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doi:10.2204/iodp.proc.346.107.2015

Biostratigraphy

At Site U1426, a ~397 m thick succession of Pliocene–Holocene sediment was recovered. Nannofossils are present shallower than 182.3 m CSF-A but are absent or rare deeper. Nannofossil Zones CN15/NN21 through CN13b/NN19 are recognized. Planktonic foraminifers are mainly confined to the upper part of the succession (shallower than ~172 m CSF-A), exhibiting moderate to poor preservation from ~172 to ~170 m CSF-A and good preservation shallower than ~99 m CSF-A. Planktonic foraminiferal assemblages shallower than ~172 m CSF-A generally indicate temperate to subarctic environments with intermittent incursions of subtropical species. The planktonic foraminiferal biostratigraphy spans Zones PF6 (Pliocene) through PF9 (Late Pleistocene). Benthic foraminifers occur intermittently throughout the Pliocene to Pleistocene succession, showing marked variations in abundance and preservation. The overall assemblage composition indicates bathyal paleodepths. The highly variable composition of the assemblages suggests fluctuating organic fluxes to the seafloor with episodic oxygen depletion and intense carbonate dissolution, particularly during the Pliocene. Radiolarians are generally common to abundant in the sequence, although they are rare or absent deeper than ~289 m CSF-A. The radiolarian biostratigraphy spans the Larcopyle pylomaticus Zone (Pliocene) through the Botryostrobus aquilonaris Zone (Late Pleistocene). Diatom preservation is good throughout the succession. Nevertheless, extreme diatom dissolution occurs deeper than 392.1 m CSF-A. Overall, diatom abundances are high through the succession and contain few/rare warm-water diatom species (Azpeitia nodulifera, Fragilariopsis doliolus, and Thalassiosira leptopus). High abundances of Chaetoceros spores and Paralia sulcata throughout the succession indicate a productive coastal environment. The diatom biostratigraphy spans the interval from Zones NPD 7B (Pliocene) to NPD 12 (Late Pleistocene). Twenty six datums, including 6 from calcareous nannofossils, 4 from planktonic foraminifers, 10 from radiolarians, and 6 from diatoms, are documented at this site. The nannofossil, planktonic foraminifer, radiolarian, and diatom datums generally agree with only some minor inconsistencies. The integrated calcareous and siliceous microfossil biozonation is shown in Figure F19, and microfossil datums are listed in Table T4. A biostratigraphic age-depth plot is provided in Figure F20. See “Stratigraphic correlation and sedimentation rates” for a discussion of sedimentation rates at Site U1426.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on the analysis of core catcher and split-core section samples from Holes U1426A and U1426B. Sixty of the 103 samples studied contain nannofossils (Table T5).

Nannofossils are present in Pleistocene sediment shallower than 182.3 m CSF-A in Hole U1426A with sporadic barren intervals throughout (Table T5; Fig. F21). Samples are generally barren deeper than 182.3 m CSF-A, with the exception of sporadic occurrences in core catcher samples from Cores 346-U1426A-37H, 40H, 41H, and 42H (between 298.4 and 317.3 m CSF-A). Preservation is generally moderate to good in Hole U1426A, with the best preservation occurring within the Pleistocene (Table T5). Only the uppermost four core catcher samples (i.e., shallower than 35.2 m CSF-A) from Hole U1426B were studied. Nannofossil preservation is moderate to good, and the assemblages are comparable to those documented in the upper portion of Hole U1426A. Marked variations in nannofossil abundances occur through the uppermost ~182 m of the succession.

Nannofossil diversity at Site U1426 is higher than at previous Sites U1422–U1425. The nannofossil assemblage consists of 23 taxa, including Braarudosphaera bigelowii, Calcidiscus leptoporus, Calcidiscus macintyrei, Coccolithus pelagicus, Dictyococcites spp., Emiliania huxleyi, Florisphaera profunda, Gephyrocapsa caribbeanica, Gephyrocapsa margerelii/muellerae, Gephyrocapsa oceanica, Gephyrocapsa spp. (>4 µm), Gephyrocapsa spp. large (>5.5 µm), Gephyrocapsa spp. (<4 µm), Helicosphaera carteri, Helicosphaera sellii, Pontosphaera japonica, Pontosphaera spp., Pseudoemiliania lacunosa, Reticulofenestra asanoi, Reticulofenestra minuta, Reticulofenestra minutula, Reticulofenestra spp., and Umbilicosphaera sibogae.

