Previous   |    Next

doi:10.2204/iodp.proc.346.103.2015

Biostratigraphy

At Site U1422, a ~205 m thick succession of Pliocene to Holocene sediment was recovered. Calcareous nannofossils are rare and sporadically distributed in the upper 40 m of the sequence. Planktonic foraminifers are rare to absent, with moderate to poor preservation throughout most of the succession but are abundant in thin calcareous layers observed at 24–46 m CSF-A. No in situ planktonic foraminifer zones were documented because of assemblage reworking. The radiolarian assemblages show moderate to good preservation except in the interval between 52 and 78 m CSF-A. The radiolarian biostratigraphic zonation ranges from the Cycladophora sakaii Zone (Pliocene) to the Botryostrobus aquilonaris Zone (Late Pleistocene). The diatom assemblage is generally moderate to well preserved; however, there are several intervals in which valve preservation becomes poor to moderate. The diatom stratigraphy spans the interval from Zone NPD 12 (Pliocene) to NPD 7 (Late Pleistocene). The nannofossil, radiolarian, and diatom datums and zonal schemes generally agree with only some minor inconsistencies. The integrated calcareous and siliceous microfossil biozonation is shown in Figure F16, with microfossil datums shown in Table T4. An age-depth plot including biostratigraphic datums is shown in Figure F17. See “Stratigraphic correlation and sedimentation rates” for a discussion of sedimentation rates at Site U1422. Benthic foraminifers occur intermittently throughout the succession and show generally poor preservation. The overall assemblage composition indicates lower bathyal to abyssal paleodepths. The rare occurrence of calcareous microfossils reflects dissolution at the seafloor. The rare occurrence of agglutinated benthic foraminifers is probably due to unfavorable dysoxic conditions at the sediment/water interface.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on analysis of core catcher and split-core section samples from Hole U1422C and split-core section samples from Hole U1422D. Depth positions and age estimates of biostratigraphic marker events are shown in Table T5. Most of the samples studied at Site U1422 lack nannofossils (Figs. F16, F18). Nannofossils are abundant in short and sporadic Pleistocene intervals (Samples 346-U1422C-3H-4, 126 cm [19.87 m CSF-A], 3H-5, 15 cm [20.26 m CSF-A], 5H-4, 66 cm [38.17 m CSF-A], and 346-U1422D-3H-7, 26 cm [21.37 m CSF-A]). With the exception of those samples, nannofossils are generally rare and etched when present. Nannofossils are absent below Sample 346-U1422C-7H-CC (62.17 m CSF-A) through significant portions of the Pliocene to early Pleistocene, with the exception of rare occurrences in Core 346-U1422C-19H (148.17–149.24 m CSF-A; Table T5).

In some samples, calcareous nannofossils co-occur with foraminifers (e.g., Samples 346-U1422C-4H-1, 59 cm [24.20 m CSF-A], 4H-5, 95 cm [30.54 m CSF-A], and 5H-4, 66 cm [38.17 m CSF-A]); however, the majority of samples with foraminifers lacked calcareous nannofossils (Fig. F18). Low-diversity nannofossil assemblages include Braarudosphaera bigelowii, Coccolithus pelagicus, Emiliania huxleyi, Gephyrocapsa caribbeanica, Gephyrocapsa muellerae, Gephyrocapsa oceanica, small Gephyrocapsa (<4 µm), Pontosphaera spp., and Reticulofenestra minutula. Nannofossil Zones CN15/NN21 are recognized using the base of E. huxleyi (Samples 346-U1422C-3H-5, 15 cm [20.26 m CSF-A], and 346-U1422D-3H-6, 115 cm [20.76 m CSF-A]). Nannofossil Zones CN14b/NN20 are also recognized based on the presence of G. oceanica and the absence of Pseudoemiliania lacunosa (24.19–38.17 m CSF-A in Hole U1422C and 21.36–28.5 m CSF-A in Hole U1422D). The downhole extension of Zones CN14b/NN20 is not constrained because of the absence of nannofossils below 38.17 and 28.5 m CSF-A in Holes U1422C and U1422D, respectively.

