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doi:10.2204/iodp.proc.346.110.2015

Biostratigraphy

At Site U1430, ~250–265 m of Miocene to Holocene sedimentary section was recovered. Nannofossils are generally absent with the exception of a few intervals where nannofossils are rare and exhibit poor preservation. One calcareous nannofossil datum, the first occurrence (FO) of Emiliania huxleyi, is documented. Diatoms are generally common and well preserved. Thirteen diatom datums are recorded. High abundances of Chaetoceros spores are indicative of a productive paleoenvironment. Laminations near 231 m CSF-A in Hole U1430A are found to contain different diatom assemblages in the light and dark colored layers. Radiolarians are generally abundant throughout the entire succession and are mostly well preserved. A total of 22 radiolarian datums are documented, including the last occurrence (LO) of Pentactinosphaera hokurikuensis (15.0 Ma) near 256.3 m CSF-A in Hole U1430B. The abundance of planktonic foraminifers is variable through the succession, ranging from rare to dominant; preservation is generally poor to moderate. The planktonic foraminiferal assemblages are characteristic of temperate to subarctic environments and primarily consist of Globigerina bulloides, Neogloboquadrina pachyderma (sinistral and dextral), and the Neogloboquadrina kagaensis group. Two datums (LO of N. kagaensis group and coiling change in N. pachyderma from dextral to sinistral) are identified. Benthic foraminifers are generally moderately to well preserved and abundant within the Pleistocene interval (shallower than ~54 m CCSF-A, as defined in the “Methods” chapter [Tada et al., 2015b]). Alternating peak abundances in Cassidulina and Uvigerina suggest elevated but fluctuating organic export fluxes to the seafloor through the Middle to Late Pleistocene. Deeper in the succession, most samples are barren or rarely consist of an impoverished assemblage dominated by a few agglutinated species. The overall composition of benthic foraminifer assemblages at Site U1430 indicates middle bathyal paleodepths from the late Pliocene to the Pleistocene. The integrated siliceous and calcareous microfossil biozonation is shown in Figure F26 with microfossil datums listed in Table T4. A biostratigraphic age-depth plot is provided in Figure F27. See “Stratigraphic correlation and sedimentation rates” for a discussion of sedimentation rates at Site U1430.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on analysis of 55 core catcher and split-core section samples from Holes U1430A (40 samples) and U1430B (15 samples). Nannofossils are virtually absent throughout the entire succession. A few nannofossil-bearing intervals are present between 3.5 and 137.0 m CSF-A (Samples 346-U1430A-1H-CC, 2H-CC, 4H-CC, 6H-3W, 75 cm, 6H-CC, and 15H-CC) in Hole U1430A and between 8.2 and 27.6 m CSF-A (Samples 346-U1430B-1H-CC, 2H-CC, and 3H-CC) in Hole U1430B (Table T5; Fig. F28). When nannofossils are present, they are generally rare and poorly preserved.

Nannofossil diversity at Site U1430 is lower than at any other site drilled during IODP Expedition 346. The nannofossil assemblage consists of only seven taxa, including Braarudosphaera bigelowii, Calcidiscus leptoporus, Coccolithus pelagicus, E. huxleyi, Gephyrocapsa oceanica, small Gephyrocapsa spp. (<4 µm), and Helicosphaera carteri.

The base of nannofossil Zones CN15/NN21 is recognized (Fig. F26) based on the FO of E. huxleyi in Hole U1430A. The underlying strata are included in nannofossil Zones CN14b/NN20 based on the absence of Pseudoemiliania lacunosa, but the base of these zones (i.e., the LO of P. lacunosa) is not identified and their zonal assignment is tentative because of the overall scarcity of nannofossils.

Radiolarians

A total of 39 samples from Holes U1430A (33 samples), U1430B (2 samples), and U1430C (4 samples) were prepared for radiolarian analyses (Table T6).

Hole U1430A

In Hole U1430A, radiolarians are generally common to abundant in the sequence, although they are rare or absent deeper than 249.7 m CSF-A (Sample 346-U1430A-30X-CC) (Fig. F28). Twenty two radiolarian datums included in the Eucyrtidium inflatum Zone (middle Miocene) through the Botryostrobus aquilonaris Zone (Late Pleistocene) are found in this hole (Table T4; Fig. F26).

