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doi:10.2204/iodp.proc.348.102.2015

Biostratigraphy and paleomagnetism

Biostratigraphy

During Expedition 348, calcareous nannofossils were systematically studied to assign preliminary ages to cuttings and core samples collected from Holes C0002M, C0002N, and C0002P.

Timescale and biohorizons

Biostratigraphic zones of calcareous nannofossils of sedimentary sequences recovered during Expedition 348 mainly follow the timescale used by biostratigraphic studies carried out during Expeditions 315, 316, and 338 (Expedition 315 Scientists, 2009a; Expedition 316 Scientists, 2009; Strasser et al., 2014a). The review compiled by Raffi et al. (2006) was used to assign ages for the biostratigraphic data. Calcareous nannofossil biostratigraphic zone determination was based on the studies of Martini (1971) and Okada and Bukry (1980) with zonal modification by Young (1999). Astrochronological age estimates for the Neogene rely on the geologic timescale developed by the International Commission on Stratigraphy (ICS) in 2013 (Cohen et al., 2013). The timescale and biostratigraphic zones of calcareous nannofossils are summarized in Figure F13 and Table T8.

Downhole contamination is a common occurrence in riser drilling cuttings and is a potential problem for recognizing a zonal boundary defined by a first occurrence (FO) datum because such a boundary may appear significantly stratigraphically lower than in situ. In order to avoid this problem, a last occurrence (LO) datum, if available, is designated to approximate the zonal boundary; otherwise the biozone was combined with adjacent zones. An additional criterion was used to resolve the reworking of zonal markers (e.g., a datum was defined by the continuous occurrence of a taxon, whereas sparse occurrence was considered reworked).

Calcareous nannofossils

Taxonomic remarks

Identification of calcareous nannofossils followed the taxonomy compilation of Perch-Nielsen (1985). Species from the genus Gephyrocapsa are common Pleistocene biostratigraphic markers. Problems in identification can occur because species of the genus show wide variation in morphological features and size (Su, 1996). Young (1999) suggested size-defined morphological groups of this genus, and this approach was adopted during this shipboard study, including Gephyrocapsa spp. medium I (>3.5 to <4 µm), Gephyrocapsa spp. medium II (≥4.5 to <5.5 µm), and Gephyrocapsa spp. large (≥5.5 µm). Additionally, Reticulofenestra pseudoumbilicus is identified by specimens having a coccolith 7 µm in length in the upper part of its range (the lower Pliocene).

Methods

Standard smear slides were made from cuttings samples at 10 m spacing and core catcher samples within the cored interval with the use of photocuring adhesive as a mounting medium. Calcareous nannofossils were examined using standard light microscope techniques under crossed polarizers and transmitted light at 250× to 2500× magnification with a Zeiss Axio Imager A1m. Preservation and abundance of nannofossils from the core and cuttings samples investigated were recorded in the J-CORES database. The classification of calcareous nannofossil species preservation was based on the following:

  • VG = very good (no evidence of dissolution and/or overgrowth).
  • G = good (slight dissolution and/or overgrowth; specimens are identifiable to the species level).
  • M = moderate (exhibits some etching and/or overgrowth; most specimens are identifiable to the species level).
  • P = poor (severely etched or overgrown; most specimens cannot be identified at the species level and/or generic level).

Relative abundances of nannofossil assemblages were based on observations in two traverses at 1250× magnification. Samples were further observed for zonal markers and rare species. Group abundance (at 250× magnification) and relative abundance of individual calcareous nannofossil species (at 1250× magnification) are estimated using the following scale:

  • D = dominant (>50% or >50 specimens per field of view [FOV]).
  • A = abundant (>15%–50% or >10 to 50 specimens per FOV).
  • C = common (>5%–15% or 1 to 10 specimens per FOV).
  • F = few/frequent (1%–5% or >1 specimen per 1–10 FOV).
  • R = rare (<1% or >1 specimen per 20 FOV).
  • T = trace (<0.1% or <1 specimen per 20 FOV).
  • B = barren (0; this degree is used only for the group abundance).

Paleomagnetism

Paleomagnetic and rock magnetic investigations on board the Chikyu during Expedition 348 were primarily designed to determine the characteristic remanence directions for use in magnetostratigraphic and structural studies. Routine measurements on archive halves were conducted with the superconducting rock magnetometer (SRM).

Laboratory instruments

The paleomagnetism laboratory on board the Chikyu houses a large (7.3 m × 2.8 m × 1.9 m) magnetically shielded room, with its long axis parallel to the ship transverse. The total magnetic field inside the room is ~1% of Earth’s magnetic field. The room is large enough to comfortably handle standard IODP core sections (~150 cm). The shielded room houses the equipment and instruments described in this section.

