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doi:10.2204/iodp.proc.303306.111.2006

Paleomagnetism

Archive halves of all cores recovered at Site U1312 were measured on the three-axis cryogenic magnetometer at 5 cm intervals. The natural remanent magnetization (NRM) was measured before (NRM step) and after stepwise alternating-field (AF) demagnetization in peak fields of up to 20 mT. Cores 306-U1312A-1H through 9H and 19H through 21H were AF demagnetized at peak fields of 10 and 20 mT. Cores 306-U1312A-10H through 18H and 22H through 25H were AF demagnetized at a peak field of 20 mT. All cores from Hole U1312B were AF demagnetized at a peak field of 20 mT.

Downcore variations in magnetic intensity in Holes U1312A and U1312B are shown in Figure F15. Data associated with intervals identified as physically disturbed were removed. NRM intensities after 20 mT AF demagnetization are in the range of 10^–3 A/m above 40 mbsf and below 220 mbsf in both holes (Fig. F15). Intensities are one order of magnitude lower (in the range of 10^–4 A/m) in the 40–100 mbsf depth interval and two orders of magnitude lower (in the range of 10^–5 A/m) between 100 and 210 mbsf. Inclination and declination data (after 20 mT AF demagnetization) are shown in Figure F16. Declination values have been corrected using Tensor tool data, which were available starting with Cores 306-U1312A-3H and 306-U1312B-2H. Both holes display similar directional changes, although the top 75 m of Hole U1312A was affected by severe core deformation because of poor weather conditions, leading to considerable data editing.

The distribution of inclination values at Site U1312, before AF demagnetization, reflects mostly downward drill string overprint (Fig. F17). The distribution obtained after AF demagnetization at 20 mT can be characterized as a combination of two log-normal distributions centered at approximately –50° and +65°, respectively. These values are close to the expected values for a geocentric axial dipole (I_GAD = ±61.7°), which suggests that most of the drill string overprint was removed at 20 mT. Nevertheless, the distributions are large due to the presence of low-intensity intervals affected by noise. The distributions are also slightly offset toward more positive values, possibly due to the presence of some intervals that are still affected by the drill string overprint.

The magnetostratigraphy was constructed based on the succession of polarity reversals recorded at Site U1312 (Fig. F18). The Brunhes/Matuyama reversal occurs at 18.40 mbsf in Hole U1312A and 16.95 mbsf in Hole U1312B. The next normal polarity interval is the Jaramillo Subchron, which corresponds to the 20.90–24.80 mbsf depth interval in Hole U1312B. The reversals present in the underlying sediments were more difficult to characterize. In conformity with the biostratigraphy (see “Biostratigraphy”), the Gauss/Matuyama boundary was placed at 51.60 mbsf in Hole U1312B. Identification of the Olduvai Subchron in the preceding depth interval (25–51 mbsf) was nearly impossible. There are a few short intervals with directions corresponding to normal polarities in this depth range, but these may reflect remaining drill string overprint as the signal is highly variable and thus noisy. Among these intervals, the one that is the most compatible with the biostratigraphy is located at the top of Core 306-U1312-6H and has a length of ~45 cm. If this short interval does correspond to the Olduvai Subchron, however, its length is considerably shortened and sedimentation rates must be very low (i.e., ~0.25 cm/k.y.). The Gauss/Gilbert boundary is tentatively placed at 72.2 mbsf in Hole U1312B, although a significant part of the Gauss interval (Chron C2An) is missing due to coring-induced deformation in Core 306-U1312-8H. The magnetostratigraphy is unclear in the depth interval associated with low NRM intensities (gray region in Fig. F18). It becomes interpretable again below ~200 mbsf, where a long interval of normal polarity is tentatively identified as Chron 5, which would place the bottom of Hole U1312A at ~11 Ma.

Depths of the polarity transitions identified in the two holes at Site U1312 and their possible correlation to the magnetic polarity timescale (Cande and Kent, 1995) are listed in Table T17.

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