Nannofossil Zones CN15/NN21 through CN13b/NN19 are recognized (Fig. F19) based on the first occurrence (FO) of E. huxleyi, the last occurrence (LO) of P. lacunosa, and the FOs of G. oceanica and G. caribbeanica. Additionally, the LOs of R. asanoi at 78.8 m CSF-A (Sample 346-U1426A-9H-CC), H. sellii at 117.7 m CSF-A (Sample 13H-CC), and C. macintyrei at 173.7 m CSF-A (Sample 20H-2W, 75 cm) with the presence of G. oceanica spp. large (>5.5 µm) from 117.3 to 117.7 m CSF-A (Samples 13H-CC to 14H-1W, 75 cm) provide good age control for the Pleistocene sequence. The pervasiveness of barren or sparse samples deeper than 182.3 m CSF-A (Sample 20H-CC) prevents further zonal assignments.

Radiolarians

A total of 46 core catcher samples from Hole U1426A were prepared for radiolarian analysis. Radiolarians are generally common to abundant shallower than 289.2 m CSF-A (Sample 346-U1426A-35H-CC), although they decrease deeper than this depth (Table T6; Fig. F21).

Ten biostratigraphic markers were found in Hole U1426A (Table T4). These span the L. pylomaticus Zone (Pliocene) through the B. aquilonaris Zone (Late Pleistocene) (Fig. F19). Based on the dominance of Stylochlamydium venustum, Sample 346-U1426A-1H-CC (2.4 m CSF-A) belongs to the S. venustum Interval Biozone (0.012–0.015 Ma) of Itaki and Ikehara (2003), equivalent to the last deglaciation. Pleistocene datums include the LOs of Amphimelissa setosa (0.08 Ma) and Spongodiscus sp. (0.29 Ma) at 12.1 m CSF-A (Sample 346-U1426A-2H-CC) and 31.6 m CSF-A (Sample 4H-CC), respectively. Eucyrtidium matuyamai is present at 117.7 m CSF-A (Sample 13H-CC), corresponding to the E. matuyamai Zone (1.03–1.98 Ma). The LO of Axoprunum acquilonium (1.2–1.7 Ma) is at 182.3 m CSF-A (Sample 20H-CC). Sample 22H-CC (201.2 m CSF-A) contains a single specimen of Lamprocyrtis heteroporos that suggests that it is older than 1.98 Ma. The Pleistocene/Pliocene boundary is close to the FO of Cycladophora davisiana (2.7 Ma) at 261.2 m CSF-A (Sample 29H-CC) and the LO of Hexacontium parviakitaensis (2.7 Ma) at 265.8 m CSF-A (Sample 30H-CC). The FO of H. parviakitaensis (3.9–4.3 Ma) occurs at 378.1 m CSF-A (Sample 55H-CC). The Siphocampe arachnea group is abundant from 382.7 (Sample 56H-CC) to 387.4 m CSF-A (Sample 56H-CC), suggesting that their acme zone is between 4.46 and 4.71 Ma.

Warm-water assemblages, including common Dictyocoryne profunda, Dictyocoryne truncatum, Didymocyrtis tetrathalamus, Euchitonia furcata, and the Octopyle/Tetrapyle group, are intermittent shallower than 172.4 m CSF-A (Sample 346-U1426A-19H-CC), suggesting interglacial inflow of the TWC during the early to Late Pleistocene. Sporadic occurrences of these species between 182.3 (Sample 20H-CC) and 349.8 m CSF-A (Sample 49H-CC) suggest reduced influence of the TWC during the late Pliocene to early Pleistocene.

Diatoms

Diatom biostratigraphy was based on smear slides from core catcher samples. Fifty seven core catcher samples were examined, and six datums were identified (Tables T4, T7). The LO of Neodenticula koizumii (2.0 Ma) at 127.17 m CSF-A (Sample 346-U1426A-14H-CC) marks the boundary between the base of Zone NPD 10 and the top of Zone NPD 9 (Yanagisawa and Akiba, 1998). The FO of Neodenticula seminae (2.4 Ma) in Sample 29H-CC (261.18 m CSF-A) and the LO of Neodenticula kamtschatica (2.6–2.7 Ma) in Sample 33H-CC (279.912 m CSF-A) mark the boundary between the base of Zone NPD 9 and the top of Zone NPD 8 (Yanagisawa and Akiba, 1998). The FO of N. koizumii (3.4–3.9 Ma) at 312.53 m CSF-A (Sample 41H-CC) defines the boundary between Zones NPD 8 and NPD 7Bb (Yanagisawa and Akiba, 1998). The LO of Thalassiosira temperei (5.44 Ma) in Sample 48H-CC and the FO of Shionodiscus oestrupii (5.56 Ma) at 359.3 m CSF-A (Sample 51H-CC) mark the boundary between Zones NPD 7Bb and NPD 7Ba (Yanagisawa and Akiba, 1998).

