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doi:10.2204/iodp.proc.346.105.2015

Biostratigraphy

In Hole U1424A, a ~159 m thick succession of Pliocene to Holocene sediment was recovered. Calcareous nannofossils are intermittent between 5 and 30 m CSF-A. Planktonic foraminifers are rare to absent with moderate to poor preservation below 40 m CSF-A, yet are mainly common to abundant in core catcher samples and thin calcareous layers shallower than 40 m CSF-A. Benthic foraminifers are absent shallower than 33 m CSF-A and are occasionally present below this depth. They show generally poor preservation. The overall assemblage composition indicates lower bathyal to abyssal paleodepths. Ostracods are extremely rare. The rare occurrence of calcareous microfossils probably reflects dissolution at the seafloor. The rare occurrence of agglutinated benthic foraminifers is probably due to unfavorable dysoxic conditions at the sediment/water interface. The radiolarian assemblages show good preservation except in the interval between 54 and 64 m CSF-A. Radiolarian biostratigraphic zonation ranges from the Larcopyle pylomaticus Zone (Pliocene) to the Botryostrobus aquilonaris Zone (Late Pleistocene). Diatom preservation is good throughout the succession. Overall, diatom abundance is low in most of the upper part of the succession and increases in the lower part of the succession. The diatom stratigraphy spans the interval from Zone NPD 12 (Pliocene) to NPD 7 (Late Pleistocene). Freshwater diatom species and opal phytoliths (land plant silica concretions), which might be related to freshwater input and/or wind transportation, are found sporadically. The radiolarian and diatom datums generally agree, with only some minor inconsistencies. The integrated calcareous and siliceous microfossil biozonation is shown in Figure F20, with microfossil datums shown in Table T4. An age-depth plot including biostratigraphic and paleomagnetic datums is shown in Figure F21. See “Stratigraphic correlation and sedimentation rates” for a discussion on sedimentation rates at Site U1424.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on analysis of core catcher and split-core section samples from Holes U1424A and U1424B. Only 9 of the 41 samples studied from Site U1424 contain nannofossils (Table T5).

Nannofossils are discontinuously present in Pleistocene sediment from 4.92 to 28.93 m CSF-A in Hole U1424A (Table T5). All samples below 32.26 m CSF-A in Hole U1424A and all core catcher samples from Hole U1424B lack nannofossils (Fig. F22). The calcareous nannofossil biostratigraphy is similar to the findings of Rahman (1992) at Site 794 (ODP Leg 127), of which Site U1424 was a redrill.

Nannofossil diversity at Site U1424 is low, and the assemblage is almost exclusively composed of Noelaerhabdaceae species belonging to the genera Gephyrocapsa and Reticulofenestra. The nannofossil assemblage consists of nine taxa, including Coccolithus pelagicus, Gephyrocapsa caribbeanica, Gephyrocapsa muellerae, Gephyrocapsa oceanica, Gephyrocapsa spp. (>4 µm), Gephyrocapsa spp. (<4 µm), Reticulofenestra minuta, Reticulofenestra minutula, and Syracosphaera spp. Preservation is generally moderate, although it becomes good in Sample 346-U1424A-2H-7, 53 cm (16.05 m CSF-A), which is dominated by small Gephyrocapsa (<4 µm) nannofossils.

Only one nannofossil subzone is recognized (Fig. F20) in Hole U1424A. Nannofossil Zones CN14b/NN20 are recognized based on the absence of Emiliania huxleyi and Pseudoemiliania lacunosa and the presence of both G. caribbeanica and G. oceanica. The pervasiveness of barren samples below 32.26 m CSF-A does not allow the base of these zones to be identified.

Radiolarians

A total of 17 core catcher samples from Hole U1424A were prepared for radiolarian analyses. Radiolarians are generally common to abundant in the sequence, although they are rare between 54.3 and 64.0 m CSF-A (Samples 346-U1424A-6H-CC through 7H-CC) (Table T6). Radiolarians are also common to abundant in the foraminifer residue >63 and >150 µm (see “Benthic foraminifers” for more details).

