Paleomagnetic studies conducted during Leg 177 comprised long-core magnetic remanence measurements of archive-half sections before and after alternating-field (AF) demagnetization. In addition, a small number of magnetic remanence measurements were made on discrete samples collected from the working halves of cores.
Long-core remanence measurements and AF demagnetizations were performed using a long-core cryogenic magnetometer (2G Model 760-R) with an in-line AF demagnetizer capable of reaching peak fields of 80 mT. Archive halves were measured for all core sections not disturbed by drilling. The number of demagnetization steps applied to each core section was controlled by time constraints and core flow through the laboratory, rather than magnetic properties of the cores themselves. A six-step demagnetization scheme (taking about 20 min) was applied to most 1.5-m-long core sections. This standard measurement scheme involved stepwise demagnetization at 0 (natural remanent magnetization), 5, 10, 15, 20, and 25 mT. At Hole 1090C, a three measurement scheme (0, 10, and 20 mT) was applied to increase core flow through the laboratory. At all other holes, either a four-, five-, or six-step measurement scheme was used. At Site 1091, archive halves were demagnetized at peak fields of 30 mT. At all other sites, the peak demagnetizing fields were 20 or 25 mT. The low peak demagnetization fields ensure that the archive halves remain useful for shore-based high-resolution (U-channel) magnetic studies. We used a 5-cm sample interval for long-core remanence measurements, starting 20 cm above the core-section top and ending 20 cm below the core-section base. The large leader and trailer distance (20 cm) was used to allow future deconvolution of the long-core data. For shipboard analysis, we disregarded measurements within 10 cm from the ends of each section.
Discrete samples were collected from working halves in standard (6 cm3) plastic cubes, with the arrow on the sampling box pointing upcore. The sampling frequency was generally one sample per core section at one hole per site. Intervals of drilling-related core deformation were avoided. The discrete samples will be analyzed on shore to (1) ground-truth the shipboard polarity stratigraphies, and (2) gain insight into the magnetic properties of the sediments and the mineralogy of the remanence carriers. The shipboard long-core magnetometer was used to measure the remanence of a small number of discrete samples, using a measurement tray designed for six samples.
Where magnetic cleaning successfully isolated the primary (characteristic) component of remanence, paleomagnetic inclinations were used to define polarity zones. All cores were, to a greater or lesser extent, affected by a steep downward viscous remanent magnetization attributed to the drill string. At most Leg 177 sites, this secondary magnetization could be removed by peak demagnetization fields in the 10- to 15-mT range. However, higher coercivity secondary magnetizations, tentatively attributed to the effects of reduction diagenesis particularly at Sites 1091 and 1093, were more difficult to remove and were not always removed at the peak demagnetization fields utilized on board (25 or 30 mT). The revised time scale of Cande and Kent (1995) was used as a reference for the ages of Cenozoic polarity chrons.
Tensor core orientation data were available for Holes 1088B, 1088C, 1089A, 1089B, 1089C, 1090D, 1090E, 1091A, 1091B, 1093A, 1093B, 1093C, and 1094A. At the other holes, the Tensor tool was not employed because of bad weather conditions or time constraints. At Site 1093, one of the two Tensor tools malfunctioned resulting in sporadic orientation data. At Hole 1094A, repeated failure of the Tensor tool resulted in one out of every five cores being oriented.
The magnetic susceptibility was measured for each whole-core section as part of the MST analysis. The MST susceptibility meter (a Bartington MS2C meter with an 88-mm coil diameter and a 0.565-kHz frequency) was set on SI units and the output values were stored in the JANUS database. To convert the database values into SI units of volume magnetic susceptibility, they should be multiplied by 10-5 and by a correction factor that takes 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 ~0.63 (see also "Physical Properties"; Blum, 1997). We confirmed that this correction factor was appropriate by comparing the MST magnetic susceptibility data from ODP Leg 162 with U-channel data from the same cores. The U-channel susceptibility data were measured at Gif-sur-Yvette (France) on a susceptibility track that has been carefully calibrated.