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doi:10.2204/iodp.proc.347.108.2015 PaleomagnetismMagnetic susceptibility measurements and simplified analyses of the natural remanent magnetization (NRM) were made on discrete specimens of known volume and mass (see “Paleomagnetism” in the “Methods” chapter [Andrén et al., 2015]). A total of 41 discrete samples were taken from Holes M0064A (16 samples), M0064C (14 samples), and M0064D (11 samples) at intervals of ~25 and 50 cm. The magnetic properties of this relatively small sample population varied. Magnetic susceptibility (χ) is relatively uniform between 8 and 2 mbsf, comprising Subunits IIIb (top) and IIIa and Units II and I. From ~8 to ~12 mbsf, χ ranges between ~0.024 × 10–6 m3/kg and 0.39 × 10–6 m3/kg within Subunits IVa and IIIb. The intensity of the NRM also varied over several orders of magnitude, from a maximum of 59 A/m at ~9.5 mbsf in Subunit IIIb to 0.02 × 10–3 A/m in the top part of Subunit IVa. There was also considerable variation in the lower part of Subunit IVa, the top part of Subunit IVa, and Subunits IIIa and IIIb, with enhanced NRM intensities associated with laminated clay, silt clays, and dark greenish gray sandy silt subunits. Paleomagnetic pilot samples were grouped into two categories according to their response to alternating field (AF) demagnetization. Category 1, containing samples from Subunit IVa (clast-rich stratified muddy diamicton and sandy gravel), has unstable magnetizations. Category 2, associated with Subunit IIIa, has a stable magnetic remanence but inclinations that differ between Hole M0064A (shallow) and Hole M0064C (steep). Discrete sample measurementsA total of 41 discrete paleomagnetic samples were obtained from Holes M0064A, M0064C, and M0064D at 25 and 50 cm intervals. Magnetic susceptibilityThe results of the magnetic analyses are shown in Figure F13. Magnetic susceptibility (χ) ranges between ~0.024 × 10–6 m3/kg and 0.39 × 10–6 m3/kg, and a general declining trend in χ is evident from Unit II to the top of Subunit IVa, which appears to be positively related to a decrease in sample wet density. The intensity of the NRM, however, displays a wide range over 3 orders of magnitude, suggesting variable magnetic mineralogy and/or efficiency of recording of the geomagnetic field. Only one sample was recovered from Subunit IVb (massive clast-poor sandy diamicton), and it has an intermediate χ value close to 0.2 × 10–6 m3/kg. The top of Subunit IVa has the lowest χ values of the entire core sampled. There are several samples in the top of Subunits IVa and IIIb that have significantly higher χ than the background level for these units, which is between 0.01 × 10–6 and 0.2 × 10–6 m3/kg, showing values between 0.3 × 10–6 and 0.8 × 10–6 m3/kg. Natural remanent magnetization and its stabilityThe NRM intensity spans 3 orders magnitude, between 0.02 × 10–3 A/m and 59 × 10–3 A/m. The NRM intensity displays a weak positive relationship with χ in Subunits IVa and IIIb; however, this relationship is disturbed by the occurrence of samples with relatively high NRM intensity compared to χ. Only two categories of response to AF demagnetization were observed in the pilot samples (Fig. F14). The pilot sample taken from Subunit IVb at ~27.03 meters composite depth (mcd) (Category 2), which contains a massive clast-poor sandy diamicton, displays a linear orthogonal vector that trends toward the origin during AF demagnetization, and the carrier(s) of remanence have medium to low coercivity with a residual NRM intensity of ~10% after the treatment at 40 mT. Category 2 has relatively high inclinations, between 70° and 80°. Category 1 samples do not contain a stable magnetic remanence. Paleomagnetic directionsCore sections recovered from Holes M0064A, M0064C, and M0064D are characterized by scattered inclination data and between-hole differences, such as the systematic 45° difference between Hole M0064C and Hole M0064D between 10 and 5 mbsf. 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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