Pleistocene

Although key species that define the base or top of the Stylatractus universus and Eucyrtidium matuyamai Zones are not observed, secondary datums include the LO of Lychnocanoma sakaii (0.05 Ma) at 3.5 m CSF-A (Sample 346-U1430A-1H-CC), Amphimelissa setosa (0.08 Ma) at 13.3 m CSF-A (Sample 2H-CC), Spongodiscus sp. (0.29 Ma) at 22.9 m CSF-A (Sample 3H-CC), and Axoprunum acquilonium (1.2–1.7 Ma) at 51.4 m CSF-A (Sample 6H-CC).

Pliocene

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 51.4 m CSF-A (Sample 346-U1430A-6H-CC) and 60.0 m CSF-A (Sample 7H-CC), respectively. The FO of H. parviakitaensis (3.9–4.3 Ma) occurs at 70.4 m CSF-A (Sample 8H-CC). The LO of Lipmanella redondoensis (5.06 Ma) and the FO of Larcopyle pylomaticus (base of the L. pylomaticus Zone; 5.3 Ma), which are close to the Pliocene/Miocene boundary, are at 79.9 m CSF-A (Sample 9H-CC). However, in the same sample, Cycladophora nakasekoi (LO at 7.4 Ma) is also present, suggesting reworking from Miocene strata or the presence of a hiatus. In the absence of the A. acquilonium to Lychnocanoma parallelipes Zones (5.3–7.4 Ma), a hiatus and/or very slow sedimentation interval are interpreted between 79.9 and 89.5 m CSF-A (Samples 9H-CC and 10H-CC).

Miocene

The last common occurrence of Lychnocanoma magnacoronuta (9.0 Ma) occurs at 146.5 m CSF-A in Hole U1430B (Sample 346-U1430B-16H-CC) corresponding to the base of the L. redondoensis Zone and the top of the L. magnacoronuta Zone. The FO of C. nakasekoi and the LO of Cyrtocapsella japonica (10.1 Ma) are found at 175.0 m CSF-A and 183.5 m CSF-A in Hole U1430B (Samples 19H-CC and 20H-CC), respectively. Dendrospyris uruyaensis ranges from 11.8 to 10.1 Ma and is present between 202.2 and 240.4 m CSF-A (Samples 22H-CC and 27H-CC).

The FO of L. magnacoronuta marks the base of the L. magnacoronuta Zone (11.8 Ma) at 230.7 m CSF-A in Hole U1430A (Sample 346-U1430A-25H-CC). The LO of Eucyrtidium inflata and Lithopera renzae (11.8 Ma) are in Sample 28H-CC (245.1 m CSF-A). The rapid decrease (RD) of C. tetrapera (12.6 Ma), dividing the E. inflata Zone, is observed at 249.5 m CSF-A (Sample 29H-CC). The presence of Dendrospyris sakaii at 255.1 m CSF-A (Sample 31X-CC) suggests an age older than 14.8 Ma.

Hole U1430B

Radiolarians are abundant to common at the base of Hole U1430B (251.1–256.3 m CSF-A; Samples 346-U1430B-32X-1W, 111–113 cm, and 33X-1W, 59–61 cm). Sample 32X-1W, 111–113 cm (251.1 m CSF-A), lies between the LO of D. sakaii (14.8 Ma) and the FO of Eucyrtidium asanoi (15.3 Ma). D. sakaii (14.8 to 16–16.8 Ma) and P. hokurikuensis (older than 15.0 Ma) are present in Sample 33X-1W, 59–61 cm (256.3 m CSF-A); however, E. asanoi and E. inflata (FO at 15.3 Ma) are not present.

Hole U1430C

At the bottom of the succession in Hole U1430C, the FO of D. uruyaensis and the LO of E. inflata (11.8 Ma) are found at 238.5 and 243.0 m CSF-A (Samples 346-U1430C-29H-CC and 30H-CC), respectively. The RD of C. tetrapera (12.6 Ma) is at 244.4 m CSF-A (Sample 31H-CC).

Diatoms

Diatom biostratigraphy was based on smear slides from core catcher and toothpick samples. A total of 32 core catcher and 17 toothpick samples were examined from Hole U1430A. In addition, three toothpick samples from Hole U1430B were also observed to constrain ages. Thirteen datums were identified (Table T4, T7; Fig. F29) in Hole U1430A.

