Paleomagnetic work conducted during ODP Leg 180 consisted of (1) long-core measurements of the remanent magnetization (RM) of archive-half core sections before and after alternating field (AF) demagnetization, (2) magnetic remanence measurements on discrete samples collected from the working half of core sections, (3) mean magnetic susceptibility measurement of whole- and half-core sections and of discrete samples, and (4) anisotropy of magnetic susceptibility measurement of discrete samples.
Long-core remanence measurements and AF demagnetization were performed using an automated pass-through cryogenic direct current superconducting quantum interface device (DC-SQUID) magnetometer (2G Enterprises Model 760-R) with an in-line AF demagnetizer (2G Enterprises Model 2G-600) capable of reaching peak fields of 80 mT with a 200-Hz frequency. The background noise level of the magnetometer is on the order of ~10-10 A·m2. The large volume of core material within the sensing region of the magnetometer, which is on the order of 100 cm3, permits the measurement of cores with remanent intensities as weak as ~10-5 A·m-1.
The AF demagnetization of discrete samples to peak fields of 30 mT was achieved by using the in-line demagnetizer installed on the pass-through cryogenic magnetometer.
Magnetic susceptibility was measured for all whole-core sections as part of the MST analysis (see "Physical Properties"). Raw data values were stored in the JANUS database in units of 10-5 SI. To convert to true SI volume susceptibilities, raw values should be multiplied by a correction factor to account for the volume of material that passed through the susceptibility coils. The correction factor for a standard ODP core is about 0.66 (= 1/1.5). Figures illustrating magnetic susceptibilities in the "Paleomagnetism" sections in this report have not been volume corrected (refer to "Magnetic Susceptibility," for the appropriate correction and for equipment details).
Point susceptibilities were measured for most archive-half core sections as part of the AMST analysis. The AMST susceptibility meter (a Bartington Model MS2 with an MS2F probe having a tip diameter of 15 mm; spatial resolution = ~20 mm; operating frequency = 580 Hz) was set on SI units, with one measurement (period = 2 s) taken every 5 cm. Raw data values are stored in units of 10-5 SI on the Lamont-Doherty Earth Observatory (LDEO) CD-ROM and in the ODP database (see the "Related Leg Data" contents list).
Measurements of susceptibility and its anisotropy (AMS) for discrete samples used a Geofyzika Brno Kappabridge KLY-2 magnetic susceptibility meter. Experiments of the acquisition of isothermal remanent magnetization (IRM) were made using an ASC Impulse Magnetizer Model IM-10, which imparts short-term fields ranging from 0.02 to 1.0 T. Experiments of anhysteretic remanent magnetization (ARM) were made using a DTECH Partial Anhysteretic Remanent Magnetizer in conjunction with a Schonstedt GSD-1 AF demagnetizer. A direct current (DC) field of 0.05 T was applied in a peak alternating current (AC) field of 100 mT. Data are stored on the accompanying LDEO CD-ROM and are available from the ODP data librarian.
For the shipboard study, oriented discrete samples were taken from working halves of most cores, generally at two samples per core.
For postcruise studies, oriented discrete samples were collected from working halves of APC-drilled cores at a frequency of one sample per section with occasional additional samples taken as needed from horizons of special interest. Intervals of drilling-related core deformation or other disturbance were avoided. High-resolution sampling for postcruise studies consisted of one 5-cm3 plug every 10 cm in the upper 2 m, followed by one 5-cm3 plug per section to 20 mbsf from Holes 1109A, 1110A, 1115A, and 1115B. At Hole 1109C, two 5-cm3 plugs were taken at the same frequency as at the other holes.
Oriented sediment samples were collected in 8-cm3 plastic cubes with an arrow on the bottom pointing upcore. Two different sampling methods were used: (1) very soft intervals were sampled by pushing the plastic cube directly into the sediment; and (2) stiffer intervals were sampled using an extruder. These two sampling methods yielded a 180º difference in the orientations of both the +X and +Y axes relative to the standard "up" arrow (the -Z direction) drawn on the bottom of the sample box (Fig. F13).
Minicores, having a diameter of 2.5 cm and a length of 2.2 cm with an approximate volume of 11 cm3, were drilled from working halves of indurated rocks using a water-cooled drill press. Cubes with a dimension of ~2 cm × 2 cm × 2 cm were cut from smaller pieces using a diamond-bladed saw. Minicores and cut cubes were oriented by indicating the upcore and +Y directions on the sample surface corresponding to the split face of the working-half section.
The long-core cryogenic magnetometer was used to measure remanences of discrete samples selected for shipboard study; these were measured in a tray designed for seven samples. The data are shown in the "Paleomagnetism" sections of the individual site reports and are stored in the JANUS database. The remainder of the samples will be measured and analyzed onshore as part of postcruise studies.
Orientation of APC cores relative to magnetic north was achieved with a tensor tool mounted on the core barrel. The tensor tool consists of a three-component fluxgate magnetometer and a three-component accelerometer rigidly attached to the core barrel. 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 (+X direction) on the core liner. Orientation is not usually attempted for the top two to three cores (about 20-30 mbsf) until the bottom-hole assembly is sufficiently stabilized in the sediment. The XCB- and RCB-drilled cores were not oriented using the tensor tool. Tensor data are stored in the JANUS database.
Archive halves of all core sections were measured unless precluded by drilling-related deformation. Time constraints controlled the number of demagnetization steps applied to each core section. Cores were AF demagnetized at two to five steps ranging between 5 and 30 mT. False apparent low intensities and inaccurate directions occur where the wide response function (maximum signal occurs within 10 cm on either side of the SQUID sensors) averages empty space with the core signal. Consequently, measurements within 15 cm of the ends of each section, which have been compromised by an edge effect, were omitted. Core photographs and laboratory notes were examined in order to delete data from disturbed or missing intervals. The raw data are archived in the ODP database.
Discrete sediment samples were placed into the tray such that +X and +Y were oriented parallel to the SQUID orientations (i.e., +X up, +Y to starboard, +Z into the cryogenic magnetometer). The AF demagnetization was applied at six steps ranging between 2 and 25 mT. Data were corrected for tilt where information was available.
The AMS measurement of discrete samples involved 15 oriented measurements for each sample. Acceptable data from measurements had a standard error of less than 3%. Susceptibility parameters were calculated according to Jelinek (1981).
All sediment cores were affected to some extent by a steep, vertically downward-pointing overprint attributed to the drill string. Some cores also showed a radially inward overprint manifested as declinations of 0º along the entire length of a section.
Where magnetic cleaning successfully isolated the characteristic component of remanence, paleomagnetic directions and intensities were used to define the polarity. The revised time scale of Cande and Kent (1995), as presented in Berggren et al. (1995a), was used as a reference for the ages of Cenozoic polarity chrons.