Shipboard magnetometer measurements were generally compromised by weak magnetizations, slurry intervals separating coherent blocks, and the inability to perform progressive thermal demagnetization. Therefore, we relied on extensive shore-based paleomagnetic and rock magnetic measurements of discrete samples.
Approximately 800 paleomagnetic minicores were collected from the Leg 207 sites. Cylindrical samples (12 cm3) were cut using a water-cooled nonmagnetic drill bit attached to the standard drill press and trimmed with a diamond saw. The trimmed ends of such minicores were retained for analysis of carriers of magnetization. Shipboard sampling was generally at 3-m intervals from Hole B at each site. Guided by this initial magnetostratigraphic framework, additional postcruise sampling of selected priority intervals at ~1-m spacing was performed at the ODP Bremen core repository. This second set of detailed studies was designed to delimit the interpreted polarity reversals and to enhance the reliability of magnetostratigraphy from zones that had displayed relatively poor magnetic behavior.
Magnetic measurements for the main magnetostratigraphy program were carried out in the mu-metal-shielded facility at the Laboratory for Paleo- and Rock Magnetism at the Niederlippach complex of the Institut Für Allgemeine und Angewnandte Geophysik, Ludwig-Maximilians-Universität, Munich, Germany. The natural remanent magnetization (NRM) of each sample was measured using a cryogenic 2G Enterprises direct-current superconducting quantum interference device (DC-SQUID) magnetometer. The effective background noise of this three-axis cryogenic magnetometer for a minicore is ~1 x 10–6 A/m (1 x 10–3 emu/cm3), which implies that reliable polarity information can be obtained from samples with remanent magnetizations as low as 4 x 10–6 A/m. To further increase the signal-to-noise ratio, we performed duplicate measurements whenever a sample had a remanent magnetization <8 x 10–6 A/m.
In order to remove secondary overprints and resolve the primary (characteristic) magnetization, we employed a combined treatment of alternating-field (AF) demagnetization to 5 mT followed by progressive thermal demagnetization in 30° or 50°C steps, typically from 150° to 450°C. The range and spacing of thermal steps depended upon the magnetic behavior displayed by suites of pilot samples from each lithology and stratigraphic age at each site. Magnetic susceptibility was monitored after each thermal demagnetization step >300°C to detect formation of new magnetic minerals or other anomalous changes in magnetic characteristics.
For visualizing the demagnetization behavior, computing characteristic directions, and assigning polarity ratings, we used the PALEOMAG software package, which has a combined graphical and analytical package designed for interpreting magnetostratigraphy and paleomagnetic statistics for large suites of samples (Zhang and Ogg, 2003; and as documented freeware available from Purdue University at www.eas.purdue.edu/paleomag/).
We conducted a separate study to investigate the nature of the remanence and the minerals responsible for stable remanence. A few representative specimens were selected based on differences of lithology and behavior of demagnetization characteristics through magnetic measurements for a set of rock magnetic experiments. These rock magnetic experiments were performed at the paleomagnetic laboratory of the Department of Environmental Sciences, Ibaraki University, Japan. The experiments included (1) progressive isothermal remanent magnetization (IRM) and (2) subsequent stepwise thermal demagnetization of acquired IRM.