Paleomagnetic investigations during Leg 205 had two major purposes: to investigate diagenetic dissolution of magnetic minerals in sediment and igneous rocks and to investigate microfabrics related to deformation processes in the subduction zone. These investigations need a large number of discrete samples; therefore, one or two minicore samples were taken from each section of a core. On the ship, NRM of all minicore samples, thermal demagnetization, and isothermal remanent magnetization (IRM) tests for some pilot samples were carried out as an initial approach to magnetic studies during Leg 205. These rock magnetic data will be combined with postcruise studies of anisotropy of magnetic susceptibility (AMS). In addition, postdepositional remanent magnetization of the archive half of the core was routinely measured to identify age boundaries of magnetic chrons recorded in the sediments and constrained to previous paleomagnetic polarities reported from ODP Leg 170 (Kimura, Silver, Blum, et al., 1997). The results of these measurements were used for core orientation correction and magnetostratigraphy.
Two magnetometers are installed in the paleomagnetic laboratory on the JOIDES Resolution for measurements of halved long cores and for discrete samples. A 2G Enterprises pass-through cryogenic direct-current superconducting quantum interference device rock magnetometer (model 760-R) measures the magnetic intensity of samples and can routinely demagnetize the samples up to 80 mT with a 200-Hz frequency in-line alternating-field (AF) demagnetizer (model 2G600). The cryogenic magnetometer and the AF demagnetizer are interfaced to a PC-compatible computer and are controlled by a 2G Long Core software program by Core Logic. The sensing coils in the cryogenic magnetometer can also measure seven discrete samples at 10-cm intervals. Background noise at the sensor of the cryogenic magnetometer shows 2.20 x 10-10 Am2 averaged over 10 min. Magnetic fields were also measured at the sensing point of the magnetometer by a fluxgate magnetometer (model APS 520), and fields of 2.24 x 10-5 mT, 3.94 x 10-5 mT, and 1.31 x 10-4 mT were observed on the +x-, +y-, and +z-axes, respectively. For the paleomagnetic measurement and rock magnetic tests of discrete samples, a Molspin spinner magnetometer is also available on the ship. Additional instruments used for demagnetization of samples include a DTECH AF demagnetizer (model D-2000) capable of demagnetization up to 200 mT and a Schonstedt thermal demagnetizer (model TSD-1) capable of demagnetization up to 800°C. For a test of partial anhysteretic remanent magnetization (pARM), the D-2000 is equipped with a pARM-2 system, consisting of two parallel coils mounted outside and on-axis with the AF coil of the GSD-1 demagnetizer.
Magnetic susceptibility was initially measured for all whole-core sections with the MST system (see "Physical Properties"). The susceptibility loop sensor on the MST system operates at a frequency of 565 Hz and AF intensity of 80 A/m (= 0.1 mT). The resolution of the loop is 2 x 10-6 SI on the 0.1 range. The 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. The magnetic susceptibility of the archive-half core was also measured using a point susceptibility sensor on the AMST (see "Barrel Sheet Data" in "Lithostratigraphy"). The AMST and MST units have the same type of susceptibility meter (Bartington Instruments model MS2C), but the sensor range is different. The susceptibility loop sensor for the MST system is 80 mm in diameter and has a ~5-cm-wide sensor range designed for passing through a whole-round core. The point susceptibility meter on the AMST is designed for sensitive measurement of magnetic susceptibility of archive-half cores and has a sensor range of ~1 cm. The point sensor operates at a frequency of 2 kHz and has the same resolution (2 x 10-6 SI on 0.1 range). A Geofyzika Brno Kappabridge KLY-2 magnetic susceptibility meter produces an opportunity to measure magnetic susceptibility as well as AMS of a discrete sample.
To investigate the magnetic mineralogy of discrete samples, experiments of the acquisition of IRM were carried out using an Analysis Services Company impulse magnetizer model IM-10 capable of applying magnetic fields from 0.02 to 1.35 T.
ODP core orientation designates the positive x-axis direction as the horizontal direction ("geomagnetic north" in a global coordinate reference frame) from the center of the core to the median line between a pair of lines inscribed lengthwise on the working half of each core liner (Fig. F14). Continuous measurements of NRM and remanence were made on the archive half of the core using the pass-through cryogenic magnetometer. Remanence measurements were made at demagnetization steps of 10, 20, and 30 mT at intervals of 5-10 cm.
Rock magnetic experiments were conducted on selected discrete samples to identify the magnetic minerals in sediments. Thermal demagnetization of multicomponent isothermal remanent magnetization (Lowrie, 1990) was used as the primary means of identifying the magnetic composition of magnetic minerals. For these experiments, orthogonally applied fields of 1.0, 0.3, and 0.1 T were used to generate the IRM components. The samples were then demagnetized using 15 thermal steps from 50° to 650°C.
For the study, one to two discrete oriented samples were acquired from each section of the working half of the core. Plastic cubes (8 cm3 and 1 cm3) were used for soft sediments, and 10-cm3 minicores were cut from sections of crystalline and igneous rock. Paleomagnetic measurement of some core samples was also conducted for azimuth reorientation of structure observed in the cores.
During Leg 205, only RCB coring was performed. Reorientation of the rotated portions, caused by RCB drilling, was accomplished by using paleomagnetic results on the archive half of the core. Core orientation was also performed using discrete samples from the working half of the core. Characteristic inclination and declination directions were isolated by stepwise AF demagnetization. The core orientations were rotated to 180° when reversed polarity was observed in the samples.
Some discrete samples need additional reorientation after the paleomagnetic measurement. When the discrete samples are set on the tray of the cryogenic magnetometer upside down, the system control program cannot work to correct their direction even though the program has a reorientation command. In this case, measured +x- and +y-axes must be changed into -x- and -y-axes, respectively. Otherwise, magnetic declination of the samples shows the opposite direction. The discrete samples taken from the working half should be set on the tray following the basic coordinate system for the archive-half core (Fig. F14) to prevent direction troubles caused by program errors.
The magnetic polarity timescale of Berggren et al. (1995a, 1995b) was used when interpreting observed polarity boundaries (Fig. F15). Additional geomagnetic short events (subchrons) that may prove to be useful stratigraphic markers are short reversals within long-period geomagnetic chrons such as the Brunhes (0.0-0.78 Ma)-Matuyama (0.78-2.581 Ma) polarity chrons. Because of difficulty in identifying the polarity timescale without shipboard micropaleontological data, observed polarity boundaries on this cruise were simply compared with previous paleomagnetic results from Leg 170 for confirmation of polarity age.