Archive halves of core sections from Holes 1210A and 1210B were measured on the shipboard pass-through magnetometer unless they displayed obvious and pervasive coring disturbance. Of the 27 APC cores from Hole 1210A, measurements were made from each except for Core 198-1210A-26H, which contained 1.24 m of disturbed material. From Hole 1210B, all cores were measured except Cores 198-1210B-36H, 39H, and 40H, which contained flow-in sediments. As at previous sites, most of the data from cores of the Oligocene-Paleocene and Cretaceous age are uninterpretable. This is attributed largely to deformation of these soft sediments during drilling, core recovery, or core splitting.
As before, 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; however, a few cores had intensity spikes with values higher than 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. A slow decline in intensity of about one order of magnitude was observed with depth from the surface to 220 mbsf (Fig. F12). Below this depth, an abrupt decrease in magnetization intensity occurs, with a factor of 10 reduction, in the weakly magnetic Maastrichtian and Campanian sediments (see "Lithostratigraphy"). Many cores in the Cretaceous section have intensities close to the noise level for the shipboard pass-through cryogenic magnetometer, especially in the depth range of 230-280 mbsf, where 20-mT demagnetized intensities decline to as low as ~3 x 10-5 A/m.
Paleomagnetic data acquired from the shipboard pass-through magnetometer from Site 1210 produced an interpretable magnetic polarity stratigraphy in the upper ~100 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) and to the Gilbert Chron (C3n) in both holes (Fig. F13). Directly below C3n, several normal polarity chrons were evident in both holes, but the polarity zone pattern is ambiguous. The next two polarity zones below the zone correlative to C3n-C4n were interpreted as correlative to C3An (Fig. F13).
Measurements on cores deeper than 100 mcd showed erratic inclination values and poor correlation between holes (Fig. F13), making it impossible to make any polarity interpretations for the Oligocene to Cretaceous section. As at previous sites, the problem is attributed to the soft and easily deformed nature of the sediments, which compromises the pass-through measurements.
Polarity chrons recognized in the upper 100 mcd of the section yield an age-depth curve for the upper Miocene-Pleistocene sediments (Fig. F14). The sedimentation rate was nearly constant for the last ~5 m.y., although slight variations in slope suggest an increase during Pliocene time, from 12 to 19 m/m.y., before a slight reduction to 16 m/m.y. for the last 2 m.y. Prior to 5 Ma, the sedimentation rate appears to have been about half as large (Fig. F14).