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doi:10.2204/iodp.proc.346.106.2015

Biostratigraphy

At Site U1425, a ~431 m thick succession of Miocene–Holocene sediment was recovered. Nannofossils are present in Pleistocene sediment shallower than ~56 m CSF-A but are absent or rare deeper than this depth. Radiolarians are generally common to abundant in the sequence, although they are rare or absent deeper than 351.2 m CSF-A. Twenty radiolarian datums included in the Eucyrtidium inflatum Zone (middle Miocene) through the Botryostrobus aquilonaris Zone (Late Pleistocene) were found at this site. 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. Seven diatom datums were identified, and the diatom-based stratigraphy spans the interval from Zone NPD 6A (middle Miocene) to NPD 12 (Late Pleistocene). The scarcity of freshwater diatoms combined with high abundance of phytoliths (Table T4) suggests wind transportation from land. The complete dissolution of diatoms and rare occurrence of radiolarians near the base of the succession coincides with the opal-A/opal-CT boundary transition. Planktonic foraminifers are mainly confined to the upper part of the succession (shallower than ~131 m CSF-A), exhibiting good to moderate preservation to ~66 m CSF-A and moderate to poor preservation between ~66 and 131 m CSF-A. Planktonic foraminiferal assemblages shallower than ~60 m CSF-A generally indicate cold and restricted environments. Benthic foraminifers occur intermittently throughout the Pleistocene to Miocene succession, showing marked variations in abundance and preservation. The overall assemblage composition indicates bathyal paleodepths. The highly variable composition of the assemblages suggests episodic oxygen depletion and intense carbonate dissolution at the seafloor, particularly during the Pliocene and Miocene. The radiolarian, diatom, calcareous nannofossil, and planktonic foraminifer datums generally agree, with only some minor inconsistencies. The integrated calcareous and siliceous microfossil biozonation is shown in Figure F29, and microfossil datums are shown in Table T5. A biostratigraphic age-depth plot is shown in Figure F30. See “Stratigraphic correlation and sedimentation rates” for a discussion of sedimentation rates at Site U1425.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on analysis of core catcher and split-core section samples from Holes U1425B and U1425D. Of the 116 samples studied, 39 contain nannofossils (Table T6).

Nannofossils are present in Pleistocene sediment shallower than 56.5 m CSF-A in Hole U1425B (Table T6), with barren intervals in Samples 346-U1425B-1H-CC, 2H-CC, 4H-CC, and 5H-2W, 25–26 cm. The majority of samples deeper than 64.8 m CSF-A lack nannofossils (Fig. F31). Sporadic occurrences are observed in core catcher samples from Cores 346-U1425B-12H through 15H, 26H, 28H, and 43H (between 104.6 and 299.2 m CSF-A). Only selected intervals were studied in Hole U1425D. Nannofossil preservation is moderate to good in Samples 346-U1425D-4H-2W, 75 cm, 4H-5W, 75 cm, and 4H-6W, 75 cm. Samples are devoid of calcareous nannofossils downhole from Samples 346-U1425D-13X-CC through 19H-CC (113.8–158.6 m CSF-A) and from Samples 59H-CC through 72H-1 (384.77–430.95 m CSF-A).

Nannofossil diversity at Site U1425 is low but higher than at previous Sites U1422–U1424. The nannofossil assemblage consists of 21 taxa, including Braarudosphaera bigelowii, Calcidiscus leptoporus, Calcidiscus macintyrei, Coccolithus pelagicus, Emiliania huxleyi, 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, Pontosphaera spp., Pseudoemiliania lacunosa, Reticulofenestra minuta, Reticulofenestra minutula, Reticulofenestra spp., and Umbilicosphaera sibogae. Reticulofenestra pseudoumbilica is interpreted as a reworked species in Sample 346-U1425B-5H-7, 33–34 cm. Preservation is generally poor or moderate, although nannofossils exhibit good preservation at 19.4, 23.4, and 35.4 m CSF-A (Samples 346-U1425B-3H-1, 108–109 cm, 3H-4, 60 cm, and 4H-6, 9–10 cm, respectively).

Nannofossil Zones CN15/NN21 through CN13b/NN19 are recognized (Fig. F29) 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. The pervasiveness of barren samples deeper than 56.6 m CSF-A (Sample 346-U1425B-6H-CC) prevents further zonal assignments.