Superconducting rock magnetometer

The long-core 4 K SRM (2G Enterprises, model 760) uses a Cryomech pulse tube cryocooler to achieve the required 4 K operating temperatures without the use of liquid helium. The SRM system is ~6 m long with an 8.1 cm diameter access bore. A 1.5 m split core liner can pass through a magnetometer, an alternating field (AF) demagnetizer, and an anhysteretic remanent magnetizer. The system includes three sets of superconducting pickup coils, two for transverse moment measurement (x- and y-axes) and one for axial moment measurement (z-axis). The noise level of the magnetometer is <10–7 A/m for a 10 cm3 volume of rock. The magnetometer includes an automated sample handler system (2G804) consisting of aluminum and fiberglass channels designated to support and guide long core movement. The core itself is positioned in a nonmagnetic fiberglass carriage that is pulled through the channels by a rope attached to a geared high-torque stepper motor. A 2G600 sample degaussing system is coupled to the SRM to allow automatic demagnetization of samples up to 100 mT. The system is controlled by an external computer and enables programming of a complete sequence of measurements and degauss cycles without removing the long core from the holder.

Spinner magnetometer

A spinner model SMD-88 (Natsuhara Giken Co., Ltd.) magnetometer was utilized during Expedition 348 for remanent magnetization measurement. The noise level is ~5 × 10–7 mAm2, and the measurable range is from 5 × 10–6 to 3 × 10–1 mAm2. Two holders are prepared for the measurements, one (small or short) for the weak samples and the other (large or tall) for the strong samples. Five standard samples with different intensities are prepared to calibrate the magnetometer. Standard 2.5 cm diameter × 2.2 cm long samples taken with a minicore drill or 7 cm3 cubes can be measured in three or six positions with a typical stacking of 10 spins. The whole sequence takes ~1 and 2 min, respectively.

Alternating field demagnetizer

The DEM-95 (Natsuhara Giken Co., Ltd.) AF demagnetizer is set for demagnetization of standard discrete samples of rock or sediment. The unit is equipped with a sample tumbling system to uniformly demagnetize up to a peak AF of 180 mT.

Thermal demagnetizer

The TDS-1 (Natsuhara Giken Co., Ltd.) thermal demagnetizer has a single chamber for thermal demagnetization of dry samples over a temperature range from room temperature to 800°C. The chamber holds up to 8 or 10 cubic or cylindrical samples, depending on the exact size. The oven requires a closed system of cooling water, which is conveniently placed next to the shielded room. A fan next to the µ-metal cylinder that houses the heating system is used to cool samples to room temperature. The measured magnetic field inside the chamber is <20 nT.

Pulse magnetizer

The MMPM 10 (Magnetic Measurement, Ltd., UK; www.magnetic-measurements.com) pulse magnetizer can produce a high–magnetic field pulse in a sample. The magnetic-field pulse is generated by discharging a bank of capacitors through a coil. A maximum field of 9 T with a pulse duration of 7 ms can be produced by the 1.25 cm diameter coil. The other coil (3.8 cm diameter) generates a maximum field of 2.9 T.

Anisotropy of magnetic susceptibility

A Kappabridge KLY 3S (AGICO, Inc.), designed for anisotropy of magnetic susceptibility (AMS) measurement, is also installed on the Chikyu. Data are acquired from spinning measurements around three axes perpendicular to each other. Deviatoric susceptibility tensor can then be computed. An additional measurement for bulk susceptibility completes the sequence. Sensitivity for AMS measurement is 2 × 10–8 SI. Intensity and frequency of field applied are 300 mA/m and 875 Hz, respectively. This system also includes a temperature control unit (CS-3/CS-L) for temperature variation of low-field magnetic susceptibility of samples.

Discrete samples and sampling coordinates

Discrete cubic samples (~7 cm3) or minicores (~11 cm3) were taken, two per section, from the working halves in order to determine paleomagnetic direction primarily for magnetostratigraphy. The relation between orientations of archive section and that of discrete samples is shown in Figure F14.

Magnetic reversal stratigraphy

Whenever possible, we offer an interpretation of the magnetic polarity, with the naming convention following that of correlative anomaly numbers prefaced by the letter C (Tauxe et al., 1984). Normal polarity subchrons are referred to by adding suffixes (e.g., n1, n2, etc.) that increase with age. For the younger part of the timescale (Pliocene–Pleistocene), we often use traditional names to refer to the various chrons and subchrons (e.g., Brunhes, Jaramillo, Olduvai, etc.). In general, polarity reversals occurring at core ends have been treated with extreme caution. The ages of the polarity intervals used during Expedition 348 are a composite of four previous magnetic polarity timescales (magnetostratigraphic timescale for the Neogene by Lourens et al., 2004) (Table T9).