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doi:10.2204/iodp.proc.346.109.2015

Biostratigraphy

At Sites U1428 and U1429, two Late Pleistocene sedimentary successions (~212 and ~178 m thick, respectively) were recovered. With the exception of the sand intervals in Hole U1428A, nannofossils are generally abundant and exhibit good preservation. One calcareous nannofossil datum, the first occurrence (FO) of Emiliania huxleyi, is documented. Diatoms are generally common and exhibit good preservation. No biostratigraphically useful marker species are recorded. Variable abundances of Chaetoceros spores and Paralia sulcata indicate the occasional presence of a productive coastal environment. Freshwater diatoms show significant peaks throughout the succession. Planktonic foraminifers are abundant and well preserved, except in the sand horizons, which are barren. Planktonic foraminiferal assemblages are diverse and typical of subtropical environments. The last occurrence (LO) of Globigerinoides ruber (pink) recorded at Sites U1428 and U1429 is consistent with the calcareous nannofossil datum. Benthic foraminifers vary markedly in abundance but are generally well preserved. The overall composition of assemblages indicates upper bathyal paleodepths. Downcore changes in assemblage composition at Sites U1428 and U1429 reflect variations in organic export fluxes to the seafloor and bottom water ventilation that are probably linked to glacial–interglacial oscillations in surface productivity and ocean circulation. The integrated calcareous microfossil biozonation is shown in Figure F18 with microfossil datums presented in Table T6. A biostratigraphic age-depth plot is shown in Figure F19. See “Stratigraphic correlation and sedimentation rates” for a discussion of sedimentation rates at Sites U1428 and U1429.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on the analysis of 62 core catcher and split-core section samples from Holes U1428A (25 samples), U1428B (15 samples), and U1429A (22 samples). Nannofossils are present throughout the Pleistocene succession, but the sand intervals tend to contain few or no nannofossils. A barren sand interval is present in Hole U1428A near 91.8 m CSF-A (Sample 346-U1428A-10H-CC), and few to no nannofossils are recorded deeper than 137.3 m CSF-A (Sample 15H-CC), also corresponding to sand layers (Table T7; Fig. F20). Outside of the sand intervals, nannofossils are generally abundant and preservation is mostly good (Table T7).

Nannofossil diversity at Sites U1428 and U1429 is high. The nannofossil assemblage consists of 29 taxa, including Braarudosphaera bigelowii, Calcidiscus leptoporus, Calciosolenia murrayi, Ceratolithus cristatus, Coccolithus pelagicus, E. huxleyi, Florisphaera profunda, Gephyrocapsa caribbeanica, Gephyrocapsa muellerae, Gephyrocapsa oceanica, Gephyrocapsa spp. (>4 µm), Gephyrocapsa spp. large (>5.5 µm), small Gephyrocapsa spp. (<4 µm), Helicosphaera carteri, Helicosphaera inversa, Helicosphaera pavimentum, Helicosphaera spp., Helicosphaera wallichii, Oolithotus fragilis, Pontosphaera japonica, Pontosphaera multipora, Pontosphaera spp., Reticulofenestra minuta, Rhabdosphaera clavigera, Syracosphaera spp., Umbellosphaera irregularis, Umbellosphaera tenuis, Umbilicosphaera foliosa, and Umbilicosphaera sibogae (Fig. F21). Reworked species include Discoaster barbadiensis (LO at 34.76 Ma) in Sample 346-U1428A-1H-CC (6.32 m CSF-A); Pseudoemiliania lacunosa (LO at 0.44 Ma) in Samples 346-U1428A-12H-5W, 75 cm, 346-U1428B-13H-CC, and 346-U1429A-12H-CC (108.1, 107.2, and 109.1 m CSF-A, respectively); and Sphenolithus spp. (LO at 3.54 Ma) in Sample 346-U1428B-12H-CC (107.2 m CSF-A) (Table T7).

