PALEOMAGNETISM

Paleomagnetic studies conducted on the JOIDES Resolution during Leg 186 consisted of remanent magnetization measurements of archive-half sections and discrete samples from the working-half sections. Measurements were made before and after alternating field (AF) demagnetization. To investigate rock magnetic properties, we measured the anisotropy of magnetic susceptibility and conducted thermal demagnetization, anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM) experiments on some of the discrete samples. Magnetic susceptibility was also measured on whole-core sections using the MST device.

Instruments and Measurements

Measurements of remanent magnetization were conducted using an automated pass-through cryogenic magnetometer with a direct-current superconducting quantum interference device (2-G Enterprises Model 760-R), which has an in-line AF demagnetizer (2-G Enterprises Model 2G600) capable of producing peak fields of 80 mT with a 200-Hz frequency. Based on tests conducted during Leg 186, the background noise level of the magnetometer in the shipboard environment is about 2 10-9 Am2. For a split core, the large volume of core material within the sensing region of the magnetometer (~100 cm3) permits accurate measurements of remanent intensities greater than about 2 10-5 A/m. For discrete samples, which typically have volumes of 6-10 cm3, the magnetometer can accurately measure samples with intensities greater than about 4 10-4 A/m. The magnetometer is scheduled for tuning following Leg 186, so this noise level may be reduced for future legs.

The natural remanent magnetization (NRM) before and after AF demagnetization was routinely measured for all archive-half sections at 2- or 5-cm intervals. In interpreting the data, we avoided using the measurements within 5 cm from the ends of each section, although we saved these values for future studies that might wish to deconvolve the remanence signal. Alternating-field demagnetizations were applied at 10, 20, and 30 mT on all sections. For a few sections from each site, we did detailed demagnetization in 5-mT steps from 0 to 60 or 70 mT.

Most of the discrete samples were also measured with the cryogenic magnetometer. Typically samples were demagnetized in 10-mT steps from 0 to 60 mT, though more detailed demagnetization was conducted on a few of the samples. A few samples were stepwise thermally demagnetized, typically at 30C steps, using a thermal demagnetizer (Schonstedt Instrument Co., Model TSD-1), and then measured with the cryogenic magnetometer.

An automatic portable spinner magnetometer (Niitsuma and Koyama, 1994) was used to measure remanence magnetization of some of the discrete samples collected during Leg 186. The noise level of this magnetometer is roughly equivalent to that of the cryogenic magnetometer for the 7-cm3 discrete samples, when 10 repeat measurements are averaged. This magnetometer is equipped with an AF demagnetizer, an anhysteretic remanent magnetizer, and magnetic susceptibility anisotropy meter. Detailed stepwise AF demagnetizations were performed using this automatic magnetometer to compare with the long-core data. About one sample per section in the APC and one sample per core in the XCB and RCB intervals were measured with this portable unit.

To investigate rock magnetic characteristics of some of the discrete samples, ARM and IRM experiments were conducted and then the samples were measured using the cryogenic magnetometer. The DTECH AF demagnetizer (Model D2000) was used to impart ARMs to discrete samples, using a 100-mT peak AF and a 0.05-mT direct current field. We then progressively demagnetized the samples at 10-mT increments from 0 to 60 mT using the cryogenic magnetometer. IRMs were imparted to the discrete samples using the Analytical Services Company Impulse Magnetizer (Model IM-10) with a DC field of 1000 mT. The samples were then stepwise AF demagnetized at 10-mT increments from 0 to 60 mT using the cryogenic magnetometer. In addition, we conducted IRM acquisition experiments and thermal demagnetization of IRMs on a few samples.

Low-field magnetic susceptibility was measured for all whole-core sections as part of the MST analysis (see "Physical Properties"). The MST susceptibility meter (a Bartington Model MS-2 with an MS2C sensor; inner coil diameter = 88 mm; operating frequency = 0.565 kHz) has a nominal resolution of 2 10-6 SI (Blum, 1997). Susceptibility was determined at 2-cm intervals using a 1-s integration time and a 4-s period. The "units" option was set on SI units and the values were stored in the Janus database in raw meter units. For conversion to true SI volume susceptibilities, these should be multiplied by 10-5, and then should be multiplied by a correction factor to take into account the volume of material that passed through the susceptibility coils. Except for measurements near the ends of each section, this factor for a standard ODP core is about 0.7 (Blum, 1997). No correction was applied for any figures illustrating magnetic susceptibilities in this volume. Hence, the units are given as raw meter values. Magnetic susceptibility of discrete samples obtained from the working half was measured using a Bartington MS2 susceptibility meter with a dual frequency MS1B Sensor. Magnetic susceptibility was used as a first-order measure of the amount of ferrimagnetic material and as a correlation tool.

