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doi:10.2204/iodp.proc.314315316.122.2009 PaleomagnetismLaboratory instrumentsThe Paleomagnetism laboratory on board the Chikyu is located on the starboard side of the core processing deck. Most of the equipment is housed in 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 generally equals 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, instruments, and ancillary items described in this section. Superconducting rock magnetometerThe 760-8.1 cm long-core superconducting rock magnetometer (2G Enterprises) unit is ~6 m long. A 1.5 m split core liner passes 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 movement measurement (x- and y-axes) and one for axial moment measurement (z-axis). These pickup coils have a large volume of uniform response to a small magnetic dipole. When a sample is inserted into the pickup coil region, persistent currents are generated in all three pickup coils. To prevent magnetic noise from being picked up from sources other than samples inserted into the system, both the pickup and coil structures and the direct-current superconducting quantum interference device (DC-SQUID) sensors have superconducting shields placed around them. The noise level of the magnetometer is <10–4 mA/m for a 10 cm3 volume sample. The superconducting rock magnetometer (SRM) dewar system has a capacity of 90 L of liquid helium. 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 pull 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 300 mT with a standard air-cooled solenoid (model 2G601S) and up to 180 mT with a transverse split pair (model 2G601T). The system is completely controlled by an external computer. Because it is used with the automatic sample handler, a complete sequence of measure and degauss cycles can be completed without removing the long core from the holder. A 2G615 anhysteretic remanent magnetization (ARM) system is included to enable magnetization of rock samples during demagnetization. Magnetization is achieved by applying a direct current (DC) magnetic field in the range of 0 to ±4 Gauss during the degaussing process. Spinner magnetometerA spinner magnetometer, model SMD-88 (Natsuhara Giken Co., Ltd.) is also available for remanent magnetization measurement. The noise level is ~5 × 10–7 mAm2. The measurable range is from 5 × 10–6 to 3 × 10–1 mAm2. Five standard samples with different intensities are prepared to calibrate the magnetometer. Standard 2.5 cm diameter × 2.1 cm long samples can be measured in three or six positions, the hole sequence taking ~1 and 2 min, respectively. Remanent intensity of Expedition 315 samples prevented the use of the spinner magnetometer. Alternating-field demagnetizerThe alternating-field 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 an AF peak of 180 mT. Anhysteretic remanent magnetizationThe DTECH alternating-field demagnetizer D-2000 is available to impart ARM to discrete samples, in which a DC magnetic field is produced continuously across the AF demagnetizer coil. The user can select the demagnetization interval over which the field is applied (maximum AF = 200 mT, maximum DC field = 1.5 mT), producing partial anhysteretic remanent magnetization. Thermal demagnetizerThe 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 boat 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 ≤10 nT. Anisotropy of magnetic susceptibility systemThe Kappabridge KLY 3, designed for anisotropy of magnetic susceptibility (AMS) measurement, is also available. Data are acquired from spinning measurements around three different axes. 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 A/m and 875 Hz, respectively. This system also includes the temperature control unit (CS-3) for temperature variation of low-field magnetic susceptibility of samples. Pulse magnetizerThe pulse magnetizer MMPM10 (Magnetic Measurement Ltd.) can produce a high magnetic field. A maximum field of 9 T with a 7 ms pulse duration can be produced by the 1.25 cm diameter coil. The 3.8 cm coil generates a maximum field of 2.9 T. During Expedition 315, this apparatus was not operative. Fluxgate magnetometerThe Walker portable three-axis fluxgate magnetometer (model FGM-5DTAA) measures small ambient fields with a range of ±100 µT and a sensitivity of 1 nT. The sensor fits into small spaces, such as the sample access tube of the cryogenic magnetometer. The magnetometer was also used to monitor the total field in the shielded room and as a thermal demagnetizer. Hall-effect magnetometerA Hall-effect magnetometer (model MG-5DP), capable of measuring DC and alternating current fields over three orders of magnitude (±0.01, ±0.1, and ±1 T), is available for calibrating demagnetization coils and measuring strong DC fields. MethodsRemanent magnetization was measured using the shipboard long-core cryogenic magnetometer. Continuous core measurements were typically made at 5 cm intervals with 20 cm long headers and trailers. The response curve from the sensor coils of the cryogenic magnetometer was measured and corresponds to a region ~20 cm wide (Fig. F17); therefore, only measurements taken every 20 cm are independent from each other. Measurements at core and section ends, whole-round locations and voids, and within intervals of drilling-related core disturbance were not measured or were removed during data processing. The background noise of the instrument, as well as the liquid helium