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doi:10.2204/iodp.proc.347.110.2015

Paleomagnetism

To achieve the main objectives of the OSP paleomagnetic work we carried out basic analyses of the natural remanent magnetization (NRM) and ascertained the magnetic susceptibility of discrete specimens of known volume and mass (see “Paleomagnetism” in the “Methods” chapter [Andrén et al., 2015]). A total of 87 discrete samples were taken from Holes M0066A (17 discrete 2 cm × 2 cm × 2 cm cubes and 24 discrete 1 cm × 1 cm × 1 cm minicubes) and M0066B (27 discrete 2 cm × 2 cm × 2 cm cubes and 19 discrete 1 cm × 1 cm × 1 cm minicubes) according to the site splice, with a higher density of samples taken between 6 and 2 mbsf. Magnetic susceptibility ranges between ~0.03 × 10–6 and 0.6 × 10–6 m3/kg through the sequence, with the highest value found within the lower part of Unit II (medium sand with laminated silt and clay interbeds).

The paleomagnetic pilot samples that were recovered from Unit IV (parallel-bedded dark greenish gray massive medium sand) contain a medium to hard coercivity magnetic assemblage with ~30% of the NRM intensity remaining after the 80 mT demagnetization level. Unit III samples were more easily demagnetized, almost completely demagnetized by a 80 mT alternating field (AF). Units II and I contained weak and unstable remanent magnetizations.

Discrete sample measurements

A total of 87 discrete samples were obtained from Holes M0066A and M0066B. Samples were recovered at intervals of ~25 cm (1 cm3 minicubes) and 50 cm (standard 2 cm × 2 cm × 2 cm IODP cubes) from within the site splice.

Magnetic susceptibility

The results of the magnetic analyses are shown in Figure F11. Magnetic susceptibility (χ) normalized to sample mass ranges between ~0.03 × 10–6 and 0.6 × 10–6 m3/kg. Samples taken from Units IV and III have χ values <0.3 × 10–6 m3/kg. Overlying Unit II has χ values that reach 0.6 × 10–6 m3/kg. Subunit Ib has χ values that range between 0.1 × 10–6 and 0.6 × 10–6 m3/kg. Subunit Ia has consistently low values of 0.1 × 10–6 m3/kg.

The biplots show no relationships between χ, sediment wet density, NRM intensity, and inclination.

Natural remanent magnetization and its stability

Results of the pilot sample demagnetization (Fig. F12) indicate that a low AF of 5 mT was sufficient to remove a weak viscous remanent magnetization (VRM). Three different responses to the sequential AF demagnetization are displayed by samples from Site M0066. Category 1 includes the samples from the coarse-grained sands in Units IV and II; such samples lose 50% of their NRM intensity at alternating fields of 50 mT with a residual component left at 80 mT that requires stronger fields to completely remove it even though the univectorial trend is toward the origin of the diagram. The NRM of these samples is carried by a phase with relatively high coercivity. Category 2, which includes samples in Unit III, is typified by a paleomagnetic vector that is smoothly demagnetized up to the maximum AF demagnetization level of 80 mT, with a vector that trends toward the origin of the orthogonal projection, with medium to low coercivity. Category 3 is typical of unstable magnetic remanence and is characterized by the removal of a significant viscous remanence at the 5 mT demagnetization level but displays erratic magnetic behavior at higher levels of demagnetization.

After removal of the viscous overprint the NRM intensity of the samples recovered from Site M0066 lies predominantly in the range between 0.1 × 10–3 and 20 × 10–3 A/m with one outlier from Unit IV at 90 × 10–3 A/m, and there is no general positive relationship with χ (Fig. F11).

Paleomagnetic directions

The directions of the paleomagnetic vectors are illustrated by the inclination data in Figure F11. The inclination data from all units are scattered, with the majority of the inclination values on the positive side of the diagram and ~10% of inclinations on the negative side. Very few samples from these units have inclinations that approach the geocentric axial dipole (GAD) prediction for this site location. It is notable that the samples taken from Units IV, III, and II and Subunit Ib that have high χ values and relatively low NRM/χ plot relatively far away from the GAD prediction, with a large degree of dispersion of the inclination values. We conclude that the variable magnetic properties and scattered inclination data from this site preclude using the paleomagnetic data for relative dating purposes.

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