METHODS

Two types of paleomagnetic measurements were made on sediments collected during ODP Leg 198; pass-through measurements on half-cores and discrete sample measurements. Discrete sample cubes (2 cm 2 cm) were collected during Leg 198 to augment measurements using the shipboard pass-through magnetometer. Shipboard measurements on half-cores were made at 5-cm intervals. A total of 747 discrete samples were taken at 50-cm intervals. Discrete samples were collected from the center of the half-cores to avoid deformation at the outer edges of the core. Magnetic measurements on the cubes were performed in the magnetically shielded room at the University of Florida (USA) using a 2G Enterprises cryogenic magnetometer. The samples were step-wise alternating-field (AF) demagnetized using a D-Tech D2000 AF demagnetizer. Magnetization component directions were determined using the method of Kirschvink (1980), applied to the 20- to 60-mT peak field demagnetization interval.

The astrochronology developed for Sites 1207–1211 was based on cycles seen in reflectance data (L*) measured shipboard on a purpose-built track. Reflectance of visible light from soft-sediment cores was measured using a spectrophotometer at 2.5-cm intervals and provided a high-resolution record of color variations for visible wavelengths (400–700 nm). L* reflectance represents "lightness" of the sediment, which is usually controlled by changes in percent carbonate.

The initial age model for each site was based on correlation of the polarity zone sequence to the timescale of Cande and Kent (1995). Power spectra using the Blackman-Tukey method with a Bartlett window from the Analyseries software of Paillard et al. (1996) indicate the presence of obliquity and eccentricity peaks. The reflectance data were then tuned to the astronomic solutions for obliquity from Laskar et al. (1993). This allowed astronomically calibrated ages to be assigned to the polarity reversal boundaries at Sites 1207–1211. Site 1212 was not included in the astrochronology, as it contains a hiatus at 4–5 Ma.