All archive halves of core sections from Hole 1207A that did not show a large degree of drilling-related deformation were measured on the shipboard pass-through magnetometer. Of the top 21 cores from Hole 1207A, only Core 198-1207A-19H was excluded owing to drilling disturbance. Measurements were also made on the least disturbed sections from the RCB cores from Hole 1207B: Cores 198-1207B-1R through 3R. Natural remanent magnetization (NRM) was measured for each core section, followed by two to four steps of alternating-field (AF) demagnetization up to a maximum peak field of 20 mT. The maximum peak field was set at this level in order not to compromise the archive sections for possible shore-based (u-channel) studies. Magnetization intensities of NRM and magnetization intensities after AF demagnetization at peak fields of 20 mT are in the 10-1-10-2 A/m range near the seafloor but fall to 10-3-10-4 A/m at 30 mbsf (Fig. F28). This decrease may be the result of diagenetic reduction of remanence-carrying magnetite in the upper part of the sedimentary section. The plot of magnetic remanence intensity vs. depth shows several oscillations between 30 and 100 mbsf, probably indicating variation in the amount or preservation of magnetic minerals. These variations are mimicked by the curve of iron content vs. depth (see "Inorganic Geochemistry").
In those parts of the record where the remanence intensity is slightly higher (above ~5 x 10-4 A/m at the 20-mT AF demagnetization step), it is possible to interpret the 20-mT AF demagnetized inclination and declination values in terms of polarity zones. In regions of low magnetization intensity, however, the record is uninterpretable. The cores provide a clear record of C1n (Brunhes Chron), C1r.1n (Jaramillo Subchron), and C2n (Olduvai Subchron) in the upper 30 mbsf (Fig. F29). Below this depth, however, the record is compromised by low remanence intensities, which average about one order of magnitude above the noise level of the shipboard magnetometer (the noise level of the magnetometer corresponds to a magnetization intensity of ~3 x 10-5 A/m). We tentatively interpret a thick reverse polarity interval at 100-110 mbsf as C3r (base of the Gilbert Chron) and a thick normal polarity zone at 141-152 mbsf as C5n (Fig. F29). Other polarity zones are shown in Figure F29 but are uncertain because the polarity zone pattern is nonunique owing to intervals of uninterpretable polarity or erratic directions (colored gray in the figure) and at least one significant unconformity (interpreted from biostratigraphic data to be at the base of C5n). Shore-based studies should allow the polarity record for Hole 1207A to be refined.
An age-depth curve for the upper 150 mbsf (Fig. F30) implies a mean interval sedimentation rate of 14-21 m/m.y. for the last 7 m.y. Prior to 7 Ma, the mean interval sedimentation rate is only ~5 m/m.y. This age-depth curve agrees well with biostratigraphic datums, but this is partly because of the use of biostratigraphic datums to aid interpretation of polarity zones in the polarity record. The figure shows that only a few polarity chron interpretations are considered robust (solid circles).
Measurements from Cores 198-1207B-1R through 3R show a stable magnetization component with mean magnetization intensity after AF demagnetization at peak fields of 20 mT of ~5 x 10-4 A/m. Normal polarity magnetizations are consistent with deposition in the late Campanian, during C33n or C32n. One short, 30-cm reversed interval was found in Section 198-1207B-2R-4.