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doi:10.2204/iodp.proc.347.111.2015 PaleomagnetismMagnetic susceptibility measurements and simplified analyses of the natural remanent magnetization (NRM) were made on discrete specimens of standard volume and known mass (see “Paleomagnetism” in the “Methods” chapter [Andrén et al., 2015]). A total of 23 discrete samples were taken from Holes M0067A (9 samples) and M0067B (14 samples) at increments of ~50 cm. Magnetic susceptibility (χ) ranged between 0.6 × 10–6 and 1.2 × 10–6 m3/kg in the lower part of Unit II. The χ of the upper part of Unit II and Unit I are low and relatively uniform, ~0.012 × 10–6 m3/kg. The intensity of the NRM varied between a maximum of 13 × 10–3 A/m at a depth of ~12 mbsf in Unit II and 0.5 × 10–3 A/m in the bottom part of Unit I. It must be noted here that the measurements presented from Cores 347-M0067B-5H and 6S (9.7 mbsf to base of hole at 10.9 mbsf) are not true depths but apparent depths. Core 5H from 9.7 to 10.70 mbsf recorded 252% recovery, giving an apparent bottom depth of 12.22 mbsf. In line with other disciplines, no compression factor was applied to the data from this interval during initial processing. Measurement depths from Unit I and the upper part of Unit II (0–9.7 mbsf) are not affected by core expansion. Paleomagnetic pilot samples were grouped into two categories according to their response to alternating field (AF) demagnetization. The first category, which contains samples from the lower part of Unit II (mostly medium-grained, massive gray sand with minor silt content), is defined by unstable magnetizations. The second category, associated with the upper part of Unit II and Unit I, has a stable magnetic remanence and the inclination agrees with the geocentric axial dipole (GAD) prediction of 71°. Discrete sample measurementsA total of 23 discrete paleomagnetic samples were obtained from Holes M0067A and M0067B at 25 and 50 cm intervals. Magnetic susceptibilityThe results of the magnetic analyses are shown in Figure F14. Magnetic susceptibility ranges between 0.012 × 10–6 m3/kg (Unit I) and ~1.2 × 10–6 m3/kg (Unit II). Natural remanent magnetization and its stabilityThe NRM intensity ranges between 0.5 × 10–3 and 12.8 × 10–3 A/m. The restricted number of samples display similar NRM intensities but very different magnetic stability. Only two categories of response to AF demagnetization were observed in the pilot samples (Fig. F15). The pilot sample taken from Unit II at ~10.01 mbsf, which contains medium-grained massive gray sand and minor silt content, displays a linear orthogonal vector that trends toward the origin during AF demagnetization from NRM to 50 mT, and the carrier(s) of remanence have medium to low coercivity, with a residual NRM intensity of ~20% after AF demagnetization at 50 mT. The sample displays erratic behavior at stronger demagnetization levels, and Category 1 samples do not contain a very stable magnetic remanence. The Category 2 pilot sample that represents Units II and I has low coercivity but good paleomagnetic stability. The sample lost 50% of its NRM magnetization at an AF of 20 mT, and the orthogonal diagram also depicts the removal of a weak viscous remanent magnetism (VRM) at low fields (i.e., ~5–10 mT), with a definite univectorial behavior and a linear trend toward the origin. Paleomagnetic directionsSamples taken from the lower part of Unit II are characterized by scattered and general negative inclinations. In contrast, the directions of samples in the upper part of Unit II and Unit I agree well with the predicted GAD inclination values (Fig. F14). We conclude that the geomagnetic field has not been recorded sufficiently well at this site to allow the data to be used for relative paleomagnetic dating. |
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