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doi:10.2204/iodp.proc.347.109.2015

Paleomagnetism

In accordance with the main objectives of the OSP paleomagnetic work, magnetic susceptibility measurements and rudimentary analyses of natural remanent magnetization (NRM) were made on discrete specimens of known volume and mass (see “Paleomagnetism” in the “Methods” chapter [Andrén et al., 2015a]). Discrete samples were taken from Holes M0065A (89 specimens), M0065B (19 specimens), and M0065C (9 specimens) for a total of 117 samples. Magnetic susceptibility ranges between 0.03 × 10–6 and 0.43 × 10–6 m3/kg through the sequence, with values greater than 0.2 × 10–6 m3/kg confined to Subunit IIIc (an interval varying from coarse fine to medium sand) and the upper part (16–13 mbsf) of Subunit IIIa. Paleomagnetic pilot samples recovered from Units I and II, considered to be Holocene in age, have inclinations that are consistent with the geocentric axial dipole (GAD) inclination value (i.e., 71°). The pilot samples located in Subunit IIIc, which is likely to be late glacial and contains slumped material, are characterized by poor magnetic stability, and this unit contains scattered inclination values between 0° and 30°.

Discrete sample measurements

A total of 117 discrete samples were obtained from Holes M0065A and M0065C. Samples were recovered at intervals of ~50 cm from inside the site splice.

Magnetic susceptibility

The results of the magnetic analyses are shown in Figure F17. Magnetic susceptibility (χ), which was normalized to sample mass, ranges between 0.03 × 10–6 and 0.43 × 10–6 m3/kg. Samples taken from Subunits IIIc and IIIb have χ values between 0.1 × 10–6 and 0.43 × 10–6 m3/kg. The overlying Subunit IIIa has χ values between ~0.1 × 10–6 and 0.15 × 10–6 m3/kg, with a distinct peak at depths between 16 and 13 mbsf of 0.1 × 10–6 and 0.3 × 10–6 m3/kg.

The χ of Unit II is variable and includes one interval (13–10 mbsf) in which the values decrease upcore from ~0.1 × 10–6 to 0.05 × 10–6 m3/kg. From 10 mbsf to the top of the core (i.e., at ~2 mbsf; Unit I), values of χ are <0.1 × 10–6 m3/kg and are relatively constant.

Sediment wet density and χ are not related to each other, although χ ranges over half an order of magnitude, which suggests that changes in the magnetic mineralogy and/or grain size are determining χ. Two trends are apparent in the biplot of χ versus NRM intensity, one that indicates high χ/NRM ratios and one that indicates low χ/NRM ratios. The samples from Subunit IIIc have high χ/NRM ratios.

Natural remanent magnetization and its stability

Results of the pilot sample demagnetization (Fig. F18) indicate that an alternating field (AF) of 5 mT is sufficient to remove a weak viscous remanent magnetization (VRM). Four different responses to the sequential AF demagnetization are displayed by samples from Site M0065. Category 1 includes the samples of the grayish brown silty clay with planar lamination that characterizes Subunit IIIc, and they lose 50% of their NRM intensity at AF <15 mT, with a small residual component left at 40 mT that is unaffected by more intense demagnetization levels. The univectorial diagram does not trend to the origin of the diagram. Category 2, which includes a sample from Subunit IIIa at 23.52 mbsf, is typified by a paleomagnetic vector that is moderately “hard,” indicating high coercivity and demagnetized up to the maximum AF demagnetization level of 80 mT, with a vector that trends toward the origin of the orthogonal projection. Category 3 has a “softer” magnetic behavior, with the removal of a significant viscous remanence at the 5 mT AF demagnetization level. The sample demagnetizes relatively easily and loses 50% of its magnetization between 30 and 40 mT. It is univectorial and trends to the origin of the orthogonal plot. Category 4 shows the behavior of a specimen that is characterized by low coercivity and a quite “soft” behavior of demagnetization. About 50% of the magnetization is removed by 20 mT, and the univectorial diagram shows a distinct linear behavior, trending toward the origin of the diagram.

After removal of the viscous overprint the NRM intensity of the samples recovered from Site M0065 lies in the range between 0.07 × 10–3 and 150 × 10–3 A/m and there is a positive relationship with χ in Subunits IIIa, IIIb, and IIIc, in which large peaks in NRM intensity are reflected in the χ data, and also in Units II and I (Fig. F17).

Paleomagnetic directions

The directions of the paleomagnetic vectors are illustrated by the inclination data in Figure F17. The inclination data from Unit III are scattered, with many positive shallow values and few negative values. Only two samples from these three units approach the GAD prediction for this site location. In contrast, the inclination data from Units II and I group closer to the GAD prediction, but there is a bias toward higher values (i.e., between 14 to 12 mbsf spanning the boundary between Subunit IIIa and Unit II). It is notable that the samples taken from 10 to 2 mbsf Unit I, which have relatively low χ values, plot within a few degrees of the GAD prediction. The variable magnetic properties downhole and different categories of response to AF demagnetization probably preclude using the paleomagnetic data for relative dating purposes.