The base of nannofossil Zones CN15/NN21 is recognized (Fig. F18) based on the FO of E. huxleyi in all three holes drilled at Sites U1428 and U1429. Although E. huxleyi is generally common to dominant when present, its distribution is discontinuous, particularly near the beginning of its range (Table T7). As such, E. huxleyi may occur deeper than Section 346-U1429A-13H-4W (114.6 m CSF-A), although it was not observed in the studied core catcher samples. The underlying strata are included in nannofossil Zones CN14b/NN20 based on the absence of P. lacunosa, but the base of these zones (i.e., the LO of P. lacunosa) is not identified.

Radiolarians

A total of 26 core catcher samples from Holes U1428A and U1429A were prepared for radiolarian analyses. Radiolarians are generally abundant throughout the section (Table T8; Fig. F20).

Buccinosphaera invaginata and Collosphaera tuberosa, which are key species of the tropical radiolarian zonal scheme, are scarce in Holes U1428A and U1429A (Table T8); therefore, datum levels are not specified. Because Stylatractus universus, which became extinct at 0.44 Ma in the Pacific, is absent in the whole section, the section may be placed in Zones RN16 (Middle Pleistocene) and RN17 (Late Pleistocene) in the low-latitude radiolarian zonation (Fig. F18).

Radiolarian assemblages in Holes U1428A and U1429A are mainly composed of subtropical species (e.g., Dictyocoryne profunda, Dictyocoryne truncatum, Didymocyrtis tetrathalamus, Euchitonia furcata, Hymeniastrum euclidis, and the Octopyle/Tetrapyle group). In addition to those radiolarians, Actinomma boreale, Actinomma leptodermum, Cycladophora davisiana, and Lithomelissa setosa are common.

Based on counts of >200 specimens in 19 core catcher and mudline samples from Hole U1429A, significant changes in the relative abundance of seven radiolarian groups are documented in Figure F22. The Octopyle/Tetrapyle group fluctuates significantly through the section, with a similar trend to the sediment color index L* (e.g., high abundances of this taxon coincide with high L* values). Such increasing abundance of this radiolarian group suggests higher nutrient supply from the shelf area of the East China Sea (Chang et al., 2003). In contrast, A. boreale and A. leptodermum show low abundances coinciding with high L* values. Common occurrences of both species might be associated with the presence of cold water at any depth in the water column as demonstrated in the shallow water of the Norwegian fjords (Bjørklund, 1974), the Arctic Seas (Itaki et al., 2003), and the deep water of the marginal sea between the Korean Peninsula and the Japanese Islands (Itaki, 2003).

Diatoms

Diatom biostratigraphy was based on smear slides from core catcher and toothpick samples. A total of 37 core catcher samples were examined (18 from Hole U1428A and 19 from Hole U1429A). In addition, two toothpick samples from Hole U1429A were also observed. Because no biostratigraphically useful marker species were found, age assignments were not possible. Preservation is good in the upper 63 m CSF-A in Hole U1428A, becoming moderate toward the base of the succession. In Holes U1428A and U1429A, preservation is good throughout the succession. Significant amounts of reworked diatom specimens are found in both sites, and no diatoms were observed in the sand samples (Table T9; Fig. F23).

Diatom abundances decrease from abundant to rare toward the base of the succession at Sites U1428 and U1429. Chaetoceros spores and P. sulcata abundances vary from barren to abundant throughout the succession (Table T9; Fig. F23), indicating the occasional presence of a productive coastal environment. Diatom freshwater species show significant peaks in abundance at both sites.

Planktonic foraminifers

Planktonic foraminifers were examined in core catcher samples from Holes U1428A and U1429A (41 samples). Relative abundance of taxa and visual estimates of assemblage preservation are presented in Table T10. Planktonic foraminifers are abundant and well preserved, accounting for >99% of the total foraminiferal assemblage, except in samples from sand horizons, which are barren (Samples 346-U1428A-15H-CC, 18H-CC, 21H-CC, 24H-CC, and 26H-CC) (Fig. F20).

