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doi:10.2204/iodp.proc.347.103.2015

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., 2015]). The sediment samples recovered from Site M0059 display a wide range of magnetic properties. Magnetic susceptibility varies by approximately an order of magnitude and includes one diamagnetic sample recovered from the Cretaceous limestone (Unit VII). The response of pilot samples to sequential alternating field (AF) demagnetization reflects, to a large degree, the variation in magnetic susceptibility and indicates that a combination of low-, medium-, and high-coercivity minerals at this site contribute to the total magnetization and that some samples contain at least two overlapping components. Some samples recovered from Subunits Ib and Ia acquired a gyroremanent magnetization (GRM) during sequential AF demagnetization, which is characteristic of, but not exclusive to, diagenetic greigite (Fe₃S₄). The majority of paleomagnetic samples recovered from Units VI–III carried a normal polarity magnetization, but the average inclination is ~30°–40° shallower than the 71° predicted by a geocentric axial dipole (GAD) model at the site latitude. The paleomagnetic samples recovered from Subunits Ib and Ia carry normal polarity NRMs that vary approximately ±10° around the GAD model prediction, with the exception of some shallow or reversed samples that are associated with sediment disturbance and/or samples taken from core catchers. Cores were not oriented with respect to an azimuth, and the relatively low density of sampling restricted the utility of the horizontal component of the NRM vector (declination).

Discrete sample measurements

A total of 454 discrete samples were obtained from Site M0059. Samples were recovered at intervals of ~50 cm from inside and outside the site splice to obtain denser coverage. The sample set consisted of 118 samples from Hole M0059A, 56 from Hole M0059B, 162 from Hole M0059C, 114 from Hole M0059D, and 4 from Hole M0059E.

Magnetic susceptibility

Magnetic susceptibility results are shown in Figure F22. Magnetic susceptibility (χ, which is normalized to sample mass) ranges between –4.7 × 10^–9 m³/kg (the sample of Cretaceous limestone at the base of Hole M0059B) and 0.6 × 10^–6 m³/kg (Sample 347-M0059B-25P-1, 20 cm). This section, which is between 117.3 and 117.9 mbsf in Unit VI, is considered to have been disturbed by coring and may not be in situ (see “Lithostratigraphy” for further details).

Samples taken from the bottom of Unit III between 83.2 and 80 mbsf have relatively high χ, which is followed by a general declining trend to the top of this unit. This trend is in agreement with the upward fining of this gray clay. The general trend of an uphole decrease in χ continues in Subunit Ia, although the interval between the bottom of Subunit Ib and a depth of 21 mbsf is characterized by frequent peaks in χ, which are shared between samples taken from different holes.

Sediment wet density and χ are positively related (Fig. F22), but there is considerable scatter in the data set. This scatter suggests that sediment consolidation and dilution of one ferrimagnetic component are not the only determinants of χ. It is most likely that variations in χ reflect sediment provenance and magnetic grain size and/or postdepositional precipitation of ferrimagnetic minerals.

Natural remanent magnetization and its stability

Results of the pilot sample demagnetization (e.g., Fig. F23) indicate that an AF of 5 mT is sufficient to remove a weak viscous remanent magnetization (VRM). After removal of this overprint, the pilot samples displayed varied responses to the sequential demagnetization, with three principal categories. Category 1 indicates the presence of a NRM carried by a mineral with high coercivity (median destructive field of NRM > 60 mT) and is characteristic of Unit III. The magnetization vector trends toward the origin after the 5 mT step, and 50% of the NRM intensity is removed at an AF of 80 mT. Category 2 indicates the presence of a mineral with low coercivity (median destructive field of NRM = 20–25 mT) and is characteristic of Unit III through Subunit Ia. After removal of a weak overprint by the 5 mT AF treatment, the magnetization vector trends toward the origin and the NRM intensity is reduced to <5%. Category 3 is characteristic of samples in Subunits Ib and Ia that have relatively high χ (between 51.68 and 16.00 mbsf). In this category the median destructive field of NRM is ~45 mT, and almost 20% remains after the application of an 80 mT AF. In addition, the magnetization vector trends toward the origin up to 40 mT AF, after which it veers into a plane that is perpendicular to the last applied demagnetization axis. This behavior is characteristic of the acquisition of a GRM, which can be indicative of the presence of diagenetic greigite (Fe₃S₄) in the samples.

After removal of the viscous overprint, the NRM intensity of the samples recovered from Site M0059 lies in the range between 0.07 × 10^–3 and 120 × 10^–3 A/m, and there is a positive relationship with χ in Subunits Ib and Ia (Fig. F22). The relationship between NRM intensity and χ is less obvious in Unit III, in which large peaks in NRM intensity are not reflected in the χ data.

Paleomagnetic directions

Palaeomagnetic samples recovered from Unit VI (between 149 and 123 mbsf) have inclinations that are widely scattered around 0°, and these are associated with high magnetic susceptibility and low NRM intensity. These data suggest that the paleomagnetic field has not been reliably recorded by these sediments. Similarly, the majority of samples recovered from Unit III, which is composed of laminated clay, show strong evidence of inclination shallowing. Some samples, however, show steep inclinations close to the GAD prediction.

The significant change in lithology that marks the bottom of Subunit Ib is accompanied by inclinations that vary within 10° either side of a GAD prediction. The absence of independent time control points, however, does not allow for a detailed comparison to the FENNOSTACK regional master curve (Snowball et al., 2007). On the other hand, it may be possible that the interval of steepest inclinations between ~18 and 16 mbsf corresponds to the period of steep inclination experienced in Fennsocandia between 3090 and 2590 cal y BP, which is delimited by inclination features ε and ε¹, respectively, in FENNOSTACK.

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