Similar distribution, abundance, preservation, and diversity patterns were documented by Muza (1992) for Site 799 (Leg 127) drilled near Site U1425 on the Yamato Rise. At Site 799, calcareous nannofossils occur intermittently in the upper 81 m of the section. The sporadic occurrence of nannofossils in this interval is attributed to CCD fluctuations. The lack of nannofossils and other calcareous microfossils deeper than this depth is attributed by Muza (1992) to in situ dissolution and production of authigenic carbonates based on the presence of monospecific assemblages comprising overgrown specimens of the highly dissolution resistant species C. pelagicus (Fig. F32) concurrent with high percentages of small (1–12 µm) carbonate grains. The carbonate grains may have come from in situ dissolution of nannofossils in these horizons, and C. pelagicus may be all that remains of an assemblage that was originally as diverse as those documented in the upper part of the section.

Radiolarians

A total of 73 core catcher samples from Holes U1425B (60 samples) and U1425D (13 samples) were prepared for radiolarian analyses (Table T7).

Hole U1425B

In Hole U1425B, radiolarians are generally common to abundant, although they are rare or absent deeper than 351.2 m CSF-A (Sample 346-U1425B-54-CC; Fig. F31). Nineteen radiolarian datums included in the E. inflatum Zone (middle Miocene) through the B. aquilonaris Zone (Late Pleistocene) were identified in Hole U1425B (Table T5; Figs. F29, F30).

Pleistocene datums

Although key species that define the base or top of the Stylatractus universus and Eucyrtidium matuyamai Zones are not present, secondary datums include the LO of Lychnocanoma sakaii (0.05 Ma) at 8.8 m CSF-A (Sample 346-U1425B-1H-CC), Spongodiscus sp. (0.29 Ma) at 18.6 m CSF-A (Sample 2H-CC), and Axoprunum acquilonium (1.2–1.7 Ma) at 66.2 m CSF-A (Sample 7H-CC).

Pliocene datums

The Pleistocene/Pliocene boundary is close to the FO of Cycladophora davisiana (2.7 Ma) and the LO of Hexacontium parviakitaensis (2.7 Ma) at 75.6 m CSF-A (Sample 346-U1425B-8H-CC). The FO of H. parviakitaensis (3.9–4.3 Ma) is at 122.5 m CSF-A (Sample 17H-CC). The FO and LO of Dictyophimus bullatus, which define the top and base of the D. bullatus Zone (3.9–4.3 to 4.4 Ma), are at 131.2 m CSF-A (Sample 19H-CC) and 136.7 m CSF-A (Sample 20H-CC). The Siphocampe arachnea group is abundant at 146.3 m CSF-A (Sample 21H-CC), suggesting the acme zone of this species group is between 4.46 and 4.71 Ma. The FO of Lipmanella redondoensis (5.06 Ma) is at 156.0 m CSF-A (Sample 22H-CC).

Miocene datums

The FO of Larcopyle pylomaticus (base of the L. pylomaticus Zone, 5.3 Ma), which is close to the Pliocene/Miocene boundary, is at 175 m CSF-A (Sample 346-U1425B-24H-CC). Sample 25H-CC (184.5 m CSF-A) corresponds to the A. acquilonium Zone (5.3–6.1 Ma) lying between the FO of L. pylomaticus (175 m CSF-A; Sample 24H-CC) and the LO of Lychnocanoma parallelipes (194.1 m CSF-A; Sample 26H-CC). The rapid increase of Lithelius barbatus (7.1 Ma), which defines the base of the L. barbatus Zone, is at 221.3 m CSF-A (Sample 29H-CC). The FO of A. acquilonium (7.1 Ma) is at 231 m CSF-A (Sample 30H-CC). The base of the L. parallelipes Zone (the FO of L. parallelipes; 7.4 Ma) is at 244 m CSF-A (Sample 33H-CC), occurring with the LO of Cycladophora nakasekoi (7.4 Ma) at 253.4 m CSF-A (Sample 34H-CC). The LO of Lychnocanoma magnacoronuta (9.0 Ma) is at 322.7 m CSF-A (Sample 48H-CC), corresponding to the base of the L. redondoensis Zone and the top of the L. magnacoronuta Zone. Although the base of the L. magnacoronuta Zone (11.8 Ma) should be marked by the FO of L. magnacoronuta at 366 m CSF-A (Sample 56X-CC), it might be present at deeper levels because the occurrence of C. nakasekoi is younger than 10.1 Ma in Samples 57X-CC and 58X-CC (369.5–378.4 m CSF-A). Sample 60H-CC (402.5 m CSF-A) lies below the LO of Eucyrtidium inflata (11.8 Ma) and is younger than the rapid decrease (RD) of Cyrtocapsella tetrapera at 12.6 Ma.