Planktonic foraminiferal assemblages are diverse and typical of subtropical environments. Common species include Globigerina bulloides, Globigerinoides glutinata, G. ruber, Globigerinoides sacculifer, Globorotalia menardii, Globorotalia truncatulinoides, Neogloboquadrina dutertrei, Neogloboquadrina incompta, Neogloboquadrina pachyderma (dextral), Orbulina universa, and Pulleniatina obliquiloculata (Fig. F24). The LO of G. ruber (pink), which is recorded at 0.12 Ma in the Pacific and Indian Oceans, occurs between Samples 346-U1428A-5H-CC (44.83 m CSF-A) and 6H-CC (54.7 m CSF-A), between Samples 346-U1428B-5H-CC (40.62 m CSF-A) and 6H-CC (50.1 m CSF-A), and between Samples 346-U1429A-7H-CC (61.38 m CSF-A) and 8H-CC (71.0 m CSF-A). The species Globigerinella calida is recorded in Samples 346-U1428A-1H-CC through 14H-CC (6.3–129.9 m CSF-A), except for Sample 10H-CC. In Hole U1429A, this species is observed intermittently in Samples 346-U1429A-1H-CC through 16H-CC (6.4–137.6 m CSF-A) (Table T10). The FO of this species, recorded at 0.22 Ma in the South Pacific (Chapronière et al., 1994), may occur earlier in the East China Sea, as this species was found in Pliocene sediment from the Okinawa-jima Island, southern Japan (Hanagata, 2004). The species Globorotalia flexuosa is observed in Samples 346-U1428A-14H-CC (129.88 m CSF-A), 346-U1429A-7H-CC (61.4 m CSF-A), and 346-U1429A-21H-CC (175.8 m CSF-A). However, the LO of this species is difficult to constrain, as it is extremely rare at Sites U1428 and U1429. The co-occurrence of G. ruber (pink) and G. flexuosa in Samples 346-U1428A-14H-CC, 346-U1428B-14H-CC, and 346-U1429A-21H-CC indicates an age younger than 0.4 Ma for these samples. Based on these occurrences, the planktonic foraminiferal Zone PT1b (Pleistocene) is recognized, which is in agreement with the calcareous nannofossil biostratigraphy (Fig. F18; Table T6).

G. ruber and G. bulloides show relatively constant abundances except for a few horizons, where foraminifers are rare (Fig. F24; Table T10). In modern conditions, P. obliquiloculata is more abundant along the edge of the continental shelf within the main path of the Kuroshio Current (Xu and Oda, 1999). The absence of P. obliquiloculata in Samples 346-U1428A-7H-CC (63.8 m CSF-A) and 346-U1429A-8H-CC (71.0 m CSF-A) suggests that these intervals correspond to glacial episodes, when the warm water inflow from the Pacific through the Okinawa Trough is restricted due to topographic changes during lowstands. In the East China Sea, Globigerina quinqueloba shows higher abundance near river mouths, where surface seawater is characterized by low temperature (<13°C in winter and <27.5°C in summer) and low salinity (<34‰ in winter and <32‰ in summer) (Xu and Oda, 1999). This species occurred more abundantly (~30% of assemblages) at ~19–10 ka during the last deglaciation. In Sites U1428 and U1429, this species exhibits relatively high abundance in several horizons (Fig. F24), which may correspond to sea level lowstands. Detailed quantitative assemblage study is required to clarify the timing and amplitude of these assemblage changes.

Benthic foraminifers

Benthic foraminifers were examined in core catcher samples from Holes U1428A (23 samples) and U1429A (19 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 sample recovered in Hole U1428A was 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.

Benthic foraminifers vary markedly in abundance but are generally well preserved throughout the ~212 and ~178 m thick Pleistocene successions recovered in Holes U1428A and U1429A (Figs. F20, F25; Table T11). However, Samples 346-U1428A-15H-CC through 18H-CC (137.3–153.83 m CSF-A), 346-U1428A-24H-CC through 26H-CC (172.91–182.9 m CSF-A), and 346-U1429A-13H-CC (116.42 m CSF-A), which correspond to sand horizons, are barren. Sample 346-U1428A-10H-CC, which is from a reworked ash layer, contains only rare benthic specimens. A total of 89 benthic foraminiferal taxa were identified. Table T11 summarizes the downcore distribution of benthic foraminifers in core catcher samples from Holes U1428A and U1429A. Figures F26 and F27 illustrate characteristic taxa.

