PALEOMAGNETISM

Paleomagnetic investigations conducted on the JOIDES Resolution during Leg 192 consisted of routine measurements of natural remanent magnetization (NRM) and of magnetic susceptibility of sedimentary and igneous rocks. We measured NRM on all archive split cores of recovered material and on discrete samples of sedimentary and basement rocks taken from the working halves. Stepwise alternating-field (AF) demagnetization was carried out on most archive half cores and on some discrete samples in an attempt to isolate the characteristic remanent magnetization (ChRM). A few discrete samples were thermally demagnetized in order to isolate the ChRM and study the magnetic mineralogy. We measured magnetic susceptibility for whole cores, archive-half core sections, and, in a few cases, discrete samples. We compared magnetic properties with biostratigraphic and lithostratigraphic units.

We followed the remanence of archive-half sections and oriented discrete samples from working-half sections using a 2-G Enterprises pass-through cryogenic direct-current superconducting quantum interference device rock magnetometer (model 760R). This pass-through cryogenic magnetometer is equipped with an in-line AF demagnetizer (2-G model 2G600) that allows demagnetization of samples up to peak fields of 80 mT. The practical limit on the resolution of natural remanence of core samples is imposed by the magnetization of the core liner itself (~0.01 mA/m). The magnetometer and AF demagnetizer are interfaced to a PC-compatible computer and are controlled by the 2G Long Core software by National Instruments. A Molspin spinner magnetometer was also available on the ship for measuring the remanence of discrete samples. For stepwise demagnetization of discrete samples, the laboratory contains an AF demagnetizer (model D-2000 by DTech Inc.) and a thermal demagnetizer (model TSD-1 by the Schonstedt Instrument Co.) capable of demagnetizing specimens to 200 mT and 700°C, respectively. An Analytical Services Company (ASC) model IM-10 pulse magnetizer (capable of pulsed fields from 0.02 to 1.35 T) and a PARM-2 system by DTech Inc. were available for isothermal and anhysteretic remanent magnetization (IRM and ARM, respectively) acquisition studies of discrete samples.

We measured magnetic susceptibility for all whole-core sections at 4-cm intervals with a susceptibility meter attached to the MST (see "Physical Properties"). The susceptibility values were stored in the Janus database as raw data in units of 10^-5 SI. The true SI volume of susceptibilities should be multiplied by a correction factor to account for the volume of material that passed through the coils. The standard correction factor for ODP core is ~0.66. We routinely measured the magnetic susceptibility of archive halves of sediment cores using the AMST (see "Lithostratigraphy") at 4-cm intervals. Some archive halves were measured using a 2-cm interval. The AMST, which permits measurements only at evenly spaced intervals along each section of core, automatically recorded the measurements. For the two types of susceptibility measurements (MST and AMST), we used the same type of magnetic susceptibility meter (Bartington Instruments model MS2) but with a different sensor. The sensor for whole-core measurements (MS2C) has an 80-mm inner diameter, and the core passes through the sensor coil. The AMST has a cylindrical tip probe (MS2F), and the sensor provides a depth of investigation approximately equal to its diameter (20 mm). A Geofyzika Brno Kappabridge KLY-2 magnetic susceptibility meter was available for magnetic susceptibility measurements of discrete samples.

We followed the standard ODP core orientation convention (Shipboard Scientific Party, 1991 [fig. 8], and 1997 [fig. 8]) for paleomagnetic work during Leg 192. This convention can be described as follows. The z-axis is downhole parallel to the core. The x-axis forms a line perpendicular to the split face of the core and is directed into the working half (Fig. F6). The x-axis is used as the reference "geomagnetic north" for the definition of magnetic declination values. Discrete sample cubes and minicores were marked with an arrow in the negative-z (uphole) direction on the plane representing the split surface of the working half. The plane marked with the arrow is the y-z plane. Because only the rotary core barrel (RCB) was used for drilling, we were unable to use the Tensor tool, which mounts on the advanced hydraulic piston corer (APC) core barrel to orient cores.

Discrete samples were taken from soft sediment using standard plastic cubes (7 cm³), with an arrow pointing in the uphole direction. We did not take samples from portions of the core that were highly disturbed by drilling. To reduce deformation of the sediment while pressing in the plastic cubes, we cut the core with a thin stainless steel spatula before pressing the cubes into the sediment. Cylindrical minicores (12 cm³) were drilled from hard sediment and igneous rocks using a water-cooled nonmagnetic drill bit attached to a standard drill press. Minicores were oriented in the same manner, with an arrow pointing in the uphole direction. We typically took discrete samples at an increment of one or two per core, and some were used for onboard pilot demagnetization studies. However, some discrete samples were taken from sedimentary rocks at smaller increments to examine geomagnetic reversals, key geologic boundaries, or other intervals of interest. Several discrete samples per flow unit were taken from igneous rocks.

We analyzed the NRM of the archive-half sections on the cryogenic magnetometer at 5-cm intervals for sedimentary material and at 1-cm intervals for igneous rocks when continuous pieces longer than 15 cm were available. To isolate characteristic magnetizations, archive halves were AF demagnetized at 5, 10, and 15 mT for sediments. Basalt samples were generally stepwise AF demagnetized up to 50 mT: a few samples were demagnetized to 80 mT. The number of demagnetization steps and the peak field used varied depending on the lithology, the NRM intensity, and the amount of time available. To isolate stable remanence, we applied stepwise thermal demagnetization of up to 620°C to some discrete samples of the volcaniclastic rocks. We performed some IRM acquisition experiments on sediments to define their dominant magnetic mineralogy.

We obtained the characteristic remanent magnetizations of both sediment and basalt samples using principal-component analysis (Kirschvink, 1980) on the results of the AF demagnetization. We determined the stability of remanence levels within the archive cores and the discrete samples by both Zidjerveld (1967) plots and equal-area stereographic projections.

Magnetozones were defined by selected directional data from the cryogenic magnetometer. Typical selection criteria were as follows:

1. Intensity of NRM after AF demagnetization at 15 mT stronger than 2 × 10^-4 A/m and hence above the noise level of the magnetometer in a condition of rough sea,
2. Inclination > +10° or < -10°,
3. At least three consecutive values (which corresponds to a minimum of 10-cm length of split core) of the same polarity, and
4. No significant core disturbance.

Positive and negative inclinations were defined as reversed and normal magnetic polarities, respectively. We then interpreted the magnetic polarity stratigraphy using constraints from the biostratigraphic data (see "Biostratigraphy"). We used the timescales of Berggren et al. (1995) for the Cenozoic polarity boundaries and Gradstein et al. (1995) (see "Biostratigraphy") for the Mesozoic polarity boundaries.

We calculated mean inclination values at each site using the method of Kono (1980). These inclinations were used to obtain paleolatitudes based on the assumption of an axial dipole field using the equation

tan (paleolatitude) = tan (inclination). (2)

We compared the results from the remanent magnetization and low-field susceptibility measurements with lithologic units and/or geologic structures based on sedimentary, petrologic, and structural features (see "Lithostratigraphy," "Igneous Petrology," and "Structural Geology").