Hole U1425D

Deeper than 375.6 m CSF-A in Hole U1425D (Sample 346-U1425D-57H-CC), radiolarians are rare or absent. The FO of L. magnacoronuta (11.8 Ma) is at 375.6 m CSF-A (Sample 57H-CC). In the same sample, Hexacontium akitaensis is present, suggesting that the interval was deposited from 11.7 to 10.7 Ma (Kamikuri, 2010), in the lower part of the L. magnacoronuta Zone. E. inflatum occurs from 384.8 (Sample 59H-CC) to 404.4 m CSF-A (Sample 65H-CC), indicating the E. inflatum Zone (11.8–15.3 Ma). The RD of C. tetrapera that divided the E. inflatum Zone at 12.6 Ma is at 390.7 m CSF-A (Sample 61H-CC). Key biostratigraphic species were absent deeper than 408 m CSF-A (Sample 66H-CC).

Diatoms

Diatom biostratigraphy was based on smear slides from core catcher samples. Sixty core catcher samples were examined, and seven datums were identified (Tables T4, T5). The LO of Neodenticula koizumii (2.0 Ma) at 75.63 m CSF-A (Sample 346-U1425B-8H-CC) marks the boundary between the base of NPD 10 and the top of NPD 9 (Yanagisawa and Akiba, 1998), the LO of Neodenticula kamtschatica (2.6–2.7 Ma) at 94.82 m CSF-A (Sample 10H-CC) marks the boundary between the base of NPD 9 and the top of NPD 8 (Yanagisawa and Akiba, 1998), and the FO of N. koizumii (3.4–3.9 Ma) at 104.61 m CSF-A (Sample 12H-1) gives the boundary between the base of NPD 8 and the top of NPD 7B (Yanagisawa and Akiba, 1998). The LO of Thalassiosira jackonsii (4.81 Ma) is in Sample 23H-CC, and the last common occurrence (LCO) of Thalassionema schraderi (7.67 Ma) at 244.03 m CSF-A (Sample 33H-CC) marks the boundary between the base of NPD 7A and the top of NPD 6B (Yanagisawa and Akiba, 1998). The boundary between the base of Zone NPD 6B and the top of Zone NPD 6A (Yanagisawa and Akiba, 1998) is given by the LO of Denticulopsis katayamae (8.7 Ma) at 313.37 m CSF-A (Sample 46H-CC). The LO of Denticulopsis dimorpha (9.3 Ma) at 336.91 m CSF-A (Sample 51H-CC) marks the base of Zone NPD 6A and the top of Zone NPD 5A (Yanagisawa and Akiba, 1998).

Diatom preservation is good throughout the succession. Overall, diatom abundance is low (0%–20%) in most of the upper part of the cored interval and increases (20%–60%) in the lower part of the succession (Fig. F31). Abundance >60% occurs in Samples 346-U1425B-8H-CC through 26H-CC and 29H-CC through 51H-CC. Diatom identification was not possible in Samples 27H-CC and 28H-CC because of the presence of very fine and abundant volcanic glass. Complete dissolution of diatoms is found at the bottom of the succession, with an absence of valves and rare (<2 fragments in two slide transects), highly dissolved fragments (Samples 52H-CC through 61X-CC [last sample recovered]). This dissolution also appears in Hole U1425D in Samples 346-U1425D-46H-CC through 72H-1 (last sample recovered) and may be the consequence of the opal-A/opal-CT boundary transition.

The absence of significant abundance of freshwater diatoms combined with high abundance (>5 specimens found in two slide transects) of phytoliths (Table T4) suggest wind transport from land.