Core Orientation

The standard ODP paleomagnetic coordinate system was used. In this system, +x is vertical upward from the split-core surface of archive halves, +y is left along the split-core surface when looking upcore, and +z is downcore (Fig. F7).

Most of the few APC cores collected during Leg 186 were azimuthally oriented using the Tensor tool. The Tensor tool consists of a three-component fluxgate magnetometer and a three-component accelerometer rigidly attached to the core barrel. The information from both sets of sensors allows the azimuth and dip of the hole to be measured as well as the azimuth of the double-line orientation mark on the core liner. Orientation is not usually attempted for the top two or three cores (~20-30 mbsf) until the bottom hole assembly is sufficiently stabilized in the sediment. The output from the Tensor tool, which contains a variety of angles including the inclination angle of the hole and the magnetic toolface (MTF) angle, is archived in the Janus database. The inclination angle of the hole is a measure of deviation of the hole from vertical and the MTF angle gives the angle between the magnetic north and the double-line orientation mark on the core liner. The core liner is always cut such that the double lines are at the bottom of the working half.

Using the ODP coordinate system for the archive and working halves or for samples taken from them, the measured remanent declination can then be corrected to magnetic north by adding the MTF angle and can be further corrected to true north by adding the deviation of magnetic north from true north, the latter of which can be estimated from the International Geomagnetic Reference Field (IGRF) coefficients. The equation is

D T = DO + AMTF + MIGRF , (1)

where D T is the Tensor tool corrected or true declination, DO is the observed declination output from the cryogenic magnetometer, AMTF is the MTF angle, and MIGRF is the deviation of magnetic north from true north. At Sites 1150 and 1151, we used -7.7 for the MIGRF correction. Specific details concerning orientation of APC cores from these sites are discussed in the site chapters.

Sampling Methods and Orientations
for Discrete Samples

Oriented discrete samples were taken from the working half of each section (typically one per section for shipboard analyses) using a cube with flattened corners (the Japanese paleomagnetism cube) and a stainless-steel extruder or using a 2 cm 2 cm 2 cm plastic cube (the French cube) and an aluminum extruder. The volume of both cubes is ~7 cm3. In lithified intervals, we used the rock saw to cut the sample, which was then placed in the Japanese cube such that the orientation of the sample in the plastic cube was the same as would have been attained with the extruder. Nine samples from APC cores were collected by pressing the Japanese cube directly into the sediment. The two sampling methods yield a 180 difference in the orientations of both the +x and +y axes (Fig. F7); therefore, we have carefully marked the labels on those eight sample cubes with a red dot. In all cases, the arrow on the cubes points upcore.

About 10% of the discrete samples were placed in the Japanese cubes and were measured with the spinner magnetometer. These data are not available in the Janus database but are provided in tables within the site chapters. The remaining samples, including all of those in the French cubes, were measured in the cryogenic magnetometer using a tray with seven sample holders. The first sample holder is 20 cm from the top of the tray, and the others are spaced every 20 cm down the tray. To properly orient the French cube in the standard ODP magnetic coordinate system would require that the lid of the cube be placed down in the sample holders. Owing to the tight fit, which can damage the sample holders, we instead placed the bottom of the cube in the sample holder. Thus, the declination is rotated 180 from its proper position. In all plots, we have made this correction, but the raw data in the Janus database are uncorrected. We provide a table of the corrected values in the site chapters.

Magnetostratigraphy

We encountered several types of secondary magnetization acquired during coring, which sometimes hampered magnetostratigraphic interpretation. The most common was a steep downward-pointing overprint attributed to the drill string. For the Leg 186 cores, we also observed a slight bias for 0 declinations, which has been observed on many previous cruises and has been interpreted as a radially-inward overprint. Other overprints related to reduction diagenesis or tectonic deformation were observed. Details of these and other magnetic complexities are presented in the site chapters.

Where AF demagnetization successfully isolated the characteristic component of remanence, paleomagnetic inclinations were used to define polarity zones. The declination provided additional constraints for those few APC cores oriented with the Tensor tool. Ages for reversals (Fig. F6) are from the revised Cenozoic time scale of Cande and Kent (1995), as presented in Berggren et al. (1995).

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