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doi:10.2204/iodp.proc.346.104.2015 BiostratigraphyAt Site U1423, a ~250 m thick succession of Pliocene to Holocene sediment was recovered. Calcareous nannofossils are generally rare and sporadically distributed in the upper 80 m. Planktonic foraminifers are rare to absent with moderate to poor preservation throughout most of the succession but are abundant in the upper part of the succession above 74.19 m CSF-A. However, the regional zonal scheme only has limited application, as few planktonic foraminiferal datums were identified. Radiolarians are generally common to abundant with rare occurrences at 83.6 m CSF-A. The radiolarian biostratigraphic zonation ranges from the Larcopyle pylomaticus Zone (Pliocene) to the Botryostrobus aquilonaris Zone (Late Pleistocene). The diatoms are well preserved with abundances ranging from 2%–5% to >60%. Diatom mats and oozes are found in the Pliocene samples. The diatom stratigraphy spans the interval from Zone NPD 7 (Pliocene) to NPD 12 (Late Pleistocene). The nannofossil, radiolarian, and diatom datums and zonal schemes generally agree, with only minor inconsistencies. The integrated calcareous and siliceous microfossil biozonation is shown in Figure F15, and microfossil datums are shown in Table T4. A preliminary age-depth plot including biostratigraphic datums is shown in Figure F16. See “Stratigraphic correlation and sedimentation rates” for a discussion on sedimentation rates at Site U1423. Benthic foraminifers occur intermittently throughout the succession, exhibiting marked changes in abundance, preservation, and species distribution. The overall assemblage composition indicates bathyal paleodepths. However, the assemblage composition also reflects substantial variations in organic export flux to the seafloor and deepwater oxygenation. Calcareous nannofossilsCalcareous nannofossil biostratigraphy is based on the analysis of core catcher and split-core section samples from Holes U1423A–U1423C. Thirty of 76 samples studied at Site U1423 contain nannofossils (Table T5). Nannofossils are present in Pleistocene sediment from 26.8 to 76.06 m CSF-A in Hole U1423A (Samples 346-U1423A-4H-1, 50–51 cm, to 9H-2, 75–76 cm) and from 28.10 to 77.36 m CSF-A in Hole U1423B (Samples 346-U1423B-4H-4, 53–54 cm, to 9H-5, 75–76 cm). Nannofossils are absent in Hole U1423A between 83.60 and 207.02 m CSF-A (Samples 346-U1423A-9H-CC to 22H-CC), with the exception of a short interval from 142.78 to 150.24 m CSF-A (Samples 16H-2, 98–99 cm, to 16H-CC), where rare to abundant nannofossils are observed (Fig. F17). These samples have been disturbed by drilling operations (see “Lithostratigraphy”) and thus represent assemblages that are not in situ. Samples from Hole U1423B were barren below 82.87 m CSF-A (Sample 346-U1423B-10H-2, 130–131 cm) with the exception of a short interval containing rare nannofossils between 136.45 and 151.91 m CSF-A (Samples 16H-3, 85–86 cm, to 17H-CC). All samples from Hole U1423C are devoid of calcareous nannofossils. Nannofossil diversity at Site U1423 is low, although it is higher than at previous Site U1422. The nannofossil assemblage consists of relatively few taxa including Braarudosphaera bigelowii, Calcidiscus leptoporus, Calcidiscus macintyrei, Coccolithus pelagicus, Emiliania huxleyi, Gephyrocapsa caribbeanica, Gephyrocapsa muellerae, Gephyrocapsa oceanica, Gephyrocapsa spp. (>4 µm), Gephyrocapsa spp. (<4 µm), Helicosphaera carteri, Helicosphaera spp., Pontosphaera japonica, Pontosphaera multipora, Pontosphaera spp., Pseudoemiliania lacunosa, Reticulofenestra minuta, Reticulofenestra minutula, Reticulofenestra spp., and Syracosphaera spp. Helicosphaera sellii is present in the disturbed interval (Sample 346-U1423A-16H-2, 98–99 cm; 142.79 m CSF-A), and Cyclicargolithus floridanus is documented as a reworked species in Samples 6H-2, 94–95 cm, and 6H-CC. Preservation is generally poor to moderate with sporadic occurrences of good preservation in samples with high nannofossil abundances (e.g., Sample 4H-4, 92–93 cm; 31.68 m CSF-A), composed almost entirely of small Gephyrocapsa (<4 µm). Two nannofossil zones are