Radiolarians

A total of 31 core catcher samples from Hole U1422C were prepared for radiolarian analyses. Radiolarians are generally common to abundant in the sequence (Fig. F18), although they are rare or absent between 52.3 and 77.8 m CSF-A (Samples 346-U1422C-6H-CC through 9H-CC).

In Hole U1422C, key species that define the base or top of the Stylatractus universus and Eucyrtidium matuyamai radiolarian zones were not observed. Six secondary datums, however, were found (Table T4). The last occurrence (LO) of Late Pleistocene datums Amphimelissa setosa (0.08 Ma) and Spongodiscus sp. (0.29 Ma) are observed in Samples 346-U1422C-2H-CC (14.3 m CSF-A) and 3H-CC (24.0 m CSF-A), respectively. The LO of Axoprunum acquilonium is in Sample 10H-CC (87.8 m CSF-A) just below the poor preservation interval between 52.3 and 77.8 m CSF-A (Samples 6H-CC through 9H-CC). The Pleistocene/Pliocene boundary is close to the first occurrence (FO) of Cycladophora davisiana (2.7 Ma) in Sample 16H-CC (135.1 CSF-A) and the LO of Hexacontium parviakitaensis (2.7 Ma) in Sample 18H-CC (144.4 m CSF-A). At the bottom of Hole U1422C (205.6 m CSF-A, Sample 31H-CC), the estimated age is younger than 3.9–4.3 Ma, based on the occurrence of H. parviakitaensis and the absence of Dictyophimus bullatus.

Radiolarian faunal assemblages are closely related to surface and deepwater oceanographic conditions (e.g., Casey, 1977). For example, Dictyocoryne truncatum and the Octopyle/Tetrapyle group are typical warm-water radiolarians related to the TWC (Itaki et al., 2007). Sporadic occurrences of these species in Hole U1422C (Table T6) suggest minor TWC influence since 4 Ma. Ceratospyris borealis, a subarctic Pacific species (Motoyama and Nishimura, 2005), is one of the most common species in the sequence, especially shallower than Sample 346-U1422C-20H-CC (153.8 m CSF-A).

Diatoms

Diatom biostratigraphy is based on smear slides from core catcher samples. A total of 31 core catcher samples were examined, and five datums were identified (Table T4). Although diatoms were identified and counted across at least an entire coverslip transect, the number of transects was increased as needed because identification was not always possible because of the high clay content, too high diatom content, and/or high level of fragmentation. Diatom species counts are shown in Table T7. Diatoms are absent or rare in the upper 100 m CSF-A, becoming dominant, abundant, or common between 100 and 210 m CSF-A (Fig. F18). This shift coincides with the Pliocene–Pleistocene transition. Overall, five diatom zones (Fig. F16) were found: NPD 12 (Samples 346-U1422C-1H-CC through 5H-CC), NPD 11/10 (Samples 6H-CC through 12H-CC), NPD 9 (Samples 13H-CC through 20H-CC), NPD 8 (Samples 21H-CC through 25H-CC), and the top of NPD 7 (Sample 26H-CC). The poor preservation of diatoms in the upper 100 m prevents the identification of Zones NPD 11 and NPD 10, and the LO of Actinocylus oculatus datum was not found. The base of Hole U1422C is younger than 5.56 Ma (because of the occurrence of Shionodiscus oestrupii), with the oldest age estimate obtained from Sample 346-U1422C-25H-CC, which is younger than 3.4–3.0 Ma (FO of Neodenticula koizumii).

Reworked Neodenticula kamtschatica valves were found in Samples 346-U1422C-5H-CC, 7H-CC, and 11H-CC. This reworking was also noted by Koizumi (1992) at Site 795, the closest location to Site U1422 for which reference information is available. Therefore, the presence of this species in the poorly preserved samples (346-U1422C-1H-CC, 2H-CC, 4H-CC, and 6H-CC to 10H-CC) and in Zone NPD 8 samples (21H-CC to 25H-CC) was not considered for biostratigraphic purposes.

