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doi:10.2204/iodp.proc.347.106.2015 PaleomagnetismMagnetic susceptibility measurements and rudimentary 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., 2015a]). A total of 290 discrete samples were taken from Holes M0062A (161 samples), M0062C (40 samples), and M0062D (89 samples) according to the site splice, with a higher density of samples taken in the upper 9 m. Magnetic susceptibility ranges between 0.07 × 10–6 and 0.55 × 10–6 m3/kg through the sequence, with the highest value found within Unit II, a well-sorted dark gray fine to medium sand. The lowest values are confined to a 0.5 m thick interval of greenish black/very dark greenish gray laminated silty clay in Subunit Ia (~8.24–7.74 mbsf, Hole M0062A). The majority of paleomagnetic pilot samples that were recovered from Unit II carried an intense low-coercivity (easily demagnetized) NRM that reached as high as >300 × 10–3 A/m, with one outlier at >800 × 10–3 A/m. The inclination values obtained from Unit II clustered between 0° and 30°. These scattered data indicate that the glaciofluvial or fluvial environment represented by Unit II did not allow the geomagnetic field to determine the orientation of minerals that acquire magnetic remanence, although it is likely that the mechanical sorting of minerals in this environment has caused enrichment of the relatively dense magnetic component (most likely magnetite). Subunits Ib and Ia are characterized by less intense NRM with medium coercivity (~30–40 mT). The transition from Unit II to Subunit Ib witnesses a reduction in NRM intensity to <200 × 10–3 A/m, and the inclination of Subunits Ia and Ib varies around the geocentric axial dipole (GAD) prediction of 74°. Some samples, particularly in the lower part of Subunit Ib, acquired a relatively intense gyroremanent magnetization (GRM) during AF demagnetization, suggesting the presence of authigenic greigite (Fe3S4) (Snowball, 1997). Discrete sample measurementsA total of 290 discrete samples were obtained from Site M0062, restricted to core sections included in the site splice. Samples were recovered at intervals of ~50 cm from inside the site splice between 35.9 and 9 meters composite depth (mcd). The sampling density in the upper 9 m was increased to 5 cm intervals. Magnetic susceptibilityThe results of the magnetic analyses are shown in Figure F16. Magnetic susceptibility (χ), which was normalized to sample mass, ranges between 0.07 × 10–6 and 0.55 × 10–6 m3/kg within the hole. Samples taken from Unit II and Subunit Ib have χ values that range between 0.15 × 10–6 and 0.55 × 10–6 m3/kg. Overlying Subunit Ia has χ values that cluster around 0.2 × 10–6 m3/kg, with the exception of an interval of low χ (0.1 × 10–6 m3/kg) between 8 and 7.5 mcd. It is notable that the group of samples in the upper 0.35 m from Hole M0062C has low and variable χ. The scatter plots of sediment wet density against χ and χ against NRM intensity contain sufficient scatter to suggest that the magnetic properties change with depth, probably as a function of grain size and/or magnetic mineralogy. Natural remanent magnetization and its stabilityResults of the pilot sample demagnetization (Fig. F17) indicate that the carrier of NRM alters with depth. Three categories can be identified. Category 1, which is typical of Unit II, is characterized by removal of almost half of the NRM intensity by the 5 mT alternating field (AF) with <20% remaining after demagnetization at 20 mT. The residual vector is not stable but trends away from the origin at AF fields >50 mT. Category 2 is common to Unit I and is typified by a single component of NRM that trends toward the origin during AF demagnetization, with ~10% remaining after the application of an AF of 80 mT. Category 3, which includes some samples close to the transition between Unit II and Subunit Ib and the lower half of Subunit Ia reveals curvilinear vectors that preclude the identification of a stable remanence. This behavior is caused by the acquisition of a GRM and is characteristic of greigite (Fe3S4) (Snowball, 1997). Paleomagnetic directionsThe directions of the paleomagnetic vectors are illustrated by the inclination data in Figure F16. The inclination data derived from Unit II are unreliable as a record of the ancient geomagnetic field because of the considerable scatter within 0° and 30°. At the transition to Subunit Ib, the inclination steepens to approach a GAD prediction (74°) and varies a few degrees through Unit I, with the exception of four outliers. Declination data (not shown in the figure) are highly scattered because of the high latitude of the site and the fact that the cores were not oriented to an azimuth. The low inclinations and likely presence of a chemical remanent magnetization (CRM) carried by greigite in Unit II do not allow for reconstructions of paleomagnetic secular variation (PSV). The inclination data from Subunit Ia contain secular variability and may contain a relatively high resolution record of PSV that can be used for relative dating purposes. |
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