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doi:10.2204/iodp.proc.314315316.210.2012

Methods and materials

Sites and samples

Pliocene–Pleistocene calcareous nannofossils from sediments at Site C0004 and in Hole C0008A in the Nankai Trough were analyzed. Downcore sample spacing is about ~150 to ~300 cm; however, additional samples were analyzed for intervals where a biostratigraphic event horizon was located based on initial shipboard analysis.

Site C0004

Two holes were drilled at Site C0004 on the shallow portion of the megasplay fault system during Expedition 316. Hole C0004C (33°13.2278′N, 136°43.3312′E; water depth = 2627.0 m) was cored from the seafloor to 134.97 meters below seafloor (mbsf), through the slope sediments into the upper part of the accretionary prism; and Hole C0004D (33°13.2190′N, 136°43.3287′E, water depth = 2630.5 m) was cored from 100 to 398.79 mbsf, from the upper part of the accretionary prism and across its lower boundary into the sediments below (see the “Expedition 316 Site C0004” chapter [Expedition 316 Scientists, 2009a]). Nannofossils from these two holes were analyzed.

Site C0008

Three holes (C0008A, C0008B, and C0008C) were drilled at Site C0008. Hole C0008A (33°12.8229′N, 136°43.5997′E; water depth = 2751 m) was cored from the seafloor to 272.46 m (see the “Expedition 316 Site C0008” chapter [Expedition 316 Scientists, 2009b]). This hole recovered the longest sediment sequence and was selected for biostratigraphic analysis.

Preparation and observation

Standard smear slides were made for all samples. In addition, a concentration method was applied to obtain more nannofossils from fine sand or silt samples before making smear slides. About 1–3 g sediment were put in a beaker with distilled water and stirred for several seconds to suspend samples. The suspension was left sitting for a few minutes so that sand-sized particles settled to the bottom first, and nannofossils were concentrated in the upper part of the suspension, from which several drops were taken to make smear slides.

Calcareous nannofossils were examined under crossed polarizers and transmitted light at 500×–1000× magnification using a Zeiss Axio Scope A1 in the micropaleontological laboratory of the School of Marine Geosciences, China University of Geosciences (Beijing).

The degrees of preservation of calcareous nannofossil species were noted as follows:

• VG = very good preservation (no evidence of dissolution and/or overgrowth).

• G = good preservation (slight dissolution and/or overgrowth; specimens are identifiable to the species level).

• M = moderate preservation (exhibit some etching and/or overgrowth; most specimens are identifiable to the species level).

• P = poor preservation (severely etched or with overgrowth; most specimens cannot be identified at the species and/or generic level).

Group abundance (at 250× magnification) and relative abundance of individual species (at 1000× magnification) are estimated based on a seven-category scheme:

• D = dominant (>50% or >50 specimens per field of view [FOV]).

• A = abundant (15%–50% or 10–50 specimens per FOV).

• C = common (5%–15% or 1–10 specimens per FOV).

• F = few/frequent (1%–5% or >1 specimen per 1–10 FOVs).

• R = rare (<1% or >1 specimen per 20 FOVs).

• T = trace (<0.1% or <1 specimen per 20 FOVs).

• B = barren (0; this level is used only for group abundance).

Biostratigraphic zonations and timescales

The Neocene timescale followed Lourens et al., (2004). The biostratigraphic zonation of calcareous nannofossils is based upon the schemes of Martini (1971) and Okada and Bukry (1980), modified by Young (1998). The orbitally turned ages for biostratigraphic datums were compiled mainly from Raffi et al. (2006).

A summary of the timescale and nannofossil zones used in this study is given in Figure F3 and Table T1.

Taxonomic remarks

Several species of genus Gephyrocapsa are commonly used as Quaternary biostratigraphic markers. The Gephyrocapsa species show a wide range of variation in sizes and other morphological features, causing problems in identification (Su, 1996). Size-defined morphological groups of this genus, suggested by Young (1998) (i.e., Gephyrocapsa spp. medium [>3.5 µm], Gephyrocapsa spp. medium I [>3.5–<4 µm], Gephyrocapsa spp. medium II [≥4.5–<5.5 µm], and Gephyrocapsa spp. large [≥5.5 µm]), were used during shipboard and shore-based studies. Reticulofenestra pseudoumbilicus is identified by reticulofenestrid specimens having a maximum coccolith length >7 µm in its uppermost range (lower Pliocene), after Young (1998). Identification of other calcareous nannofossils mainly followed the compilation of Perch-Nielsen (1985).

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