The LO of Proboscia curvirostris (0.3 Ma) in Sample 346-U1430A-3H-CC (22.93 m CSF-A) marks the boundary between the base of Zone NPD 12 and the top of Zone NPD 11 (Yanagisawa and Akiba, 1998). The LO of Neodenticula koizumii (2.0 Ma) at 52.7 m CSF-A (Sample 7H-1W, 100–101 cm) marks the boundary between the base of Zone NPD 10 and the top of Zone NPD 9, and the LO of Neodenticula kamtschatica (2.6–2.7 Ma) in Sample 346-U1430A-7H-CC (61.0 m CSF-A) marks the boundary between the base of Zone NPD 9 and the top of Zone NPD 8. The FO of N. koizumii (3.4–3.9 Ma) at 62.9 m CSF-A (Sample 8H-2W, 75 cm) defines the boundary between Zones NPD 8 and NPD 7Bb. The FO of N. kamtschatica (6.57 Ma) is in Sample 346-U1430A-9H-3W, 14 cm, and the FO of Shionodiscus oestrupii (5.56 Ma) is found in Sample 9H-CC (79.9 m CSF-A), marking the boundary between Zones NPD 7Bb and NPD 7Ba. The LO of Thalassionema schraderi (7.67 Ma) at 98.9 m CSF-A (Sample 11H-CC) separates Zones NPD 7A (base) and NPD 6B (top). The FO of T. schraderi (8.5 Ma) occurs in Sample 13H-CC (118.0 m CSF-A). The boundary between Zones NPD 6B and NPD 6A is marked by the LO of Denticulopsis katayamae at 8.7 Ma (137.0 m CSF-A; Sample 15H-CC). The LO of Denticulopsis dimorpha (9.3 Ma) marks the boundary between Zones NPD 6A and NPD 5D at 156.0 m CSF-A (Sample 17H-CC). The LO of Denticulopsis hustedtii (10.2 Ma) is in Sample 20H-CC (183.5 m CSF-A), and the FO of this species (at 11.2 Ma) is in Sample 25H-CC (230.7 m CSF-A). Finally, the FO of Denticulopsis simonsenii (13.1 Ma) is found in Sample 27H-CC (240.4 m CSF-A), marking the boundary between Zones NPD 5A and NPD 4Bb. In Sample 346-U1430B-31X, 94 cm, Actinocyclus ingens var. nodus is found. This species occurrence is placed within the Denticulata lauta Zone NPD 4A (Baldauf and Barron, 1980; Yanagisawa and Akiba, 1998) between 14.4 and 15.9 Ma.

Diatom preservation is good throughout the succession, except for the upper 20 m and the bottom 50 m (Fig. F28; Table T7). Overall, diatom abundances are high (20% to >60%) through the succession. The high abundance (20%–60%) of Chaetoceros spores indicates a productive environment.

Dark and light colored laminations found in Sample 346-U1430A-25H-CC (Fig. F29) contain different diatom assemblages. Both layers are defined as diatom oozes (reflecting high productivity); however, the light colored layers have larger centrics. This may be due to an increase of nutrients from terrestrial origin and/or an increase in upwelled water flux. Further analysis, statistical and geochemical, will help to assess this hypothesis. In order to better investigate if any particular species assemblages have a significant pattern, 22 toothpick samples from light colored layers in Cores 346-U1430A-22H through 25H were taken in order to perform postcruise multivariate statistical analysis.

Planktonic foraminifers

Planktonic foraminifers were examined in core catcher samples from Holes U1430A (32 samples), U1430B (7 samples), and U1430C (7 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-U1430A-5H-CC and shallower, Sample 346-U1430B-6H-CC and shallower, and Sample 346-U1430C-6H-CC and shallower). Their abundances vary from rare to dominant. Sample 346-U1430A-6H-CC (42.1 m CSF-A) and deeper, Sample 346-U1430B-7H-CC (42.1 m CSF-A) and deeper, and Sample 346-U1430C-7H-CC (42.1 m CSF-A) and deeper are barren (Table T8; Fig. F28). Preservation is moderate to poor because of frequent fragmentation, except for Sample 346-U1430C-4H-CC (33.6 m CSF-A), which contains well-preserved tests. The frequent fragmentation may indicate partial diagenetic dissolution.