boil-off, seem to be amplified by the ship’s movement compared to shore-based instruments, and the first background noise estimate was ~10–10 Am2. The relatively large volume of core material within the sensing region compensates for the relatively high background noise, and with very few exceptions sediment magnetization was well above instrumental noise level. Sampling coordinatesThe standard IODP core coordinate system was used, where +x is the vertical upward direction when the core (archive half) is on its curved side, +y is the direction to the left along the split-core surface when looking upcore, and +z is the downcore direction (Fig. F18). Coordination of the ship’s long-core magnetometer is shown in Figure F18, along with standard IODP core coordination. The “flipping” function of the control software (Long Core version 3.4) enables 180° rotation of the x- and y-axes about the z-axis. By switching, working and archive halves can be measured in the same coordination. AF demagnetization on the archive halves was performed routinely with the inline AF demagnetizer at typical fields of up to 20 or 30 mT in order to avoid compromising future shore-based paleomagnetic studies. Occasionally we reached 80 mT, the maximum AF attainable field, on samples taken from working halves from intervals where a more precise direction was needed for structural correction purposes. Such a high AF value was required to overcome the drilling-induced overprint (see “Paleomagnetism” in the “Expedition 315 Site C0001” chapter). Paleomagnetic core reorientationAzimuthal orientation of drilled core material is of great importance when modeling directional properties of rock formations. Paleomagnetism can be used to determine the core azimuth by providing a reference direction from the drilled rocks. Paleomagnetic core reorientation has been used successfully for a number of years (e.g., Fuller, 1969; Kodama, 1984; Shibuya et al., 1991; Pares et al., 2007, in press). 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 the length of the core. Provided that the reference magnetic pole is known, the orientation of the paleomagnetic vector is then used to restore the core azimuth. The horizontal component of the mean characteristic remanent magnetization (ChRM) direction makes an angle with the reference line, which specifies the rotation of the core relative to the geographic coordinates. To restore core orientation to geographic coordinates, the mean paleomagnetic direction is computed for samples sharing a common reference line. We have systematically used the archive half, which is marked at the bottom with a single reference line (the working half has a double line). After visual inspection of the AF demagnetization plots, we determined whether the blanket demagnetization at the highest peak (typically 20 mT) truly reflects the ChRM of the sediments. Measurements made on the uppermost and lowermost 20 cm have been typically disregarded to avoid end-core effects. Our assumptions are as follows:
Physical properties of the cored sediment, including voids, drilling disturbance, and flow-in, have determined to large extent the applicability of the paleomagnetic method for core reorientation. Hence, our “mean directions” are based on the statistical analysis of individual sections (from ~30 to 150 cm long) only when there is no visual evidence for general core disturbance or twisting. Even so, we noticed seemingly undeformed core sections that showed evidence of severe core twisting (Fig. F19). We think that the origin of such core twisting, when using the HPCS, is related to the extraction of the liner from the barrel when the core is on board. In order to extract the liner, operators have to first remove the shoe, which requires twisting counterclockwise. For intervals of particular interest for structural geology (see “Structural geology”) we have taken two different approaches for core reorientation:
Magnetic reversal stratigraphyMagnetic polarity was calculated using inclination-only data from continuous core measurements. Site C0001 has a latitude of 33°14.5′, which translates into an expected inclination of ~52° for at least Neogene and Quaternary sediments. This inclination is high enough to base the polarity interpretation (normal or reversed) on the sign of the magnetic inclination (positive or negative). Typical steps used to establish magnetic reversal stratigraphy included the following:
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 315 are a composite of four previous magnetic polarity timescales (Gradstein et al., 2004) (Table T7; Fig. F16). Discrete samplesDiscrete sampling (“routine sampling”) was completed for shore-based detailed studies. On average, about one standard sample (~8 cm3) was taken every 60 cm, but the actual spacing largely depends on the properties of the core material (e.g., flow-in, coring disturbances, etc.) and the preliminary paleomagnetic record obtained with the pass-through magnetometer. A few pilot specimens were demagnetized on board using both AF and thermal demagnetization. Results are shown in the corresponding summary. Data reduction and softwareData visualization is possible because of the Long Core software that controls the SRM. However, that facility doesn't allow any computation of ChRM directions. The following programs have been used to interpret data (Tauxe, 1998):
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