recognized (Fig. F15). Nannofossil Zones CN15/NN21 are recognized based on the first occurrence (FO) of E. huxleyi (Sample 346-U1423B-2H-4, 69–70 cm; 9.29 m CSF-A), and the bases of nannofossil Zones CN14b/NN20 are recognized based on the last occurrence (LO) of P. lacunosa (Sample 346-U1423A-5H-1, 113–114 cm; 36.93 m CSF-A). RadiolariansA total of 29 core catcher samples from Holes U1423A (Samples 346-U1423A-1H-CC to 22H-CC) and U1423B (Samples 346-U1423B-22H-CC to 28H-CC) were prepared for radiolarian analysis (Table T6). Radiolarians are generally common to abundant in the entire succession (Fig. F17), although they are rare at 83.6 m CSF-A (Sample 346-U1423A-9H-CC). Nine radiolarian datums were found in Hole U1423A (Table T4). Late Pleistocene datums, the LOs of Lychnocanoma sakaii (0.05 Ma), Amphimelissa setosa (0.08 Ma), and Spongodiscus sp. (0.29 Ma), are observed at 7.2 m CSF-A (Sample 346-U1423A-1H-CC), 17.0 m CSF-A (Sample 2H-CC), and 26.6 m CSF-A (Sample 3H-CC), respectively. The LO of Axoprunum acquilonium (2.2 Ma) is at 93.2 m CSF-A (Sample 10H-CC) just below the sparse interval at 83.6 m CSF-A (Sample 9H-CC). The Pleistocene/Pliocene boundary is close to the FO of Cycladophora davisiana (2.7 Ma) at 112.1 m CSF-A (Sample 12H-CC) and the LO of Hexacontium parviakitaensis (2.7 Ma) at 131.2 m CSF-A (Sample 14H-CC). The FO of H. parviakitaensis (3.9/4.3 Ma) was found at 168.9 m CSF-A (Sample 18H-CC). Deeper than 178.3 m CSF-A (Sample 19H-CC), the Siphocampe arachnea group shows an acme zone (4.46–4.71 Ma). The FO and LO of Dictyophimus bullatus are primary datums that define the top and base of the Dictyophimus bullatus Zone. Although this species is very rare, the LO and FO datums are recognizable at 187.7 m CSF-A (Sample 20H-CC) and 197.2 m CSF-A (Sample 21H-CC). The abundant occurrence of the S. arachnea group in the absence of D. bullatus at the base of Hole U1423A (207.0 m CSF-A; Sample 22H-CC) suggests an age between 4.4 and 4.71 Ma. In the lower part of Hole U1423B, two radiolarian datums, the LO of D. bullatus (4.4 Ma) and a rapid increase (RI) of the S. arachnea group (4.71 Ma), are at 192.3 m CSF-A (Sample 346-U1423B-22H-CC) and 220.8 m CSF-A (Sample 25H-CC), respectively (Table T4). The base of Hole U1423B (249.5 m CSF-A; Sample 28H-CC) is deeper than the RI of the S. arachnea group (4.71 Ma) and shallower than the LO of Larcopyle pylomaticus, suggesting an age younger than 5.3 Ma. DiatomsDiatom biostratigraphy was based on smear slides from core catcher samples. Twenty-two core catcher samples were examined from Hole U1423A (Samples 346-U1423A-1H-CC to 22H-CC), and five NPD datums were identified (Tables T4, T7):
Sample 22H-CC, which is younger than 5.6 Ma, is the deepest sample from Hole U1423A (LO of Shionodiscus oestrupii). Ten core catcher samples were collected from Hole U1423B (Samples 346-U1423B-15H-CC to 17H-CC and 22H-CC to 28H-CC), and two datums were identified (Tables T4, T7). The samples from Hole U1423B were collected to complement the observations from Hole U1423A because the base of Hole U1423B was deeper than in Hole U1423A. Therefore, the ages in samples from Hole U1423B define the following NPD zones: the FO of Neodenticula koizumii (younger than 3.4–3.9 Ma) marks the base of NPD 8 (Sample 16H-CC), and Sample 28H-CC (NPD 7) is younger than 4.81 Ma based on the LO of Thalassiossira jacksonii. Overall, diatom abundance ranged from 2%–5% to >60% in the observed smear slides (Fig. F17). Preservation was good in all samples, and there were no signs of dissolution. Diatom valve fragmentation was normal. In addition, the ratio between preserved >50 µm centrics and <20 µm pennates was constant. Diatom range abundance is shown in Table T7. Diatom samples with abundance >60% were identified in Samples 346-U1423A-8H-CC to 14H-CC, and diatom mats were identified in Samples 346-U1423A-7H-CC, 13H-CC, 15H-CC, 20H-CC, and 21H-CC and 346-U1423B-23H-CC (Fig. F17). The comparison of these samples with qualitative abundances of Chaetoceros spores, an upwelling indicator, suggests different high-productivity systems: one dominated by Chaetoceros spores that is analogous to modern upwelling systems and