As at Site 1207, all archive halves of core sections from Hole 1208A that did not show a large degree of drilling-related deformation were measured on the shipboard pass-through magnetometer. Many of the measured cores from the Paleogene and Cretaceous part of the section, however, are in poor condition, due to either drilling disturbance or disturbance induced during the splitting of core sections. In addition to the 20 APC cores (198-1208A-1H through 20H), measurements included 18 of 21 XCB cores (only Cores 198-1208A-34X through 36X were not measured). Many of the XCB cores showed "biscuiting" (fragmentation) into segments several centimeters to tens of centimeters in length. This deformation has the potential to render the pass-through magnetometer data useless, owing to the averaging of divergent magnetic vectors when several "biscuits" and/or intervening drilling slurry are in the instrument sensing region. Many XCB cores, however, displayed consistent remanence inclination data that implied the measurements were worthwhile.
The natural remanent magnetization (NRM) of core sections was measured at 5-cm intervals and at 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 measured. NRM measurements typically ranged over two orders of magnitude in intensity, from 10-3 to 10-1 A/m, but several spikes in cores from near the bottom of the hole reached values as high as 5.8 A/m (Fig. F18). NRM inclinations uniformly showed steep downward directions indicative of a drill-string induced overprint. This overprint was progressively removed with AF demagnetization, typically disappearing by the 15- to 20-mT demagnetization step. After 20-mT demagnetization, magnetization intensities mostly ranged from 10-4 to 10-2 A/m. In the upper 90 mbsf, magnetization intensities declined by about a factor of 10, with short wavelength variations superimposed (Fig. F18).
The AF demagnetized magnetization directions have inclination values clustering around ±55°, the geocentric axial dipole value for the location of Site 1208. Changes between positive and negative inclination were compared with declination values to identify magnetic reversals. In the APC cores (0-185 mbsf), the data give an expanded magnetostratigraphic record, showing all of the polarity chrons in the Cande and Kent (1995) timescale from C3n (Gilbert) to C1n (Brunhes) (Fig. F19). Magnetic chron depths indicate that the sedimentation rate increased through the Miocene and Pliocene-Pleistocene, eventually reaching rates as high as 40 m/m.y. during the last 3 m.y. (Fig. F20). Pliocene-Pleistocene magnetic records of comparable resolution are available for the Atlantic realm; however, this record is unique for the Pacific.
In the XCB-cored section, below 185 mbsf, the polarity record is more difficult to interpret because the cored record is not continuous and the data are noisier as a result of the core deformation. Nevertheless, we interpret a long reversed polarity interval just below 200 mbsf as Chron C3r and a long normal polarity interval at ~275 mbsf as Chron C5n. Other intervals of normal and reversed polarity are interpreted between C2r and C5n and correlated to other Miocene polarity chrons, but with less certainty (Fig. F19).