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doi:10.2204/iodp.proc.322.102.2010

Paleomagnetism

Paleomagnetic and rock magnetic investigations on board the Chikyu during Expedition 322 were primarily designed to determine the characteristic remanence directions for use in magnetostratigraphic and tectonic studies. Routine measurements on archive halves could not be conduced with the superconducting rock magnetometer (SRM) because of the reason described in "Superconducting rock magnetometer," so paleomagnetic measurements were performed only on discrete minicores and cube samples taken from the working halves.

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 SRM (2G Enterprises, model 760) unit 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 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.

During the repair work of thrusters on the Chikyu, the SRM was warmed up to room temperature. In February 2009, the SRM was cooled down to liquid helium (He) temperature to prepare for Expeditions 319 and 322, and it was found that the y-axis superconducting quantum interference device (SQUID) was malfunctioning. Before the beginning of Expedition 319, the SRM was warmed up to room temperature and sent back to the manufacturer. The y-axis SQUID was replaced with a new one in the factory during Expedition 319, and the SRM was transported from the manufacturer, set up, refilled with liquid He, and cooled down to liquid He temperature during the port call at Yokkaichi before Expedition 322. During the port call, a leakage of He was found in the high vacuum surrounding the helium dewar, and liquid He evaporated more quickly than usual. Before starting the measurement of Expedition 322 archive halves, liquid He in the dewar of the SRM was almost empty and the SQUID stopped working. Thus, for the paleomagnetic study of Expedition 322, we could only measure remanent magnetization of discrete samples with the spinner magnetometer (see below). It is worthy to point out that the helium gauge in the reservoir reads 15% when it actually is empty. This nonzero gauge problem should be fixed before the next IODP expedition in addition to the problem of He leakage.

Spinner magnetometer

A spinner magnetometer, model SMD-88 (Natsuhara Giken Co., Ltd.) was utilized during Expedition 322 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 Natsuhara 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. Sedimentary rocks recovered from Expedition 322 all have natural remanent magnetization well above the spinner's minimum limit, making the spinner a unique workhorse during Expedition 322. Basaltic rocks were measured with the large holder; however, one of the basaltic rocks could not be measured with an error "Too strong specimen" after demagnetization at ~5 mT. Because the spinner could not give stable readings after this measurement, the actual reason for the error cannot be determined.

Alternating-field demagnetizer

The AF demagnetizer DEM-95 (Natsuhara Giken Co., Ltd.) 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. Most of the paleomagnetic samples from Expedition 322 sites were progressivly demagnetized with this instrument to isolate various magnetic components of the samples. Whenever possible, demagnetization was continued up to 180 mT until an unambiguous and reliable determination of polarity of the stable component of magnetization had been achieved.

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 sister samples in relation to the AF demagnetized samples were step-wisely heated to determine magnetic components and their unblocking temperatures.

Pulse magnetizer

The pulse magnetizer MMPM 10 (Magnetic Measurement, Ltd., UK; www.magnetic-measurements.com) 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 7 ms pulse duration 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. We used this apparatus with the larger diameter coil during Expedition 322 to impart magnetic fields to three axes of a sample before progressive thermal demagnetization.

Anisotropy of magnetic susceptibility

The Kappabridge KLY 3S (AGICO, Inc.), designed for anisotropy of magnetic susceptibility (AMS) measurement, is also available 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 the temperature control unit (CS-3/CS-L) for temperature variation of low-field magnetic susceptibility of samples. During Expedition 322, only a limited number of samples were subjected to AMS measurements because the science party needed to concentrate on the nonautomatic measurements of remanent magnetization for shipboard magnetostratigraphy with the spinner magnetometer at many demagnetization steps. For the samples without AMS measurements, only bulk susceptibility was measured on discrete paleomagnetic 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 actual spacing depended on the properties of the core material (e.g., to avoid flow-in, coring disturbances, etc.) and the distribution of interbedded lithologies. For the most part, paleomagnetic sampling concentrated on hemipelagic mud(stone) as the dominant lithology. The orientation of discrete samples is shown in Figure F16.