Freshwater species (Table T7) were found in Samples 346-U1422C-1H-CC, 2H-CC, 11H-CC, 17H-CC, 25H-CC, 29H-CC, and 30H-CC. Opal phytoliths (land plant silica concretions) were found in Cores 346-U1422C-10H, 14H, and 16H. The presence of these two indicators might be related to either freshwater input and/or wind transportation. Chaetoceros spores, which are upwelling indicators, were found in Samples 346-U1422C-2H-CC, 3H-CC, 5H-CC, 11H-CC to 17H-CC, 19H-CC, 21H-CC, and 23H-CC to 31H-CC.

Highly fragmented large centric diatoms (>50 µm) were found from 110 to 210 m CSF-A. These fragments are very well preserved, and their fragmentation could be the result of bottom transport rather than dissolution. However, significant numbers of >150 µm centric diatoms belonging to the Coscinodiscus and Arachnodiscus genera (Fig. F19) were found in the >150 µm residue from the foraminifer samples (346-U1422C-5H-CC, 6H-CC, and 12H-CC to 31H-CC). The fact that such large diatoms were not found in the smear slides may imply breakage of the specimens when making these. However, this fragmentation causes a biased number of counts, as the diatom species can often be identified but cannot be counted as a whole valve as it does not contain >50% of the valve area. Therefore, the counts are biased toward the smaller pennate diatoms that can be easily found at 1000× magnification. In order to assess this fragmentation, a simple fragmentation index was calculated to qualitatively assess its impact. The index is calculated by dividing the number of centrics by the number of pennate diatoms counted. The closer the index is to zero, the higher the fragmentation, assuming that no intact centrics are found. The fragmentation index always needs to be complemented with information either from the foraminifer sample or from microscopic observation at 630× magnification in order to verify the presence of large centrics.

The presence of specimens with a diameter >150 µm belonging to the genera Arachnodiscus and Coscinodiscus are only found in the foraminifer sample residue (>150 µm), as these are not observable on the microscopic slides because they break when the slides are prepared. Some Coscinodiscus marginatus >50 and >100 µm were found in the smear slides from Samples 346-U1422C-3H-CC, 9H-CC, 13H-CC, 16H-CC, 20H-CC, 24H-CC, 25H-CC, and 28H-CC to 30H-CC. Nevertheless, in order to identify the species found in the foraminifer sample residue >150 µm, some specimens were observed with the scanning electron microscope (Fig. F19). These large diatoms were also found in the mudline sample from Core 346-U1422D-1H.

Planktonic foraminifers

Planktonic foraminifers were examined in core catcher samples from Holes U1422A (1 sample), U1422B (1 sample), U1442C (30 samples), and U1422D (2 samples) and were supplemented by toothpick samples from Hole U1422C (13 samples) after the core sections were split. Additionally, mudline samples recovered in Holes U1422C and U1422D were investigated. Planktonic foraminifers are confined to the upper part of the succession recovered at Site U1422 (Fig. F18; Cores 346-U1422C-1H to 8H and Hole U1422D). Within this interval, the assemblages are impoverished and generally poorly preserved with evidence of intense dissolution and/or pyritization. In the 35 (~30 cm³) samples examined from the core catchers, only 2 samples contain >35 specimens and 5 samples contain 5 or fewer specimens (Table T8). All other core catcher samples are barren. Of the 13 toothpick samples examined from thin carbonate-rich layers, 4 samples contained >70 specimens, 1 sample contained 44 specimens, and 1 had only 2 specimens. All other toothpick samples were barren. The abundance of planktonic foraminifers per standardized volume is, thus, generally much higher in the thin carbonate-rich layers in the upper part of the succession (to 58.66 m CSF-A; Sample 346-U1422C-7H-5, 55–56 cm) than in core catcher samples. However, most of the thin carbonate-rich horizons are interpreted to be turbiditic layers (see “Lithostratigraphy”), which mainly contain reworked and size-sorted planktonic foraminiferal tests.