Diatom preservation is good throughout the succession. Nevertheless, extreme diatom dissolution occurs deeper than 392.1 m CSF-A. This dissolution most likely reflects the opal-A/opal-CT boundary.

Overall, diatom abundances are high (20%–60% and >60%) through the succession (Fig. F21; Table T7). At this site, rare (<2%) to few (2%–5%) abundances of warmer water diatom species (A. nodulifera, F. doliolus, and T. leptopus) are also present (Table T7). The high abundances (from 20% to 60%) of Chaetoceros spores and P. sulcata indicate a productive coastal environment. Diatoms are also present in the foraminifer residue >63 and >150 µm (see “Benthic foraminifers” for more details).

Planktonic foraminifers

Planktonic foraminifers were examined in core catcher samples from Hole U1426A (57 samples). Relative abundance of taxa and visual estimates of assemblage preservation are presented in Table T8. Planktonic foraminifers are mainly confined to the upper part of the succession (Sample 346-U1426A-19H-CC; 172.4 m CSF-A and shallower) (Fig. F21). They are generally abundant, except for one barren horizon (Sample 3H-CC; 22.1 m CSF-A) and rare occurrences in Samples 10H-CC (88.8 m CSF-A) and 16H-CC (145.3 m CSF-A). Sample 20H-CC (182.3 m CSF-A) and deeper is mainly barren (Table T8; Fig. F21), except for Samples 37H-CC (298.4 m CSF-A) and 40H-CC (307.8 m CSF-A), which contain relatively abundant assemblages. Preservation is good shallower than 98.72 m CSF-A (Sample 11H-CC), becoming moderate to poor deeper than 107.3 m CSF-A (Sample 12H-CC) because of frequent fragmentation and pyritization. Partial dissolution and recrystallization of foraminiferal tests are most prominent in Sample 58H-CC (392.1 m CSF-A).

Planktonic foraminiferal assemblages shallower than Sample 346-U1426A-19H-CC (172.4 m CSF-A) are characteristic of temperate to subarctic environments with intermittent incursions of subtropical species. They mainly consist of Globigerina bulloides, Neogloboquadrina pachyderma (sinistral and dextral), and the Neogloboquadrina kagaensis group (N. kagaensis and N. inglei) with rare occurrences of Globigerina umbilicata, Globigerina quinqueloba, Globigerinita glutinata, Globigerinoides ruber, Globigerinoides sacculifer, Globorotalia inflata, Globorotalia praeinflata, Globorotalia menardii, Globoturborotalita woodi, Neogloboquadrina dutertrei (= Neogloboquadrina himiensis), Neogloboquadrina incompta, Orbulina bilobata, Orbulina universa, and Pulleniatina obliquiloculata. The subtropical species G. ruber and G. sacculifer occur in Sample 4H-CC (31.6 m CSF-A) but are rare deeper than this sample. The species G. ruber (pink), which ranges from 0.12 to 0.4 Ma in the Pacific and Indian Oceans, was also identified in this sample. The LO of the N. kagaensis group, which is observed at 0.7 Ma (Kucera and Kennett, 2000), is recorded in Sample 8H-CC (69.0 m CSF-A). The change in coiling direction of N. pachyderma (from dextral to sinistral) occurs between Samples 12H-CC (107.32 m CSF-A) and 13H-CC (117.7 m CSF-A), indicating the boundary between Zones PF8 and PF7 in the regional zonation for this marginal sea (Maiya, 1978). The FO of G. praeinflata in Sample 40H-CC (307.8 m CSF-A) indicates an age younger than 3.3 Ma (Lyle, Koizumi, Richter, et al., 1997). However, the position of this datum could be deeper because samples deeper than this level are devoid of planktonic foraminifers. Based on this datum, the boundary between Zones PF7 and PF6 is tentatively placed at 310.1 m CSF-A between Samples 40H-CC and 41H-CC.