Nine radiolarian datums were found in Hole U1424A (Table T4), and they could be included in the L. pylomaticus Zone (Pliocene) through the B. aquilonaris Zone (Late Pleistocene). Late Pleistocene datums include the last occurrence (LO) of Lychnocanoma sakaii (0.05 Ma) and Spongodiscus sp. (0.29 Ma) at 6.8 m CSF-A (Sample 346-U1424A-1H-CC) and 16.5 m CSF-A (Sample 2H-CC). A single specimen of Eucyrtidium matuyamai was found at 44.9 m CSF-A (Sample 5H-CC). This taxon defines the top and bottom of the E. matuyamai Zone (1.03–1.98 Ma) by its occurrence interval. As the LO of Axoprunum acquilonium (1.2–1.7 Ma) co-occurs with E. matuyamai (Sample 5H-CC), together they suggest an age between 1.2 and 1.98 Ma. The Pleistocene/Pliocene boundary is close to the first occurrence (FO) of Cycladophora davisiana (2.7 Ma) at 83.0 m CSF-A and the LO of Hexacontium parviakitaensis (2.7 Ma) at 92.4 m CSF-A (Sample 9H-CC). The FO of H. parviakitaensis (3.9–4.3 Ma) occurs at 111.5 m CSF-A (Sample 12H-CC). The Siphocampe arachnea group is abundant at 140.2 m CSF-A (Sample 15H-CC), suggesting their acme zone between 4.46 and 4.71 Ma. The base of Hole U1424A (158.9 m CSF-A; Sample 17H-CC) lies lower than the base of the S. arachnea group acme zone (4.71 Ma) and is younger than the FO of L. pylomaticus, suggesting these strata are younger than 5.3 Ma.

Diatoms

Diatom biostratigraphy is based on smear slides from core catcher samples. Seventeen core catcher samples were examined, and two datums were identified (Tables T4, T7). The LO of Neodenticula kamtschatica (>2.6–2.7 Ma) marks the top of Zone NPD 9 (Sample 346-U1424A-9H-CC) and the FO of Neodenticula koizumii (<3.4–3.9 Ma) marks the base of Zone NPD 8 (Sample 346-U1424B-14H-CC). Diatom preservation is good throughout the succession. Overall, diatom abundance is low (0%–20%) in most of the upper part of the succession and increases (20%–60%) in the lower part of the succession (Fig. F22). Abundance >60% occurs in Samples 346-U1424A-15H-CC and 17H-CC. Diatoms are not present in Samples 1H-CC, 6H-CC, and 7H-CC. Freshwater diatom species are present in Samples 1H-CC, 4H-CC, 6H-CC, 8H-CC, and 17H-CC. Opal phytoliths (land plant silica concretions) are found in Samples 5H-CC, 7H-CC, 8H-CC, 9H-CC, 12H-CC, 14H-CC, 16H-CC, and 17H-CC (Table T7). The presence of freshwater diatom species and phytoliths might be related to freshwater input and/or wind transportation. 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 Holes U1424A (17 samples), U1424B (6 samples), and U1424C (4 samples) in addition to toothpick samples from Hole U1424A (11 samples) taken after the core sections were split. Planktonic foraminifers are mainly confined to the upper part of the succession (shallower than 35.43 m CSF-A; Sample 346-U1424A-4H-CC) (Fig. F22). Eleven toothpick samples were examined (Table T8), and seven of these samples contained ~100 specimens, three samples contained ~80 specimens, and one sample had 21 specimens.

Relative abundance of taxa and estimates of assemblage preservation are presented in Table T8. Planktonic foraminifers are generally abundant shallower than Sample 346-U1424A-4H-CC (35.43 m CSF-A), especially in brown layers within Subunit IA (see “Lithostratigraphy” for more details), and are absent or rare below. Preservation is moderate to poor in most of the core catcher samples, and fragmentation and/or pyritization is common.