Planktonic foraminifers

Planktonic foraminifers were examined in core catcher samples from Holes U1425A (2 samples), U1425B (60 samples), and U1425D (18 samples). Relative abundances 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-U1425B-19H-CC; 131.20 m CSF-A and shallower; Figs. F31, F33). They are generally abundant between 8.77 and 56.56 m CSF-A (Samples 1H-CC and 6H-CC), whereas abundance varies markedly between 66.25 and 131.20 m CSF-A (Samples 7H-CC and 19H-CC). Sample 20H-CC (136.69 m CSF-A) and other samples deeper in the succession are mainly barren. Preservation is good to moderate shallower than 56.56 m CSF-A (Sample 6H-CC), becoming moderate to poor deeper than 66.25 m CSF-A (Sample 7H-CC) because of frequent fragmentation and/or pyritization. Deeper than Sample 17H-CC (122.49 m CSF-A), most of the planktonic foraminiferal tests have a yellow or orange color. These tests appear to have been partially dissolved and recrystallized when observed in the scanning electron microscope (Fig. F34).

Planktonic foraminiferal assemblages shallower than Sample 346-U1425B-6H-CC (56.56 m CSF-A) are characteristic of cold, subarctic, and restricted environments. They mainly consist of Globigerina bulloides and Neogloboquadrina pachyderma (sinistral and dextral) with rare occurrences of Globigerina umbilicata, Globigerina quinqueloba, Neogloboquadrina dutertrei (= Neogloboquadrina himiensis), Neogloboquadrina incompta, Neogloboquadrina kagaensis group (N. kagaensis and Neogloboquadrina inglei), and Neogloboquadrina cf. asanoi. The LO of N. kagaensis group (recorded at 0.7 Ma; Kucera and Kennett, 2000) is reported in Sample 4H-CC (37.61 m CSF-A). The change in coiling direction of N. pachyderma from sinistral to dextral in Sample 5H-CC (46.99 m CSF-A) corresponds to the Zone PF8/PF7 boundary in the regional zonation for the marginal sea (Maiya, 1978). The planktonic foraminiferal assemblages between Samples 11H-CC and 19H-CC (104.02 and 131.20 m CSF-A) are more diverse than in the upper part of the sequence, including Globigerinita glutinata, Globorotalia praeinflata, Globorotalia ikebei, Orbulina universa, and Orbulina suturalis, in addition to the species listed above. The FO of G. praeinflata in Sample 12H-CC (104.59 m CSF-A) indicates an age younger than 3.3 Ma (Lyle, Koizumi, Richter, et al., 1997). However, this datum is poorly constrained, as samples deeper than this level contain only rare planktonic foraminifers. Based on the FO of G. praeinflata, the Zone PF7/PF6 boundary is placed at 105.44 m CSF-A between Samples 12H-1 and 13X-CC. The occurrence of O. universa in Sample 12H-CC (104.59 m CSF-A) suggests an age older than 3.0 Ma (Miwa, 2014). The relatively continuous occurrences of O. universa and O. suturalis between Samples 14H-CC and 18H-CC (115.79 and 127.31 m CSF-A) together with G. ikebei (Sample 17H-CC) indicate that this interval corresponds to Zone PF6 (G. ikebei/O. universa Zone) of Maiya (1978).

Benthic foraminifers

Benthic foraminifers were examined in core catcher samples from Holes U1425A (2 samples), U1425B (60 samples), and U1425D (18 samples). The mudline sample recovered in Hole U1425B was 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. Core catcher samples were also examined for the presence of ostracods during shipboard preparation of benthic foraminifer samples.

Benthic foraminifers vary substantially in abundance and preservation throughout the 408 m thick Miocene to Pleistocene biosiliceous-rich succession recovered at Site U1425 (Figs. F31, F33; Table T9). Samples 346-U1425B-1H-CC and 2H-CC (8.77 and 18.58 m CSF-A) are barren. Samples 3H-CC and 19H-CC (27.04 and 131.17 m CSF-A) contain assemblages that are intermittently diverse and show marked fluctuations in abundance. Deeper than 131.17 m CSF-A (Sample 19H-CC), samples are barren or impoverished, except for Samples 42H-CC and 43H-CC (294.53 and 299.24 m CSF-A), which contain abundant taxa that exhibit low-diversity assemblages. Preservation is generally poor to moderate, except for Samples 3H-CC, 5H-CC, 12H-1, 15H-CC, and 18H-CC, where preservation is good. The assemblages consist of calcareous and agglutinated taxa, and their overall composition indicates bathyal paleodepths throughout the Miocene to Pleistocene.