Benthic foraminifers

Benthic foraminifers were examined in core catcher samples from Hole U1426A (57 samples). Samples with an average volume of ~30 cm³ were processed from all core catchers to obtain quantitative estimates of benthic foraminiferal distribution patterns downhole. The mudline samples recovered in Holes U1426A and U1426B were also investigated. To assess assemblage composition and variability, all 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 vary substantially in abundance and preservation throughout the ~397 m thick Pliocene to Pleistocene succession recovered in Hole U1426A (Figs. F21, F22; Table T9). Benthic foraminifers are generally well preserved, abundant, and diverse within the Pleistocene interval shallower than Sample 346-U1426A-19H-CC (172.43 m CSF-A) (Fig. F22). Deeper in the succession, most samples are barren or consist of impoverished assemblages that are dominated by agglutinated species (Samples 19H-CC to 36H-CC [172.4–294 m CSF-A] and 43H-CC to 55H-CC [321.7–378.1 m CSF-A]). However, more diverse assemblages with poorly preserved calcareous foraminifers occur in Samples 22H-CC (201.3 m CSF-A), 24H-CC (218.6 m CSF-A), 37H-CC to 42H-CC (298–317 m CSF-A), 47H-CC (340.5 m CSF-A), and 56H-CC to 59H-CC (383–396.9 m CSF-A). The overall composition of assemblages at Site U1426 indicates middle bathyal paleodepths throughout the Pliocene to Pleistocene.

A total of 70 benthic foraminiferal taxa were identified. Table T9 summarizes the downcore distribution of benthic foraminifers in core catcher samples from Hole U1426A. Figure F23 illustrates characteristic taxa. Sample 346-U1426A-1H-CC contains 27 tests of Elphidium. This shallow-water dweller does not occur in the other core catcher samples, suggesting that tests may have been transported downslope. Species commonly recorded through the succession include Bolivina pacifica, Cassidulina japonica, Cassidulina norcrossi, Epistominella pulchella, Globobulimina pacifica, Trifarina angulosa (= Angulogerina kokozuraensis of Kato, 1992), 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). Peak abundances in Cassidulina and Uvigerina alternate in the upper part of the succession, suggesting elevated but fluctuating food fluxes throughout the Middle to Late Pleistocene. In contrast, the agglutinated species Martinotiella communis and Miliammina echigoensis dominate in the late Pliocene to early Pleistocene (between ~173 and 294 m CSF-A). Similar distribution patterns are apparent at Sites U1422–U1425, which may be related to pervasive carbonate dissolution in the basin during this warmer climate period. However, two intervals containing calcareous taxa, including E. pulchella, Cassidulina spp., G. pacifica, Melonis spp., and Uvigerina spp. (298–317 and 383–397 m CSF-A), suggest transient periods of improved ventilation at the seafloor and reduced carbonate dissolution during the Pliocene. Comparable horizons recorded at Sites U1423 and U1425 cannot, however, be correlated because of the low resolution of shipboard samples. Overall, the highly variable composition of assemblages at Site U1426 suggests enhanced organic flux to the seafloor with marked variations in oxygenation and intense carbonate dissolution, in particular during the Pliocene. As noted previously, moderately to well-preserved diatoms and radiolarians are common to abundant in residues >150 and >63 µm throughout the succession, becoming dominant deeper than ~227 m CSF-A.

Ostracods

Core catcher samples were also examined for the presence of ostracods during shipboard preparation of benthic foraminifer samples. Ostracods are rare at Site U1426. They were observed only occasionally and always in very low numbers (1–2) throughout the sediment succession in Hole U1426A (Samples 346-U1426A-1H-CC, 4H-CC to 6H-CC, 8H-CC, 13H-CC, 15H-CC, 17H-CC, 40H-CC, and 58H-CC). Ostracod preservation ranges from poor (fragmented and showing signs of dissolution) to good (translucent single valves or carapace) (Fig. F24). The following taxa are present at Site U1426: Krithe dolichodeira Bold, Acanthocythereis dunelmensis Norman, Legitimocythereis sp., Argilloecia lunata Frydl, Loxoconcha sp. A, Loxoconcha sp. B, Falsobuntonia sp., and Propontocypris sp.

Mudline samples

The mudline sample from Hole U1426A was gently washed in order to preserve fragile, agglutinated foraminifer specimens with extremely low fossilization potential. The mudline sample contains organically cemented agglutinated species including Haplophragmoides sphaeriloculum, Hyperammina elongata, M. echigoensis, Paratrochammina challengeri, and Reophax scorpiurus (Fig. F25) as well as calcareous taxa including B. pacifica, C. japonica, Cassidulina teretis, G. pacifica, Nummoloculina sp., Procerolagena gracillima, T. angulosa, and Uvigerina sp. Most of the agglutinated and calcareous tests were stained with rose bengal. Several specimens of H. elongata bore a small test of P. challengeri or Cribrostomoides subglobosus attached in life position (Fig. F26). Diatoms are abundant, but no ostracods were recorded in the mudline sample from Hole U1426A. The mudline sample from Hole U1426B contains rare poorly preserved calcareous nannofossils. Very few specimens of G. oceanica and small Gephyrocapsa (<4 µm) were observed.

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