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doi:10.2204/iodp.proc.346.102.2015

Biostratigraphy

Preliminary age assignments for Expedition 346 were based on biostratigraphic analyses of calcareous nannofossils, planktonic foraminifers, radiolarians, and diatoms. Benthic foraminifers and ostracods were used primarily for paleoenvironmental interpretation. The biostratigraphy is tied to the geomagnetic polarity timescale (GPTS) (GTS2012) of Gradstein et al. (2012). Calcareous nannofossils, planktonic and benthic foraminifers, radiolarians, diatoms, and ostracods were examined in core catcher samples, and, where appropriate, additional samples were taken to refine biostratigraphic assignments. The preservation, abundance, and zonal assignment for selected samples and for each microfossil group were entered via DESCLogik into the LIMS database.

Calcareous nannofossils

Calcareous nannofossil zonal scheme and taxonomy

Nannofossil taxonomy follows Bown (1998) and Perch-Nielsen (1985). Bioevent ages were assigned based on the occurrence of calcareous nannofossils (dominant, present, or absent) in core catcher samples and in additional split-core sections, when necessary. Calibration of the identified events is derived mainly from Gradstein et al. (2012). The standard zonal schemes of Martini (1971) and Okada and Bukry (1980) are adopted (Fig. F8; Table T1). The first occurrences (FOs) of Gephyrocapsa oceanica and Gephyrocapsa caribbeanica denote the bases of Zones CN14a and CN13b, respectively. Ages and calibration sources for calcareous nannofossil datums are presented in Table T1.

Methods of study for calcareous nannofossils

Samples were prepared following the smear slide technique of Watkins and Bergen (2003) with Norland Optical mounting medium. Calcareous nannofossils were examined with a Zeiss polarized microscope at 1000× magnification. In addition, mudline samples were analyzed for calcareous nannofossils. Mudline samples were collected by emptying the sediment/water material from the top core liner into a bucket from which samples were pipetted for preparation of smear slides and scanning electron microscope (SEM) stubs.

Total calcareous nannofossil abundance within the sediment was recorded as

• D = dominant (>90% of sediment particles).
• A = abundant (50%–90% of sediment particles).
• C = common (10%–50% of sediment particles).
• F = few (1%–10% of sediment particles).
• R = rare (<1% of sediment particles).
• B = barren (none present).

Abundance of individual calcareous nannofossil taxa was recorded as

• D = dominant (>100 specimens per FOV).
• A = abundant (10–100 specimens per FOV).
• C = common (1–10 specimens per FOV).
• F = few (1 specimen per 1–10 FOVs).
• R = rare (≤1 specimen per 10 FOVs).

Preservation of calcareous nannofossils was recorded as

• G = good preservation (little or no evidence of dissolution and/or recrystallization; primary morphological characteristics unaltered or only slightly altered; specimens were identifiable to the species level).
• M = moderate preservation (specimens exhibit some etching and/or recrystallization; primary morphological characteristics somewhat altered; however, most specimens were identifiable to the species level).
• P = poor preservation (specimens were severely etched or overgrown; primary morphological characteristics largely destroyed; fragmentation has occurred; specimens often could not be identified at the species and/or generic level).

Radiolarians

Radiolarian zonal scheme and taxonomy

The radiolarian zonal scheme used during Expedition 346 is described in Morley and Nigrini (1995), Kamikuri et al. (2004, 2007), and Motoyama (1996, 2014) and was established for the Pacific mid to high latitudes including the region’s marginal seas. Some radiolarian datums established in the North Pacific (e.g., the rapid increase and rapid decrease of Siphocampe arachnea [Kamikuri et al., 2007]) are poorly constrained chronostratigraphically in the marginal seas’ sediments. The last occurrence (LO) of Axoprunum acquilonium is reported at 0.33 Ma in the North Pacific (Kamikuri et al., 2007; Matul et al., 2002), although this datum may be older in the marginal sea according to results from DSDP Leg 31 and ODP Leg 127 (Alexandrovich, 1992; Ling, 1992; Motoyama, 1996). At ODP Sites 794, 795, and 797, the age of this datum may be 1.2–1.7 Ma (Alexandrovich, 1992).

For the East China Sea, the tropical Pacific radiolarian zones were used (Sanfilippo and Nigrini, 1998; Nigrini and Sanfilippo, 2001; Kamikuri et al., 2009).

Expedition 342 Scientists (2012) assigned GTS2012 ages to the tropical radiolarian datums (Table T2). All other datums were converted to GTS2012 from previous geologic timescales (Table T3; Fig. F9).

Methods of study for radiolarians

Sample preparation for light microscopy observation was conducted as follows:

1. Approximately 5 cm³ of wet (core catcher) sediment was sieved and rinsed using a 45 µm mesh sieve.

2. When needed, samples were processed with 10% H₂O₂ and 15% hydrochloric acid (HCl) to remove calcium carbonate and clay infillings and resieved on a 45 µm mesh sieve.

3. Residues were dried on a slide, mounted with Norland optical adhesive, and covered with a 22 mm × 40 mm cover glass.

4. The adhesive was solidified by placing the slide under UV light for ~15 min.

5. Slides were partially examined at 50× to 400× for stratigraphic markers and other common taxa using a Zeiss Axioskop microscope.