Measurements

Stepwise demagnetization experiments were conducted on discrete paleomagnetic samples using the AF demagnetizer. For a limited number of samples, stepwise thermal demagnetization was conducted. Acquisition of isothermal remanent magnetization and its thermal demagnetization was conducted on several samples after AF demagnetizaion. In addition to standard paleomagnetic measurements, the AMS was measured on a limited number of discrete samples. For the rest of the samples, only bulk magnetic susceptibility was measured using the Kappabridge KLY 3S magnetic susceptibility meter.

Orientation of discrete samples

We planned for orientation of discrete samples relative to the geographic frame by matching the characteristic features such as fractures or bedding between X-ray CT images and oriented borehole images. However, it was not possible because of the termination of the operation without taking any borehole images at the sites during Expedition 322.

Paleomagnetic reorientation of cores

Azimuthal orientation of drilled core material 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 [VRM] or primary magnetization) with respect to a common reference line that is marked lengthwise along the core. Assuming a moderate sedimentation rate of ~5 cm/k.y. and a magnetization lock-in depth of ~10 cm, a typical sample depth interval of 2.5 cm might be enough to average the secular variation of the geomagnetic field, and the paleomagnetic direction roughly points in the direction of geographic north. During Expedition 322, the VRM or primary magnetization after removal of drilling-induced remanent magnetization above 10 mT was used to reorient the blocks with important directional structural features (see "Structural geology").

Magnetic reversal stratigraphy

Magnetic polarity was determined based on the sign of inclination. Sites C0011 and C0012 have a latitude of 32°50′N (32°45′N), which translates into an expected inclination of ±52.2° (±52.1°). This inclination is steep enough to distinguish the magnetic polarity (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. The demagnetization behavior of remanent magnetization was inspected visually on the demagnetization plots (e.g., Zijderveld plots). 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 changes are the most frequent, best-dated, and globally synchronous geophysical phenomena. Magnetostratigraphy is a tool of great promise for precise temporal correlation and accurate dating in sediments. It is based on the facts that Earth's magnetic field has occasionally reversed its polarity and that many sedimentary rocks retain a magnetic imprint of the field at the time they were deposited. Because the length of time for the geomagnetic field to flip from one polarity state to the other is only a few thousand years, the boundaries between magnetozones (stratigraphic zones of single polarity) in sections of magnetized rocks are extremely sharp, much sharper than between typical biostratigraphic zones. During Expedition 322, magnetozones were recognized on the basis of distinct intervals of magnetic reversal zones and biostratigraphic datum events. Specifically, if the approximate age of the sediment is known from biostratigraphic information, it should serve as an anchor point to allow us to focus on a particular part of the established polarity timescale, and thus to identify specific reversals in the sedimentary sequence. 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, Olduvai, etc.). In general, polarity reversals occurring at core ends have been treated with caution. The ages of the polarity intervals used during Expedition 322 are the same as those for Expeditions 315 and 316, which is the magnetostratigraphic timescale for Neogene by Lourens et al. (2004) (Table T11).

For calcareous nannofossil zonations, Raffi et al. (2006) was used as a standard timescale for the entire NanTroSEIZE, including this expedition (see "Biostratigraphy"). They rely on ATNTS2004 (Lourens et al., 2004). Although the magnetostratigraphic timescale on their figures 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) for consistency within NanTroSEIZE expeditions. We used corrected boundary ages for Chron C5ABn listed as "13.252 13.466 C5ABn" in the GPTS table used for Expeditions 315 (Expedition 315 Scientists, 2009, table T7) and 316 (Expedition 316 Scientists, 2009, table T10) with the values of 13.369 (top) and 13.605 (bottom) according to Lourens et al. (2004). 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 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 equal area projections) was conducted by using a visualization software called Progress installed on the PC controlling the spinner magnetometer described above. Principal component analysis (Kirschvink, 1980) was also performed by using Progress software to determine characteristic remanent magnetization directions. In addition, Igor Pro software (Wavemetrics Co., Inc.) was used for data analysis and plotting in combination with Microsoft Excel.

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