a different one that produces diatom mats and oozes with lower Chaetoceros spore abundances. Freshwater diatom species were absent in all samples except for rare occurrences in Samples 346-U1423A-5H-CC and 6H-CC (Table T7). Planktonic foraminifersPlanktonic foraminifers were examined in core catcher samples from Holes U1423A (22 samples), U1423B (10 samples), and U1423C (6 samples) and in toothpick samples from Hole U1423A (8 samples) and U1423B (15 samples) taken after the core sections were split. Planktonic foraminifers are mainly confined to the upper part of the succession (shallower than Sample 346-U1423A-8H-CC; 74.19 m CSF-A) (Fig. F18). Of the 23 toothpick samples examined from dark layers (Table T8), 14 samples contained >80 specimens, 1 sample contained 22 specimens, and 2 samples had <5 specimens. Taxon relative abundances and estimates of assemblage preservation are presented in Table T8. Planktonic foraminifers are abundant shallower than Sample 346-U1423A-6H-CC (54.96 m CSF-A) and are absent or rare in the interval deeper than Sample 7H-CC (64.64 m CSF-A), except in Samples 346-U1423A-16H-CC (150.20–150.24 m CSF-A), 346-U1423B-15H-CC (132.80–132.85 m CSF-A), and 346-U1423C-4H-CC (142.65–142.70 m CSF-A), where high abundance peaks occur. Preservation is moderate to good in most core catcher samples, and fragmentation and/or pyritization are common. Site U1423 planktonic foraminiferal assemblages above Sample 346-U1423A-8H-CC (74.19 m CSF-A) are characteristic of cold, midlatitude, restricted environments, consisting mainly of Globigerina bulloides and Neogloboquadrina pachyderma (sinistral) with rare occurrences of Globigerina umbilicata, Globigerina quinqueloba, Neogloboquadrina pachyderma (dextral), Neogloboquadrina dutertrei (= Neogloboquadrina himiensis), Neogloboquadrina incompta, Neogloboquadrina kagaensis group (Neogloboquadrina kagaensis and Neogloboquadrina inglei), and Neogloboquadrina cf. asanoi. In addition to these species, Globorotalia praeinflata is common in Samples 346-U1423B-15H-CC (132.80–132.85 m CSF-A) and 346-U1423C-4H-CC (142.65–142.70 m CSF-A). N. pachyderma (dextral) is dominant in Sample 346-U1423A-16H-CC (150.20–150.24 m CSF-A), where rare specimens of Orbulina universa are also found. The LO of N. kagaensis group, reported as 0.7 Ma by Kucera and Kennett (2000), was identified in Sample 346-U1423A-5H-4W, 77–78 cm (41.07–41.08 m CSF-A). The peak in planktonic foraminiferal abundance in Sample 346-U1423A-16H-CC (150.20–150.24 m CSF-A) corresponds to Zone PF6 of Maiya (1978), based on the co-occurrence of O. universa and N. pachyderma (dextral), indicating that this horizon is older than 3.0 Ma (Miwa, 2014). Benthic foraminifersBenthic foraminifers were examined in core catcher samples from Holes U1423A (22 samples), U1423B (10 samples), and U1423C (6 samples) and in toothpick samples from Holes U1423A and U1423B taken after cores were split. Mudline samples recovered in Holes U1423A and U1423B were also investigated. Samples with an average volume of ~30 cm3 were processed from all core catchers to obtain quantitative estimates of downhole benthic foraminiferal distribution patterns. To assess assemblage composition and variability, all specimens from the >150 µm fraction were picked and transferred to slides prior to identification and counting. The 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 occur intermittently through the ~250 m thick biosiliceous-rich succession recovered at Site U1423 (Fig. F18; Table T9). Abundance and diversity are generally low, except for three samples (Table T9) found to contain at least 100 specimens per 30 cm3 of sediment and to exhibit much higher diversity. Preservation varies substantially throughout the succession. The assemblages consist of calcareous and agglutinated taxa, and their overall composition indicates bathyal paleodepths throughout the Pleistocene and Pliocene. Prominent variations in the downcore distribution of benthic foraminifers appear to reflect changes in organic export flux to the seafloor and/or in oxygenation at the seafloor (Fig. F18). A total of 44 benthic foraminiferal taxa were identified. Census counts