Planktonic foraminiferal assemblages are characteristic of temperate to subarctic environments, consisting mainly of G. bulloides, N. pachyderma (sinistral and dextral), and the N. kagaensis group (N. kagaensis and Neogloboquadrina inglei) with rare occurrences of Globigerina umbilicata, Globigerina quinqueloba, Globoturborotalita woodi, and Neogloboquadrina dutertrei (= Neogloboquadrina himiensis). The LO of the N. kagaensis group, which is observed at 0.7 Ma in the North Pacific Ocean (Kucera and Kennett, 2000), is recorded between Samples 346-U1430B-4H-CC and 3H-CC (36.9 and 27.5 m CSF-A) and between Samples 346-U1430C-4H-CC and 3H-CC (33.6 and 42.2 m CSF-A). The change in coiling direction of N. pachyderma (from dextral to sinistral) occurs between Samples 346-U1430B-5H-CC (46.3 m CSF-A) and 4H-CC (36.9 m CSF-A), indicating the boundary between Zones PF8 and PF7 in the regional zonation for the marginal sea (Maiya, 1978).

Benthic foraminifers

Benthic foraminifers were examined in core catcher samples from Holes U1430A (32 samples), U1430B (8 samples), and U1430C (6 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 samples recovered in Holes U1430A and U1430B 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. Core catcher samples were also examined for the presence of ostracods during shipboard preparation of benthic foraminifer samples, but none were found.

Benthic foraminifers vary substantially in abundance and preservation throughout the ~280 m thick Miocene to Pleistocene succession recovered in Holes U1430A–U1430C (Figs. F28, F30; Table T9). Benthic foraminifers are generally moderately to well preserved and abundant within the Pleistocene interval shallower than Samples 346-U1430A-6H-CC, 346-U1430B-6H-CC, and 346-U1430C-6H-CC (53.7, 55.6, and 57.8 m CCSF-A) (Fig. F28). Deeper in the succession, most samples are barren or consist of impoverished assemblages dominated by a few agglutinated species (Samples 346-U1430A-7H-CC though 32X-CC; 63.9–280.3 m CCSF-A). The overall composition of assemblages at Site U1430 indicates middle bathyal paleodepths from the late Pliocene to Pleistocene.

A total of 38 benthic foraminiferal taxa were identified. Table T9 summarizes the downcore distribution of benthic foraminifers in core catcher samples from Holes U1430A–U1430C. Species commonly recorded through the succession include Bolivina pacifica, Cassidulina japonica, Cassidulina norcrossi, Epistominella pulchella, Globobulimina pacifica, Trifarina angulosa, 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 at Site U1430 (Fig. F30) 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 early Pleistocene to late Pliocene (between ~68 and 83 m CCSF-A). Similar distribution patterns are apparent at Sites U1422–U1426 that may be related to pervasive carbonate dissolution in the marginal sea and/or to unfavorable conditions for benthic foraminifers at the seafloor during this warmer climate period. 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 in Hole U1430A.

Mudline samples

The mudline sample from Hole U1430A was gently washed in order to preserve fragile, agglutinated foraminifer specimens with extremely low fossilization potential. The mudline sample contains planktonic foraminifers, including a few warm-water species such as Globigerinoides ruber, Globorotalia inflata, Globorotalia tumida, and Orbulina universa, reflecting the modern influence of the TWC. The mudline assemblage is dominated by agglutinated taxa including Cribrostomoides subglobosus, Haplophragmoides subsphaericum, M. echigoensis, Paratrochammina challengeri, and Reophax scorpiurus. The mudline sample also contains calcareous taxa including Amphycorina scalaris, B. pacifica, Bolivina robusta, Cassidulina japonica, Cibicidoides mundulus, Nummoloculina sp., Pyrgo spp., Pyrgoella sp., Quinqueloculina spp., Robertinoides bradyi, T. angulosa, and Uvigerina peregrina (Figs. F31, F32). Most of the agglutinated and calcareous tests are stained with rose bengal. The high abundance of agglutinated taxa and scarcity of high-productivity calcareous indicators (bolivinids, buliminids, and uvigerinids) suggests reduced organic export flux to the seafloor and more limited food resources than at the shallower Site U1427 located in a productive upper slope environment.

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