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doi:10.2204/iodp.proc.314315316.132.2009

Paleomagnetism

Paleomagnetic and rock magnetic investigations on board the Chikyu during Expedition 316 were primarily designed to (1) determine the characteristic remanence directions for use in magnetostratigraphic and tectonic studies and (2) assess the orientation and significance of magnetic fabrics. To accomplish these goals, paleomagnetic measurements were performed on all continuous pieces of archive halves and on discrete minicore and cube samples taken from the working halves.

Magnetic measurements

Remanent magnetization was measured using the shipboard 2G Enterprises (model 760R) long-core cryogenic magnetometer equipped with direct-current superconducting quantum interference devices (DC-SQUIDs) and an inline automated alternating-field (AF) demagnetizer capable of reaching a peak field of 80 mT. The response curve from the sensor coils of the cryogenic magnetometer was measured and corresponds to a region ~20 cm wide (Fig. F26); 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 taken or were removed from the data set during data processing. 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 the instrumental noise level. Archive halves were measured at 5 cm intervals. This sampling interval coincides roughly with that used on the MSCL track (~4 cm; see “Physical properties”) so that susceptibility and remanence data may easily be compared. Results from the remnant magnetization and low-field susceptibility measurements were compared with lithologic units and/or geologic structures. Remnant magnetization directions were fit using principal component analysis (Kirschvink, 1980).

In addition to standard paleomagnetic measurements, the anisotropy of magnetic susceptibility was determined for several representative discrete samples using the Kappabridge KLY 3 magnetic susceptibility meter.

Sampling coordinates

The orientation of the Chikyu’s long-core magnetometer (Fig. F27) is the standard IODP core orientation. All magnetic data are reported relative to the following core coordinates: +x (north on plots) is into the face of the archive half of the core, +y (east on plots) points toward the left side of the face of the archive half, and +z is downcore (Fig. F27). 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 using this flipping function, working and archive halves can be measured in the same coordinate system. AF demagnetization on the archive halves was performed routinely with the inline AF demagnetizer at typical fields of up to 40 mT. Occasionally we reached 60 mT on working half samples taken 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 (e.g., see “Paleomagnetism” in the “Expedition 316 Site C0004” chapter).

Paleomagnetic core reorientation

Azimuthal 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). 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 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 40 mT) truly reflects the ChRM of the sediments. The uppermost and lowermost measurements have been typically disregarded to avoid end-core effects.

Our assumptions are as follows:

• The true paleomagnetic direction points to present-day geographic north. A given section has enough measurements to average secular variation.
• Bedding is horizontal or subhorizontal.
• Core is vertical.
• Sedimentary unit is in situ and has not experienced any vertical axis rotation.

Physical properties of the cored sediment, including voids, drilling disturbance, and flow-in, have determined to a 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.

For intervals of particular interest for structural geology (see “Structural geology”) we have taken two different approaches for core reorientation:

1. Discrete samples: small cubic (8 cm³) samples were cut from the working half in order to determine paleomagnetic direction. We used the “discrete sample” option of the superconducting rock magnetometer (SRM), which allows automatic measurement of up to six samples, 20 cm apart.
2. Biscuits: for homogeneous segments containing structures that had to be reoriented (see “Structural geology”), we measured entire (up to ~15 cm) core segments. In this case, we measured the samples as “continuous samples” in the SRM, also 20 cm apart.

Magnetic reversal stratigraphy

Magnetic polarity was determined using inclination-only data from continuous core measurements. Site C0004 has a latitude of 33°13.9′N, which translates into an expected inclination of ~52.6° for at least Neogene and Quaternary sediments. This inclination is steep enough to distinguish the magnetic polarities (normal or reversed) on the sign of the magnetic inclination (positive or negative).

Typical steps used to establish magnetic reversal stratigraphy included the following:

1. Complete visual inspection for stable remanent magnetization on the demagnetization plots (e.g., Zijderveld plots) using the Long Core software.
2. Obtain remanent magnetization directions after 20 mT or highest (40 mT) demagnetization step and group them by sections.
3. Locate disturbed and “flow-in” intervals in core descriptions and discard those intervals from the data set also, for RCB cored sediments, determine the position of biscuits in order to group the paleomagnetic data accordingly.
4. Exclude the top and bottom ~15 cm (response of the pick up coils is ~20 cm) to avoid end-core edge effects.
5. Check that there are at least four consecutive data points (= measuring intervals) with the same inclination sign to define a polarity chron.

Whenever possible, we offer an interpretation of the magnetic polarity, with the naming convention consisting 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) 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 316 are the same as Expedition 315, which are a composite of four previous magnetic polarity timescales. We used the Gradstein et al. (2004) timescale (Table T10).

Discrete samples

Discrete sampling (“routine sampling”) was completed for shore-based detailed studies. On average, about one standard sample (~8 cm³) was taken every core section, but the actual spacing largely depended 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 site chapter summaries.

Data reduction and software

Data visualization (standard As-Zijderveld demagnetization plots) is possible because of the Long Core software that controls the SRM. However, that facility does not allow computation of ChRM directions. The following programs have been used to interpret data (Tauxe, 1998):

• “plotdmag” makes orthogonal and equal area projections of input demagnetization data.
• “boodi” calculates bootstrap statistics for a group of vectors.
• “plotdi” makes equal area plots of data, with uncertainties.
• “pca” calculates best-fit line through specified data.
• “incfish” estimates the Fisher mean inclination and 95% confidence bounds from inclination only data using the method of McFadden and Reid (1982).

Laboratory instruments

The Paleomagnetism laboratory on board the Chikyu houses a large (7.3 m × 2.8 m × 1.9 m) magnetic 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 long sections (~150 cm). The shielded room houses the equipment, instruments, and ancillary items described in this section.

Superconducting rock magnetometer

The 8.1 cm long-core superconducting rock magnetometer (2G Enterprises, model 760) unit is ~6 m long. A 1.5 m split core liner can pass through a magnetometer, an 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 rock 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 rocks inserted into the system, both the pickup and coil structures and DC-SQUID sensors have superconducting shields placed around them. The noise level of the magnetometer is <10^–7 mA/m for a 10 cm³ volume rock. The 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 615 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 magnetometer

A spinner magnetometer, model SMD-88 (Natsuhara Giken Co., Ltd.) is also available for remanent magnetization measurement. The noise level is ~5 × 10^–7 emu, and the measurable range is from 5 × 10^–6 to 3 × 10^–1 mA/m². 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 316 samples prevented the use of the spinner magnetometer.

Alternating-field demagnetizer

The 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 magnetization

The DTECH alternating-field demagnetizer D-2000 can be used 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 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 the samples to room temperature. The measured magnetic field inside the chamber is not >10 nT.

Anisotropy of magnetic susceptibility system

The 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 mA/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 magnetizer

The 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. We did not use this apparatus during Expedition 316, as it has not been operative since Expedition 315.

Fluxgate magnetometer

The 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 magnetometer

A 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.

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