A total of 52 benthic foraminiferal taxa were identified. Table T9 summarizes the downcore distribution of benthic foraminifers in core catcher samples from Holes U1425A, U1425B, and U1425D. Figure F35 illustrates characteristic taxa found at Site U1425. Species commonly recorded in Samples 346-U1425B-3H-CC and 19H-CC (27.04 and 131.17 m CSF-A) include Bolivina pacifica, Cassidulina laevigata, Cassidulina norcrossi, Epistominella pulchella, Globobulimina pacifica, 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). Sample 12H-1 (104.56 m CSF-A) contains a slightly more diverse assemblage including Martinotiella communis, Melonis pompilioides, Oridorsalis umbonatus, Cibicidoides mundulus, Quinqueloculina sp., and U. yabei, suggesting a transient increase in deepwater oxygenation between 4 and 5 Ma.

Deeper than Sample 346-U1425B-19H-CC (131.17 m CSF-A), the agglutinated foraminifers M. communis and Miliammina echigoensis occur sporadically together with rare calcareous taxa. Samples 42H-CC and 43H-CC (294.53 and 299.24 m CSF-A) are dominated by the organic flux–sensitive species E. pulchella and include the species G. pacifica, which is known to be tolerant of low-oxygen conditions within pore waters (Gooday, 1993; Jorissen et al., 1995, 2007; Jorissen, 1999). Samples 346-U1425B-60H-CC and 61H-CC (402.45 and 403.83 m CSF-A) and 346-U1425D-63H-CC through 67H-CC (397.66–408.86 m CSF-A) contain impoverished agglutinated assemblages including M. communis, Kareriella sp., and Spirosigmoilinella compressa. Sample 346-U1425D-65H-CC (404.42 m CSF-A) additionally contains three specimens of Uvigerina. These two intervals, which are characterized by low-diversity agglutinated assemblages and are older than 12.5 Ma, may be correlative in Holes U1425B and U1425D. However, they do not correspond to the more diversified Cibicidoides wuellerstorfi Assemblage Zone defining the “First Foram Sharp Line” (~14 Ma), which was identified at ODP Site 797 in the Yamato Basin by Nomura (1992). This more diversified calcareous assemblage was not recovered at Site U1425.

Overall, the highly variable composition of assemblages at Site U1425 suggests marked variations in oxygenation at the seafloor and intense carbonate dissolution, in particular during the Pliocene and Miocene. Moderately to well-preserved diatoms and radiolarians are common to abundant in residues >150 and >63 µm throughout the succession, becoming dominant deeper than ~134 m CSF-A. Ostracods were not found in any of the samples examined.

Mudline samples

Mudline samples from Holes U1425B and U1425C were gently washed in order to preserve fragile agglutinated specimens with extremely low fossilization potential. The mudline sample from Hole U1425B contains planktonic foraminifers that show partial dissolution. The planktonic foraminifers include N. pachyderma (sinistral), G. bulloides, G. umbilicata, and G. quinqueloba. Benthic foraminifers include mainly M. echigoensis, B. pacifica, Haplophragmoides sphaeriloculum, and Paratrochammina challengeri with rare Cassidulina teretis and Reophax scorpiusus (Fig. F36). The thin, delicate tests of B. pacifica and the fragile, organically cemented agglutinated tests of H. sphaeriloculum, P. challengeri, and R. scorpiusus have extremely low fossilization potential in contrast to M. echigoensis, which frequently occurs in Pleistocene core catcher samples at this site. The mudline sample from Hole U1425B also contains calcareous nannofossils with moderate to poor preservation. Specimens of C. pelagicus (Fig. F32) and G. oceanica were found together with broken shields of E. huxleyi (according to light microscope and scanning electron microscope observations). Diatoms are abundant, but no ostracods were recorded in the mudline sample from Hole U1425B.

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