Abundance estimates of the radiolarian assemblage are qualitative estimates of the concentration of radiolarians in individual sediment samples, using the following categories:

• A = abundant (>10,000 specimens in a sample).
• C = common (2,000–10,000 specimens in a sample).
• F = few (500–2000 specimens in a sample).
• R = rare (50–500 specimens in a sample).
• VR = very rare (<50 specimens in a sample).
• B = barren (0 specimens in a sample).

Abundance of individual radiolarian species was recorded as

• A = abundant (>16% of the radiolarian assemblage).
• C = common (4%–16% of the radiolarian assemblage).
• F = few (1%–4% of the radiolarian assemblage).
• R = rare (0.2%–1% of the radiolarian assemblage).
• P = present (<0.2%).

Preservation of the radiolarian assemblage was recorded as

• G = good (majority of specimens complete, with no or minor dissolution, recrystallization, and/or breakage).
• M = moderate (minor but common dissolution, with a small amount of breakage of specimens).
• P = poor (strong dissolution, recrystallization, or breakage; many specimens unidentifiable).

Diatoms

Diatom zonal scheme and taxonomy

Diatom biostratigraphy for Expedition 346 (Fig. F10) follows the work of Koizumi (1992) (ODP Leg 127/128), Barron and Gladenkov (1995) (ODP Leg 145), Yanagisawa and Akiba (1998), and Watanabe and Yanagisawa (2005). Although the majority of the biostratigraphy follows the work of Koizumi (1992), the zonal schemes are refined as needed.

Diatom ages for zones and datum events are calibrated to the GTS2012 timescale if there are changes from ATNTS2004 (Lourens et al., 2004) and also to be comparable with other microfossil datums (Fig. F10; Table T4). The LOs and FOs as defined by Yanagisawa and Akiba (1998) with modifications by Gladenkov (2003) are applied.

Diatoms are identified to the species level when possible. Key stratigraphic species were identified based on previous work of Koizumi (1992), Barron and Gladenkov (1995), and Yanagisawa and Akiba (1998).

Methods of study for diatoms

Diatom observations are based on smear slides from core catcher samples. If needed, additional smear slides are made from split-core sections in order to refine particular zone assignations or for age improvement. Smear slides are made by picking a small amount of unprocessed sediment with a disposable wood toothpick, spreading it on a slide, and diluting it with one or two drops of distilled water. The slide is then placed on a hot plate (30°–35°C) until the liquid on the slide evaporates. Norland optical adhesive is placed on the coverslip, and this coverslip is then glued to the slide. The adhesive was solidified by placing it under an UV lamp for 10–15 min.

Diatom total qualitative abundance is determined at 1000× magnification using phase contrast light. Observations are made on the two slide transects located at the center of the slide. This slide area is where the sediment is placed before spreading; thus, it is the area with the most consistent amount of sediment between all slides. The number of transects increases if any doubt in the species or their abundance occurs.

Diatom preservation was assessed by applying the following criteria:

• G = good (valves are intact, including the most fragile species; some breaking occurs).
• M = moderate (valves and most broken material show areolae and/or outside dissolution).
• P = poor (extreme dissolution and/or fragmentation prevents species identification).

A question mark (?) regarding preservation is used when the sample does not allow an assessment of this property.

Diatom total abundance, a qualitative measure, is based on the amount of valves found in two slide transects:

• D = dominant (>60% diatoms).
• A = abundant (20%–60% diatoms).
• C = common (5%–20% diatoms).
• F = few (2%–5% diatoms).
• R = rare (<2% diatoms).
• B = barren (none present).

Diatom relative abundance is based on the assemblages found in two slide transects:

• Ma = massive (>10 valves in two slide transects).
• A = abundant (>5 valves in two slide transects).
• C = common (3–5 valves in two slide transects).
• F = few (2 valves in two slide transects).
• R = rare (<2 valves in two slide transects).

For biostratigraphy purposes and in order to minimize biases because of the amount of sediment that is placed on the smear slides, the datums are defined by the presence of at least two specimens in two slide transects. This cutoff is chosen having taken into consideration the identification uncertainties and confidence levels discussed in Fatela and Taborda (2002) and the existence of laboratory cross contamination because of the presence of previous samples’ sediment. Therefore, only relative diatom abundances defined as common or higher are considered for dating purposes. The number of transects is increased if any doubt arises regarding species identification or if only fragments of a particular species are present. The counting procedure and definition of counting units (diatom valves) follows Schrader and Gersonde (1978).

Foraminifers

Planktonic foraminiferal zonal scheme and taxonomy

The (sub)tropical planktonic foraminiferal zonal scheme for the Neogene (M, PL, and PT zones) follows Berggren et al. (1995a, 1995b) with modifications by Wade et al. (2011). For this expedition, the zonal scheme of Maiya (1978) was used, which is based on assemblage changes in the North Pacific Ocean and Japanese onshore sections as summarized in Miwa (2014). Additionally, datums from Lagoe and Thompson (1988) were used, which were recalibrated to the timescale of Cande and Kent (1992) by Lyle, Koizumi, Richter, et al. (1997). In order to maintain consistency, the ages of datums in Lagoe and Thompson (1988), Motoyama et al. (2004), and Miwa (2014) were converted to GTS2012 for Expedition 346.