The Site U1422 planktonic foraminiferal assemblages are characteristic of cold, midlatitude, restricted environments, consisting mainly of Globigerina bulloides and Neogloboquadrina pachyderma (sinistral) with rare occurrences of Globigerina umbilicata, Globigerina quinqueloba, N. pachyderma (dextral), Neogloboquadrina dutertrei (= Neogloboquadrina himiensis), Neogloboquadrina incompta, and Neogloboquadrina kagaensis group (Neogloboquadrina kagaensis and Neogloboquadrina inglei; Fig. F20). A biostratigraphy based on planktonic foraminifers was not generated at this site, as index markers were mainly reworked. Furthermore, the assemblage zones defined by Maiya (1978) for onshore sections in northern Japan are not applicable at this site because of the lack of diagnostic species in core catcher samples. Taxon relative abundance and estimates of assemblage preservation are presented in Table T8.

Benthic foraminifers

Benthic foraminifers were examined in core catcher samples from Holes U1422A (1 sample), U1422B (1 sample), U1442C (30 samples), and U1422D (2 samples) and were supplemented by samples from Hole U1422C after cores were split. Mudline samples recovered in Holes U1422C and U1422D 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 downhole, all specimens from the >150 µm fraction were picked and transferred to slides prior to identification and counting. The presence and distribution of benthic foraminifers were 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 ~200 m thick, biosiliceous-rich succession of Pliocene to Pleistocene age (Fig. F18; Table T9). Abundance is generally low, and preservation varies significantly. The assemblages predominantly consist of rare calcareous and agglutinated taxa, and their overall composition indicates lower bathyal to abyssal paleodepths throughout the Pleistocene and Pliocene. Prominent variations in the downcore distribution of benthic foraminifers appear to reflect intense dissolution and highly dysoxic conditions at the seafloor (Fig. F21).

A total of 14 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 Hole U1422C. Species recorded include Cibicidoides robertsonianus, Eggerella bradyi, Fursenkoina bradyi, Globobulimina pacifica, Globobulimina pupoides, Melonis pompilioides, and Miliammina echigoensis (Fig. F21). The presence of F. bradyi and Globobulimina spp., taxa that are sensitive to changes in organic flux and tolerant of low oxygen conditions, suggests that changes in surface productivity and bottom water oxygenation occurred throughout the Pleistocene to Pliocene, possibly associated with glacial–interglacial and millennial-scale climate fluctuations. However, the low resolution of our shipboard data set prevents detection of variability that may occur on orbital to suborbital timescales. Well-preserved diatoms and radiolarians are common to abundant in sediment residues >150 and >63 µm throughout the succession and become dominant in most samples deeper than 78 m CSF-A. Pyritized burrows are common in most of the >150 and >63 µm residues examined.

The agglutinated species M. echigoensis is intermittently present from Samples 346-U1422C-15H-CC (132 m CSF-A) to 29H-CC (196 m CSF-A). This taxon is typically found in Pliocene onshore successions of Japan (Kato, 1992; Brunner, 1992). At ODP Sites 794, 795, and 796, the last occurrence of this species was recorded near the Pliocene/Pleistocene boundary.

Ostracods

No ostracods were found in the core catcher samples examined at this site.

Mudline samples

Mudline core top samples from Holes U1422C and U1422D were gently washed to preserve fragile agglutinated specimens with extremely low fossilization potential. The mudline sample from Hole U1422C is barren of benthic and planktonic foraminifers and mainly contains abundant and well-preserved diatoms, radiolarians, and sponge spicules. The mudline sample from Hole U1422D, which had a substantially larger volume than in Hole U1422C, contains only rare benthic foraminifers, including one specimen of Reophax, one specimen of Saccammina, and one specimen of Rhizammina, as well as a few fragile soft-bodied komokiacean-like foraminifers (Fig. F22) and tubular organically cemented agglutinated foraminifers, which were all stained with rose bengal. The mudline sample from Hole U1422D also contains a few fish teeth and abundant diatoms, radiolarians, and sponge spicules.

Top of page   |    Previous   |    Next