Previous   |    Next

doi:10.2204/iodp.proc.338.102.2014

Paleomagnetism

Paleomagnetic and rock magnetic investigations on board the Chikyu during Expedition 338 were primarily designed to determine the characteristic remanence directions for use in magnetostratigraphic and structural studies of cores. Routine measurements on archive halves were conducted with the 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 SRM (2G Enterprises, model 760) unit was upgraded from the liquid helium cooled system to the liquid helium–free cooling system “4 K SRM” in June 2011. The 4 K SRM uses a Cryomech pulse tube cryocooler to achieve the required 4 K operating temperature without the use of liquid helium. The differences between the pulse tube cooled system and the liquid helium cooled magnetometers have significant impact on the system ease of use, convenience, safety, and long-term reliability. The other parts of the SRM were not changed from the previous version. 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 measurements (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 handling 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.

Because Hole C0021B core sampling was conducted at KCC, magnetic measurements were performed using a magnetometer (2G Enterprises, model 760-3.0) at JAMSTEC, Yokosuka. The system specifications are the same as those of the system on the Chikyu, but the cooling system on the JAMSTEC magnetometer requires liquid helium.

Spinner magnetometer

A spinner magnetometer, model SMD-88 (Natsuhara Giken Co., Ltd.), was utilized during Expedition 338 for remanent magnetization measurement. The noise level was ~5 × 10^–7 mAm², and the measurable range was 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 were prepared to calibrate the magnetometer. Standard 2.5 cm diameter × 2.2 cm long samples taken with a minicore drill or 7 cm³ cubes could be measured in three or six positions with a typical stacking of 10 spins. The whole sequence took ~1 or 2 min, for three or six positions, respectively.

Alternating field demagnetizer

The DEM-95 AF demagnetizer (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.

Thermal demagnetizer

The TDS-1 thermal demagnetizer (Natsuhara Giken Co., Ltd.) has a single chamber for thermal demagnetization of dry samples over a temperature range of 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 MMPM10 pulse magnetizer (Magnetic Measurement, Ltd., United Kingdom; 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 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.

Anisotropy of magnetic susceptibility

The Kappabridge KLY 3S (AGICO, Inc.), which is 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. The 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 the applied field 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.

Discrete samples and sampling coordinates

Two discrete cubic samples (~7 cm³) or minicores (~11 cm³) were taken per section from working halves in order to determine paleomagnetic direction, primarily for magnetostratigraphy. The relation between the orientation of the archive section and that of a discrete sample is shown in Figure F22.

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 polarity intervals used during Expedition 338 are a composite of four previous magnetic polarity timescales (magnetostratigraphic timescale for Neogene by Lourens et al. [2004]) (Table T14).

Top of page   |    Previous   |    Next