The planktonic foraminiferal zonal scheme used is illustrated in Figure F11. Ages and calibration sources of planktonic foraminifer datums are presented in Tables T5 and T6. Planktonic foraminiferal taxonomic concepts selectively follow those of Maiya et al. (1976), Takayanagi et al. (1976), Kennett and Srinivasan (1983), Bolli and Saunders (1985), Toumarkine and Luterbacher (1985), Loeblich and Tappan (1988), Spezzaferri and Premoli Silva (1991), Chaisson and Leckie (1993), Leckie et al. (1993), Spezzaferri (1994), Pearson (1995), Chaisson and Pearson (1997), Pearson and Chaisson (1997), and Pearson et al. (2006). A taxonomic list of planktonic foraminifer species from Tables T5 and T6 is given in Table T7.

Benthic foraminiferal taxonomy and paleodepth determination

Taxonomic assignments follow Tjalsma and Lohmann (1983), van Morkhoven et al. (1986), Miller and Katz (1987), Thomas (1990), Van Marle (1991), Katz and Miller (1991), Kato (1992), Jones (1994), Nomura (1995), Hanagata (2003), Hanagata and Hiramatsu (2005), Kaminski and Gradstein (2005), and Holbourn et al. (2013). The generic classification of Loeblich and Tappan (1988) was used and updated in some instances, in particular for uniserial taxa (Hayward, 2002). A taxonomic list of benthic foraminifers recorded during Expedition 346 is given in Table T8.

Paleodepth estimates were based on van Morkhoven et al. (1986) using the following categories:

• Neritic = <200 m.
• Bathyal = 200–2000 m.
• Abyssal = >2000 m.

Methods of study for foraminifers

From each core catcher, 20–30 cm³ of sediment was washed with tap water over a 63 µm wire mesh sieve. Indurated samples were soaked in a 3% hydrogen peroxide (H₂O₂) solution (with a small amount of Borax added) prior to washing. In addition, mudline samples from each hole were analyzed for planktonic and benthic foraminifers. Mudline samples were collected by emptying the sediment/water material from the top core liner of each hole into a bucket and then washed with tap water over a 63 µm wire mesh sieve. Tests using rose bengal were performed to confirm the presence of living ostracods and benthic foraminifers in the mudline sample. All samples were then dried in the sieves in a low-temperature oven at ~50°C and subsequently examined under a binocular light microscope. To avoid contamination of foraminifers between samples, the sieve was thoroughly cleaned, placed into a sonicator for at least 15 min, and then carefully checked. Species identifications for planktonic and benthic foraminifers were generally made on the >150 µm size fractions. The 63–150 µm size fraction was scanned for distinctive taxa. For Expedition 346, planktonic foraminiferal species distribution and range charts are presented in each site chapter. Benthic foraminiferal assemblage composition and paleodepth estimates were based on counts of ~100 specimens from the >150 µm size fractions, where possible. Relative percentages of benthic to planktonic tests were determined by counting specimens in four adjacent quadrants in three different locations on the picking tray.

Total abundance of foraminifers within the sediment was recorded as

• A = abundant (>70% of sediment particles).
• C = common (30%–70% of sediment particles).
• R = rare (<30% of sediment particles).

The following abundance categories were estimated from visual examination of the dried sample for planktonic and benthic foraminifers:

• D = dominant (>30% of foraminiferal assemblage).
• A = abundant (10%–30% of foraminiferal assemblage).
• F = few (5% to <10% of foraminiferal assemblage).
• R = rare (1% to <5% of foraminiferal assemblage).
• B = barren.

The preservation status of planktonic and benthic foraminifers was estimated as

• VG = very good (no evidence of overgrowth, dissolution, or abrasion).
• G = good (little evidence of overgrowth, dissolution, or abrasion).
• M = moderate (calcite overgrowth, dissolution, or abrasion were common but minor).
• P = poor (substantial overgrowth, dissolution, or fragmentation).

In addition, ostracods, pteropods, fish teeth, sponge spicules, and other bioclasts were examined in the >150 µm size fractions of the core catcher samples used for foraminifer analysis.

Abundance of pteropods, fish teeth, sponge spicules, and other bioclasts was noted as

• C = common (>5 specimens per sample).
• R = rare (1–5 specimens per sample).
• B = barren.

Abundance of ostracods was noted as

• A = abundant (>30 specimens per sample).
• C = common (10–30 specimens per sample).
• R = rare (<10 specimens per sample).
• B = barren.

The preservation status of ostracods was estimated as

• VG = very good (valves translucent; no evidence of overgrowth, dissolution, or abrasion).
• G = good (valves semitranslucent; little evidence of overgrowth, dissolution, or abrasion).
• M = moderate (calcite overgrowth, dissolution, or abrasion were common but minor).
• P = poor (substantial overgrowth, dissolution, or fragmentation of the valves).

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