1, 271.5, 346.8, 365.7, 384.7, 449.9, and 487.0 m CSF-A (Figs. F16, F22; Table T10). When compared to the downhole record of the color reflectance parameter b*, which can be used to infer glacial–interglacial intervals (see “Benthic foraminifers”), abundance maxima seem to correspond to higher b* values, thus interglacial periods. In contrast, periods of ostracod abundance minima and barren intervals correspond to lowest b* values and presumably glacial conditions (Figs. F16, F22). The taxonomic composition of the ostracod assemblage and its distribution throughout the cored interval could not be established on the ship because of time limitations and was deferred to postexpedition studies. However, the main species found at Site U1427 are illustrated in Figure F24.

Mudline samples

The mudline sample from Hole U1427A was gently washed in order to preserve fragile, agglutinated foraminifer specimens with extremely low fossilization potential. The mudline sample contains a few organically cemented agglutinated species including Ammodiscus anguillae, Haplophragmoides sphaeriloculum, Miliammina echigoensis, and Reophax scorpiurus as well as abundant specimens of the calcareous taxa Bolivina decussata, B. pacifica, C. norcrossi, Epistominella sp., Fursenkoina bradyi, G. pacifica, T. angulosa, Triloculina sp., and Uvigerina sp. (Figs. F25, F26). The dominance of small (<150 µm) bolivinids and fursenkoinids in the mudline assemblage points to a high organic export flux and strongly dysoxic conditions at the seafloor and upper centimeters of the sediment.

The mudline sample from Hole U1427A also contains abundant and well-preserved diatoms. Calcareous nannofossils found in the mudline sample exhibit moderate to good preservation. Specimens of C. leptoporus, C. pelagicus, F. profunda, G. oceanica, Gephyrocapsa spp. (>4 µm), Gephyrocapsa spp. large (>5.5 µm), and Gephyrocapsa spp. (<4 µm) were identified via light microscope and scanning electron microscope. Presumably in situ large Gephyrocapsa spp. with maximum diameters >5.5 µm are present in the mudline sample. This is noteworthy because their last occurrence is documented at 1.24 Ma by Gradstein et al. (2012).

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