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doi:10.2204/iodp.proc.333.102.2012

Paleomagnetism

Paleomagnetic and rock magnetic investigations on board the Chikyu during Expedition 333 were primarily designed to determine the characteristic remanence directions for use in magnetostratigraphic and tectonic studies. Routine measurements on archive halves could not be conducted because the superconducting rock magnetometer was being repaired at the factory and a system was being upgraded during this expedition. Paleomagnetic measurements were thus only performed on discrete sediment and sedimentary rocks and basalts (~2.2 cm cubes of 7 cm³) taken from the working halves using spinner magnetometers as described below. Before the paleomagnetic measurements were taken, bulk magnetic susceptibility was measured.

Spinner magnetometer

Two spinner magnetometers, models SMD-88 and ASPIN (Natsuhara Giken Co., Ltd.), were utilized during Expedition 333 for remanent magnetization measurement. They were installed in a large magnetically shielded room (7.3 m × 2.8 m × 1.9 m) in the paleomagnetism laboratory. The total magnetic field in this room is ~1% of Earth’s magnetic field. One magnetometer (ASPIN) was borrowed from the Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) for this expedition.

The spinner magnetometers can measure magnetization from 5 × 10^–6 to 3 × 10^–1 mAm² with noise levels of ~5 × 10^–7 mAm². Sediments collected during this expedition were measured using a short-type holder for the 7 cm³ cube samples. Basalt rocks were measured using a tall-type holder for a 7 cm³ sample. The samples were measured in six different positions with data stacking of usually 10 or 20 spins and sometimes up to 64 spins, depending on the intensity of samples.

Alternating-field demagnetizer

Two alternating-field (AF) demagnetizers, DEM-95C and DEM-95 (Natsuhara Giken Co., Ltd.), were set for demagnetization of 7 cm³ discrete samples of sediment and sedimentary rocks (DEM-95 was borrowed from IFREE, JAMSTEC). The units are equipped with a sample tumbling system for AF demagnetization (maximum AF = 180 mT). The samples were demagnetized using these instruments to decipher paleomagnetic data from samples. There are various different magnetic components in samples, so stepwise AF demagnetizations were performed at five steps (Natural remanent magnetization, 5, 10, 20, and 30 mT) after remanent magnetization was measured. We carried out extra steps on a few selected samples to assess their magnetic characteristics in detail.

Thermal demagnetizer

The thermal demagnetizer TDS-1 (Natsuhara Giken Co., Ltd.) 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. Representative samples were step-wisely heated to determine magnetic components and their unblocking temperatures.

Magnetic susceptibility measurement system

Bulk magnetic susceptibility was measured using the Kappabridge KLY 3S (AGICO Inc.), designed for anisotropy of magnetic susceptibility (AMS) measurements. Sensitivity, intensity, and frequency of field applied for AMS measurements are 2 × 10^–8 SI, 300 mA/m, and 875 Hz, respectively.

Coordinate geometry of the samples

Two discrete cube samples (7 cm³) were taken for each section from the working halves. Sampling intervals are dependent on core conditions (e.g., to avoid flow-in, coring disturbances, etc.). The samples were selected carefully from undisturbed hemipelagic mud(stone) layers. Coordinate geometry of the sample is shown in Figure F16.

Paleomagnetic reorientation of cores

Azimuthal orientation of drilled cores is of prime importance when modeling directional properties of rock formations. Paleomagnetic core reorientation has been successfully used for a number of years (e.g., Fuller, 1969; Kodama, 1984; Shibuya et al., 1991). The procedure is based on determining the direction of stable remanent magnetization (either viscous remanent magnetization or primary magnetization) with respect to a common reference line that is scribed lengthwise along the core.

Magnetic reversal stratigraphy

Magnetic polarity was determined based on inclination. Sites C0011, C0012, and C0018 are at latitudes 32°50′N, 32°45′N, and 33°09′N, which translates into expected inclinations of 52.2°, 52.1°, and 52.6°, respectively. This inclination is steep enough to distinguish the magnetic polarities (normal or reversed) on the sign of the magnetic inclination (positive or negative) for at least Neogene and Quaternary sediments, even with the expected tectonic latitudinal migration. Zijderveld plots were inspected visually for behavior of remanent magnetization after demagnetization. Stable (primary) remanent magnetization directions of discrete samples were fit using principal component analysis (Kirschvink, 1980), and the obtained inclination was used for the determination of paleomagnetic field polarity.

Geomagnetic polarity reversals are well dated and globally synchronous geophysical phenomena. Magnetostratigraphy is a well-proven tool for precise temporal correlation and accurate dating in sediments. During this expedition, magnetozones were recognized on the basis of distinct intervals of magnetic reversal zones, also taking into account tephrochronologic and biostratigraphic data. Normal polarity subchrons are referred to by adding suffixes (n1, n2) that increase with age. For the younger part of the timescale (Pliocene–Pleistocene), we use traditional names to refer to the various chrons and subchrons (e.g., Brunhes, Jaramillo, etc.). In general, polarity reversals occurring at core ends have been treated with extreme caution. The ages of the polarity intervals used during this expedition are the same as those for Expeditions 315, 316, and 322, which is the magnetostratigraphic timescale for the Neogene by Lourens et al. (2004) (Table T6).

For calcareous nannofossil zonations, Raffi et al. (2006) was used as a standard timescale for the entire NanTroSEIZE, including this expedition (see “Biostratigraphy”). This timescale relies on ATNTS2004 (Lourens et al., 2004). Although the magnetostratigraphic timescale on figures in Raffi et al. (2006) incorporated slight modifications to ATNTS2004, there is no clear explanation with an appropriate table for these modifications. Because of this and the fact that there is no significant change affecting the main chron boundaries, we used ATNTS2004 (Lourens et al., 2004) to keep consistency within NanTroSEIZE expeditions. However, we need to note that the boundary ages for Chron C5ABn listed as “13.252-13.466 C5ABn” in the geomagnetic polarity timescale (GPTS) table used for Expeditions 315 (Expedition 315 Scientists, 2009, table T7) and 316 (Expedition 316 Scientists, 2009, table T10) should be replaced with the correct values of 13.369 (top) and 13.605 (bottom) according to Lourens et al. (2004) because they are using the same timescale. For the up-to-date Neogene GPTS for reference, we need to consider the discussion by Hilgen (2008), especially for Chrons C7n through C5Cn.1n between 24.1 and 15.9 Ma and for chron boundaries in the interval between 12.5 and 8.5 Ma. These might be included in the shore-based study and for future NanTroSEIZE expeditions.

Data reduction and software

Data reduction (Zijderveld demagnetization plots and Schmidt net projections) was conducted using visualization software called “Progress” installed on the PC controlling the spinner magnetometers. Principal components were analyzed using Progress to determine characteristic remanent magnetization directions based on Kirschvink (1980).

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