Planktonic foraminiferal assemblages shallower than Sample 346-U1424A-4H-CC (35.5 m CSF-A) are characteristic of cold, subarctic, and restricted environments. They mainly consist of Globigerina bulloides and Neogloboquadrina pachyderma (sinistral) with rare occurrences of Globigerina umbilicata, Globigerina quinqueloba, N. pachyderma (dextral), Neogloboquadrina dutertrei (= Neogloboquadrina himiensis), and Neogloboquadrina incompta and Neogloboquadrina kagaensis group (N. kagaensis and Neogloboquadrina inglei). Planktonic foraminifer Zones PF9 and PF8 are recognized based on the dominance of sinistral N. pachyderma shallower than 35.5 m CSF-A (Fig. F20). The single occurrence of Orbulina universa in Sample 346-U1424A-17H-CC (153.82 m CSF-A) indicates an age older than 3.2 Ma (Miwa, 2014). However, this datum is poorly constrained, as shallower samples are barren of planktonic foraminifers. The LO of N. kagaensis (= N. inglei) was found in Samples 346-U1424A-4H-2W, 135–136 cm (28.65 m CSF-A), and 346-U1424B-4H-CC (30.88 m CSF-A). This species disappeared 0.7 m.y. ago (Kucera and Kennett, 2000).

Benthic foraminifers

Benthic foraminifers were examined in core catcher samples from Holes U1424A (17 samples), U1424B (6 samples), and U1424C (4 samples) and in toothpick samples from Hole U1424A (11 samples) taken after the core sections were split. Mudline samples recovered in Holes U1424A and U1424B were also investigated. Samples with an average volume of ~30 cm³ were processed from all core catchers to obtain quantitative estimates of benthic foraminiferal distribution patterns downhole. 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 are rare or absent through the ~160 m thick biosiliceous-rich succession recovered at Site U1424 (Fig. F22; Table T9). Samples 346-U1424A-1H-CC through 8H-CC (0–73.17 m CSF-A) are barren. Deeper than Sample 346-U1424A-8H-CC (73.17 m CSF-A), abundance and diversity generally remain low or the samples are barren. Preservation is generally poor or moderate, except in Samples 346-U1424A-17H-CC and 346-U1424B-17H-CC, where preservation improves. The assemblages consist of calcareous and agglutinated taxa, and their overall composition indicates bathyal paleodepths throughout the Pleistocene and Pliocene.

A total of 17 benthic foraminiferal taxa were identified. Census counts from core catcher and split-core section samples are presented in Table T9. Figure F22 summarizes the downcore distribution of benthic foraminifers in core catcher samples from Hole U1424A. Species commonly recorded include Martinotiella communis and Miliammina echigoensis (Table T9). Samples 346-U1424A-17H-CC (158.8 m CSF-A) and 346-U1424B-17H-CC (155.02 m CSF-A) are characterized by a more diverse assemblage, indicating substantially improved oxygenation at the seafloor at ~5 Ma. The assemblage includes Cibicidoides refulgens, Eggerella bradyi, Globobulimina pacifica, Melonis barleeanus, Pullenia quinqueloba, M. echigoensis, and Quinqueloculina akneriana. These intervals do not appear to be correlative, based on hole-to-hole stratigraphic correlation (see “Stratigraphic correlation and sedimentation rates”). However, they may correspond to several transient episodes of improved deepwater circulation in the sea, although the low resolution of our sample set does not allow identification of individual events in Holes U1424A and U1424B. Sample 346-U1424C-5H-CC contains only pyritized, agglutinated foraminifers (Aschemonella spp.), which have fine-grained agglutinated walls that easily decay (Figs. F23, F24). Moderately to well-preserved diatoms and radiolarians are common to abundant in residues >150 and >63 µm throughout the succession and become dominant below ~80 m CSF-A.

Ostracods

Core catcher samples were also examined for the presence of ostracods during shipboard preparation of benthic foraminifer samples. Only one valve of Henryhowella cf. H. circumdentata (Hanai et al., 1980), a bathyal taxon, was found in Sample 346-U1424A-11H-CC (Fig. F23).

Mudline samples

Mudline samples from Holes U1424A and U1424B were gently washed in order to preserve fragile agglutinated specimens with extremely low fossilization potential. The mudline sample from Hole U1424A contains only rare, organically cemented, agglutinated foraminifers including M. echigoensis, Aschemonella sp., and Rhabdammina spp. (Fig. F23). Diatoms are abundant and planktonic foraminifers are absent in the mudline sample, suggesting that calcareous tests are being dissolved in this water depth (~2800 m).

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