The overall composition of assemblages indicates upper bathyal paleodepths throughout the Late Pleistocene. Species frequently recorded through the succession include the calcareous species Ammonia takanabensis, Amphicoryna scalaris, Bolivina robusta, Bulimina aculeata, Bulimina marginata, Chilostomella oolina, Cibicidoides lobatulus, Cibicidoides pachyderma, Globobulimina pacifica, Globobulimina pupoides, Hoeglundina elegans, Melonis barleeanus, Planulina wuellerstorfi, Pullenia bulloides, Quinqueloculina spp., and Uvigerina peregrina as well as the agglutinated species Martinotiella communis and Sigmoilopsis schlumbergeri. Downcore changes in assemblage composition at Sites U1428 and U1429 reflect variations in organic export fluxes to the seafloor and bottom water ventilation that are probably linked to glacial–interglacial oscillations in surface productivity and ocean circulation. In particular, the buliminids, globobuliminids, uvigerinids, and miliolids, which can be used to track changes in productivity and dysoxia (Jorissen et al., 2007), show marked fluctuations in abundance throughout the succession (Fig. F25). Higher abundance of these taxa during intervals of lower carbonate content and lower L* values reflect enhanced productivity, possibly associated with increased monsoonal winds during glacials. Higher frequency variability associated with millennial-scale climate events may be identified in higher resolution studies but is not detected in core catcher samples because of the coarse shipboard sampling resolution.

Ostracods

Ostracods were examined in core catchers and the mudline sample from Hole U1428A during shipboard examination for benthic foraminifers. All specimens present in the >150 µm fraction were picked and transferred to slides for identification and counting. Ostracod taxa were identified to genus level and in some cases to species level. Their preservation varies from moderate to very good, and their abundance varies from absent (deeper than 137 m CSF-A) to ~30 specimens per sample in the upper part of the record (Fig. F20). About 20 genera were recognized, including reworked shallow taxa (e.g., Aurila, Callistocythere, and Puriana) (Table T12). The general assemblage composition indicates upper bathyal paleodepths. The main genera recorded through the succession include (in order of decreasing abundance) Krithe, Legitimocythere, Bradleya, Argilloecia, Cytheropteron, and Amphileberis. Figure F28 shows characteristic ostracods found in Hole U1428A. Downhole changes in assemblage composition in Hole U1428A suggest variability in organic flux to the seafloor and bottom water ventilation. Detailed postexpedition examination will be carried out to complete the characterization of the ostracod assemblage at this site.

Mudline samples

The mudline sample from Hole U1428A was gently washed in order to preserve fragile, agglutinated foraminifer specimens with extremely low fossilization potential. The mudline sample contains rare specimens of the agglutinated species Reophax scorpiurus and Reophax spiculifer and abundant calcareous taxa including Bolivina pacifica, Bolivina subspinescens, B. aculeata, C. oolina, Fursenkoina bradyi, G. pacifica, Hyalinea balthica, P. wuellerstorfi, P. bulloides, and U. peregrina (Figs. F29, F30). The dominance of Globobulimina, Chilostomella, and Fursenkoina spp. in the mudline assemblage points to a high organic export flux and dysoxic conditions at the seafloor and upper centimeters of the sediment. The mudline sample contains few planktonic foraminifers, G. bulloides, G. calida, Globigerina aequilateralis, G. ruber, G. inflata, N. dutertrei, N. incompta, O. universa, and P. obliquiloculata.

The calcareous nannofossil assemblage recovered in the mudline sample from Hole U1428A shows high diversity and good preservation as indicated by the presence of coccospheres, as well as fragile species (e.g., E. huxleyi). Specimens found in the mudline sample include C. leptoporus, C. murrayi, E. huxleyi, F. profunda, G. oceanica, H. carteri, H. wallichii, R. clavigera, Syracosphaera cf. pulchra, U. sibogae, and small Gephyrocapsa spp. (<4 µm) (Fig. F31).

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