Archive halves of core sections from Holes 1212A and 1212B were measured on the shipboard pass-through magnetometer unless they displayed obvious and pervasive coring disturbance. Measurements were made on Cores 198-1212A-1H through 12H and on one section of Core 13H. From Hole 1212B, Cores 198-1212B-1H to 13H, 15H, 18H to 20H, and 23H were measured. Cores 198-1212B-14H, 16H, 17H, 21H, 22H, 25H, and 27H were not measured because of low recovery and/or pervasive disturbance. The results from Site 1212 are similar to other sites on southern Shatsky Rise. Cores of Pliocene and younger age yield an easily interpretable magnetic polarity stratigraphy, whereas most data from older cores are uninterpretable. This is largely attributed to deformation of the 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 or three alternating-field (AF) demagnetization steps (10- and 20-mT peak fields, with a 15-mT step added when time permitted). NRM intensity values typically stayed within a narrow range, from ~2 x 10-3 to 10-1 A/m for Cenozoic sediments, but showed a drop of an order of magnitude for the Upper Cretaceous section at depths greater than 100 meters composite depth (mcd). After AF demagnetization at 20 mT (to remove the drill string overprint), the downward trend was similar with a shift to values 5 to 10 times weaker than NRM values (Fig. F9). A decline of magnetization intensity of about one order of magnitude was observed with depth from the surface to 70 mcd. Below that, the magnetization intensity increases slightly from 80 to 105 mcd in cores around the K/T boundary and drops to levels near the sensitivity limit of the cryogenic magnetometer in Maastrichtian and Campanian ooze (Fig. F9).
Paleomagnetic data acquired from the shipboard pass-through magnetometer from Site 1212 produced an interpretable magnetic polarity stratigraphy in the upper ~50 mcd. In this Pliocene and Pleistocene section, it was possible to recognize polarity zones corresponding to the Brunhes Chron (C1n) to the base of the Gauss Chron (C2A) in both holes (Fig. F10).
Measurements on cores deeper than ~50 mcd showed erratic inclination values and poor correlation between Holes 1212A and 1212B (Fig. F10), making it impossible to make a reliable polarity interpretation for the upper Miocene to Cretaceous section. As at previous sites, the problem is attributed to pervasive coring deformation of the soft sediments of those ages. It may be possible to construct a polarity stratigraphy from carefully chosen discrete samples that are not severely affected by drilling disturbance.
Polarity chrons recognized in the upper 45 mcd of the section yield an age-depth curve for the Pliocene-Pleistocene sediments (Fig. F11). For this interval, the polarity stratigraphy suggests a near uniform sedimentation rate of ~13 m/m.y.