Archive halves of core sections from Holes 1211A and 1211B were measured on the shipboard pass-through magnetometer unless they displayed obvious and pervasive coring disturbance. Measurements were made on 18 APC cores from Hole 1211A, 19 APC cores from Hole 1211B, and 15 APC cores from Hole 1211C. As at previous sites, most of the data from cores older than Pliocene age are uninterpretable. This is largely attributed to deformation of these soft sediments during drilling, core recovery, or core splitting.
As for other Leg 198 sites, the natural remanent magnetization (NRM) of core sections was measured at 5-cm intervals, followed by measurement after two alternating-field (AF) demagnetization steps (10- and 20-mT peak fields). When time was available, additional AF demagnetization steps (usually at peak fields of 15 mT) were carried out. NRM intensity values typically stayed within a narrow range, from ~10-2 to 10-1 A/m. After AF demagnetization at 20 mT (to remove the drill string overprint), intensity values were generally from ~2 x 10-4 to 2 x 10-2 A/m (Fig. F10). A decline of magnetization intensity of about one order of magnitude was observed with depth from the surface to 160 mbsf. An interval of lower magnetization intensities (~5 x 10-4 A/m) is observed in the 50-90 mbsf interval, corresponding to a large part of the Oligocene-Eocene sedimentary sequence. Similar low intensity values are also found at the base of the section in the 140-160 mbsf (Upper Cretaceous) interval (Fig. F10).
Paleomagnetic data acquired from the shipboard pass-through magnetometer from Site 1211 produced an interpretable magnetic polarity stratigraphy only in the upper ~55 meters composite depth (mcd). In this uppermost Miocene, Pliocene, and Pleistocene section, it was possible to recognize polarity zones corresponding to the Brunhes Chron (C1n) to the base of the Gilbert Chron (C3r) in Holes 1211A and 1211B (Fig. F11). The polarity zone correlative to C3r is thinned relative to C3n, indicating increased mean sedimentation rates from the onset of C3n. Directly below C3r, several polarity chrons were evident in both holes, but the polarity zone pattern is ambiguous. The next two normal polarity zones below C3r were tentatively interpreted as correlative to C3An (Fig. F11).
Measurements on cores deeper than 50 mcd showed erratic inclination values and poor correlation between holes (Fig. F11), making it impossible to interpret polarities for the upper Miocene-Cretaceous section. As at previous sites, the problem is attributed to the soft and easily deformed nature of the sediments that compromises the pass-through measurements. We are optimistic that the sediments below 55 mcd carry a primary magnetization, and it is possible that discrete samples measured postcruise will resolve the polarity stratigraphy in much of the Cenozoic section. Magnetization intensities in the Cretaceous white ooze below 130 mbsf are very low and may not be useful for shore-based resolution of a magnetic stratigraphy.
Polarity chrons recognized in the upper 50 mbsf of the section yield an age-depth curve for the upper Miocene-Pleistocene sediments (Fig. F12). Interval mean sedimentation rates were in the range 9-13 m/m.y. for the last ~4 m.y. In the 4- to 7-Ma interval, the sedimentation rates were lower, with a mean value of ~4 m/m.y. (Fig. F12). The age-depth curves for Holes 1211A and 1211B diverge in the upper 20 mbsf, corresponding to the last 2.5 m.y., owing to major differences in the cored section in each hole in that interval. In Hole 1211B, the polarity zone correlative to the Brunhes Chron (C1n) is twice as thick as in Hole 1211A, whereas that correlative to C2n is missing from the Hole 1211B section, but not in Hole 1211A. Comparison of magnetic susceptibility measurements between holes (see "Composite Depths") shows that these differences result from gaps between cores and recored intervals in both holes. When depths are computed as meters composite depth, these differences are reconciled (Fig. F11).