from core catcher and split-core section samples are presented in Table T9. Figure F18 summarizes the downcore distribution of the more common benthic foraminiferal taxa in core catcher samples from Holes U1423A–U1423C. Common species include Bolivina pacifica, Epistominella pulchella, Eggerella bradyi, Globobulimina pacifica, Martinotiella communis, and Miliammina echigoensis (Figs. F19, F20). In Samples 346-U1423A-4H-CC and 5H-CC, the high abundances of B. pacifica and E. pulchella suggest enhanced surface productivity and higher organic export flux during deposition. B. pacifica is an ectobiont-bearing foraminifer, which also displays cytoplasmic adaptations (plasma membrane invaginations) that are thought to promote inhabitation of microxic environments (trace or 0.1 mL/L; sensu Bernhard and Sen Gupta, 1999) (Bernhard et al., 2010). Thus, the dominance of this species in Sample 346-U1423A-5H-CC indicates microxic pore water associated with intense oxygen depletion of bottom water during deposition of this sedimentary interval. Samples 346-U1423A-16H-CC (150.2 m CSF-A), 346-U1423B-15H-CC (132.8 m CSF-A), and 346-U1423C-4H-CC (142.65 m CSF-A) are characterized by a diverse assemblage indicating substantially improved oxygenation at the seafloor at ~3 Ma (Fig. F19). The assemblage includes cosmopolitan taxa such as Cibicidoides mundulus, Cibicidoides refulgens, Cibicidoides robertsonianus, Globobulimina pacifica, Hoeglundina elegans (which has an aragonitic test), Melonis barleanum, Melonis pompilioides, Pullenia quinqueloba, Oridorsalis umbonatus, and Pyrgo murrhina, as well as a few ostracods (see below). These intervals do not appear to be correlative, based on hole-to-hole stratigraphic correlation (see “Stratigraphic correlation and sedimentation rates”). They may, however, correspond to several transient episodes of improved deepwater circulation in the marginal sea, although the low resolution of our sample set does not allow resolution of individual events in Holes U1423A–U1423C. Moderately to well-preserved diatoms and radiolarians are common to abundant in residues >150 and >63 µm throughout the succession and become dominant deeper than 100 m CSF-A. Most samples deeper than 100 m CSF-A, except for Samples 346-U1423A-16H-CC (150.2 m CSF-A), 346-U1423B-15H-CC (132.8 m CSF-A), and 346-U1423C-4H-CC (142.65 m CSF-A), are either barren or impoverished, mainly containing rare agglutinated species. OstracodsCore catcher samples were also examined for the presence of ostracods during shipboard preparation of benthic foraminifer samples. Ostracods are absent from all but Sample 346-U1423A-16H-CC (150.2 m CSF-A), where three valves and one carapace belonging to Krithe spp., one carapace of Henryhowella cf. H. circumdentata (Brady, 1880), and one valve of Legitimocythere sp. were found. Preservation of the shells ranges from moderate to very good (Fig. F21). This same assemblage was found previously at Site 435 in the Japan Trench during Deep Sea Drilling Project Leg 56 (Hanai et al., 1980). All three taxa are common in bathyal environments. Mudline samplesMudline samples from Holes U1423A and U1423B were gently washed in order to preserve fragile agglutinated specimens with extremely low fossilization potential. The mudline sample from Hole U1423A contains only organically cemented agglutinated foraminifers, including Cribrostomoides subglobosus, Haplophragmoides sphaeriloculum, Hyperammina elongata, Jacullela cf. acuta, Miliammina echigoensis, Paratrochammina challengeri, and Rhabdammina sp. (Fig. F20). These organically cemented tests are extremely delicate and fragile (except for M. echigoensis) and thus have virtually no fossilization potential. Only two specimens of M. echigoensis were stained with rose bengal. These specimens exhibit much better preservation than unstained specimens. Diatoms are dominant, and planktonic foraminifers are absent in the mudline sample, suggesting that planktonic calcareous tests may be dissolved and that unfavorable conditions prevent calcareous benthic foraminifers from inhabiting the upper few centimeters of the sediment. |
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