PALEOMAGNETISM

Paleomagnetic and rock magnetic investigations aboard the JOIDES Resolution during Leg 188 included routine measurements of natural and artificial remanent magnetizations of archive-half sections and discrete samples before and after static alternating field (AF) demagnetization, low-field magnetic susceptibility (k) measurements, and a limited set of rock magnetic measurements aimed at characterizing the downcore variation in the composition, concentration, and grain size of the magnetic carriers (e.g., Verosub and Roberts, 1995).

The shipboard paleomagnetic laboratory is equipped with the following:

1. An automated pass-through cryogenic magnetometer manufactured by 2-G Enterprises (model 760-R) with an in-line, three-axis AF demagnetizer (2-G Enterprises model 2G600), capable of reaching peak fields of 80 mT at a frequency of 200 Hz. The sensing pickup coils measure the magnetic signal over an interval of ~7 cm (half-power width of the response curve), and the coils for each of the x-, y-, and z-axes have slightly different response curves (Fig. F10). The widths of the sensing regions correspond to <100 cm³ of cored material. This large volume within the sensing region permits accurate determination of remanent intensities as weak as ~10^-6 A/m, despite the relatively high background noise related to the motion of the ship. The background noise level of the magnetometer on board the JOIDES Resolution is ~3 × 10^-6 A/m;
2. A Molspin Minispin spinner magnetometer capable of measuring higher remanent intensities than the 2-G magnetometer with some loss of accuracy, for use with strongly magnetized samples;
3. A DTECH alternating field demagnetizer (model D-2000) available for demagnetization of discrete samples of rock or sediment. The unit can demagnetize five samples simultaneously at peak alternating fields of up to 200 mT. The D-2000 can also be used to impart an anhysteretic remanent magnetization (ARM), in which a direct-current (DC) magnetic field is produced continuously across the AF demagnetizer coil, or a partial ARM (pARM), in which the user selects the demagnetization interval over which the field is applied;
4. A Schonstedt Instrument Co. thermal demagnetizer (model TSD-1) capable of demagnetizing discrete samples to 800°C with a resolution and repeatability of ~1°C. The residual magnetic field inside the TSD-1 was measured using a three-axis fluxgate magnetometer (model APS 520). The mean magnetic field within the furnace chamber is 99 nT with a minimum of 27 nT at 35 cm from the external door of this chamber. Within the cooling chamber, the residual magnetic field has a mean value of 8 nT and a minimum of 4 nT at 90 cm from the furnace chamber door (Fig. F11). During the thermal demagnetization, samples were arranged in the center of the heating and cooling regions to obtain magnetic fields <100 nT during heating and <5 nT during cooling. The samples were inserted in alternate directions at each heating step to check for spurious magnetizations acquired during the thermal demagnetization experiments;
5. An Analytical Services Company model IM-10 impulse magnetizer capable of applying magnetic fields from 0.02 to 1.35 T to study stepwise and saturation isothermal remanent magnetization of discrete samples;
6. A Bartington MS2 susceptibility meter, with a dual frequency sensor, operating at 0.565 and 5.650 kHz to measure low-field susceptibility at two frequencies; and
7. An Agico KLY-2.03 Kappabridge (KLY2) magnetic susceptibility meter with an operating frequency of 920 Hz and a magnetic induction of 0.3 mT. This instrument has a sensitivity of 1 × 10^-6 SI units. The KLY2 operates at a single frequency and is particularly sensitive to the high background noise levels caused by the ship's motion.

The bulk of the remanence measurements made during Leg 188 were carried out using the shipboard pass-through cryogenic magnetometer. The standard ODP magnetic coordinate system was used (+x: vertical upward from the split surface of archive halves; +y: left along split surface when looking upcore; +z: downcore) (Fig. F12).

Natural remanent magnetization was routinely measured on all archive-half sections at 4-cm intervals with 15-cm-long headers and trailers. Measurements at core and section ends and within intervals of drilling-related core deformation were removed during data processing. AF demagnetizations were applied to cores at 10, 20, and 30 mT. The low maximum peak demagnetization fields ensured that the archive halves remain useful for shore-based high-resolution (U-channels) studies of magnetic properties.

Discrete samples were collected from the working halves in standard 8-cm³ plastic cubes with the arrow on the bottom of the sampling cube pointing upcore (-z). Our preferred strategy was to sample from the working halves at an interval of one meter; whenever possible, samples were selected from fine-grained horizons. Intervals with drilling-induced core deformation were avoided. The discrete samples were analyzed on the shipboard pass-through cryogenic magnetometer using a tray designed for measuring six discrete samples. Samples were AF demagnetized using the in-line demagnetizer installed on the pass-through cryogenic magnetometer at steps of 0, 10, 20, 30, 40, 50, 60, 70, and 80 mT. A subset of samples was thermally demagnetized using the Schonstedt TSD-1 oven. All of the samples subjected to thermal demagnetization were measured at steps of 0°, 100°, 200°, 300°, 330°, 360°, 400°, 500°, 550°, 600°, 650°, and 700°C. The samples were heated for 90 min at the first demagnetization step to ensure that they had fully dried, then for 40 min at each subsequent step to ensure that they had reached thermal equilibrium. After each step, the low-field magnetic susceptibility was measured to monitor for thermal alteration.

Magnetic susceptibility was measured for each whole-core section as part of the MST (see "Physical Properties") using a Bartington MS2 meter coupled to an MS2C sensor coil with a diameter of 88 cm, operating at 0.565 kHz. The sensor was set on SI units, and the data were stored in the Janus database in raw meter units. The sensor coil is sensitive over an interval of ~4 cm (half-power width of the response curve) (Fig. F13), and the width of the sensing region corresponds to a volume of 166 cm³ of cored material. To convert to true SI volume susceptibilities, these values were multiplied by 10^-5 and then 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, the correction factor for a standard full ODP core is ~0.67 (= 1/1.5). The end effect of each core section was not corrected.

The low-field magnetic susceptibility was also routinely measured for all the discrete samples, and the data were compared with the whole-core susceptibility log. The frequency-dependent susceptibility, fd(%) = 100 × (k_low - k_high) / k_low, was monitored to estimate the contribution of superparamagnetic contamination (Bloemendal et al., 1985). Measurements of k_high, however, were sometimes unstable and gave unusually high fd(%) values or negative values.

Further analyses were made on a selected subset of discrete samples. These analyses included the determination of the following:

1. Stepwise acquisition of isothermal remanent magnetization (IRM) in fields up to 1.3 T;
2. Coercivity of remanence (B_cr) and S-ratio (-IRM _-0.3T / IRM _1.3T) (e.g., Bloemendal et al., 1992; Verosub and Roberts, 1995) determined by progressively increasing the backfield up to 300 mT to the maximum IRM;
3. ARMs imparted by using a 100-mT AF and a 0.05-mT DC bias field; and
4. Stepwise thermal demagnetization of a composite IRM (Lowrie, 1990) at steps of 0°, 100°, 200°, 300°, 330°, 360°, 400°, 500°, 550°, 600°, 650°, and 700°C. Fields of 1.3, 0.5, and 0.12 T were applied along the x-, y-, and z-axes of a sample to distinguish between high-, intermediate-, and low-coercivity magnetic phases, respectively. After each thermal demagnetization step, the low-field magnetic susceptibility was measured to monitor for thermally induced phase changes.

Estimates of the concentration of magnetic minerals can be obtained from parameters such as k, IRM, and ARM, whereas B_cr, S-ratio, and thermomagnetic curves are more diagnostic of magnetic mineral composition.

Where magnetic cleaning successfully isolated the characteristic component of magnetization (ChRM), paleomagnetic inclinations were used to define magnetic polarity zones. Interpretations of the magnetic polarity stratigraphy, with constraints from the biostratigraphic data, are presented in the site chapters. The revised time scale of Cande and Kent (1992, 1995), as presented in Berggren et al. (1995a, 1995b), was used as a reference for the ages of Cenozoic polarity chrons.