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doi:10.2204/iodp.proc.347.107.2015

Paleomagnetism

Magnetic 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 216 discrete samples were taken from Hole M0063D at intervals of ~50 cm. The magnetic properties of the sample population were found to be extremely variable. Magnetic susceptibility (χ) gradually decreases from values of 1 × 10^–6 m³/kg in the lowermost Unit VII to <0.1 × 10^–6 m³/kg in Units I and II. The intensity of the NRM varied over 3 orders of magnitude, from a maximum of 2.6 A/m at a 55 mbsf in Unit VI to 0.3 × 10^–3 A/m in Unit I, but with considerable variation in Units III, II, and I, with enhanced NRM intensities associated with laminated subunits.

Paleomagnetic pilot samples were grouped into four categories according to their response to alternating field (AF) demagnetization. Category 1 contains samples from Units VII, VI, and V (mostly laminated clays), has shallow inclination, and contains one or more carriers of NRM with medium to high coercivity. Category 2, associated with Units IV and III, also has medium to higher coercivity carriers of the NRM but steeper inclinations than Category 1. Category 3, which is characteristic of some samples from Units III, II, and I, has distinctly low NRM intensity and acquires a gyroremanent magnetization (GRM) at AF > 50 mT. Category 4 contains pilot samples from Units II and I, and it is distinguished from the other categories by a demagnetization curve that indicates a narrow magnetic grain size distribution. This fourth category includes samples with steep inclinations and high NRM intensity. An important observation is that the discrete paleomagnetic samples taken from the upper 0.75 m of cores were disturbed (see “Stratigraphic correlation”) and have shallow and normal or reversed inclinations. It was therefore concluded that changes in inclinations and declination are highly dependent on magnetic mineralogy and identified coring disturbance.

Discrete sample measurements

A total of 216 discrete paleomagnetic samples were obtained from Hole M0063D at 50 cm intervals.

Magnetic susceptibility

The results of the magnetic analyses are shown in Figure F21. Magnetic susceptibility (χ) ranges between <0.1 × 10^–6 and 1 × 10^–6 m³/kg, and a general declining trend in χ is evident from the bottom of Unit VII to the top of Unit I, which appears to be positively related to a decrease in sample wet density. The lowermost Unit VII (clayey silty sand) contains the highest χ values, close to 1 × 10^–6 m³/kg, and the values decline through Units VI, V, and IV to reach a minimum of ~0.2 × 10^–6 m³/kg. Subunit Id has low χ values of <0.1 × 10^–6 m³/kg. There are several samples in Units III, II, and Ib that have significantly higher χ than the background level for these units, which is between <0.1 × 10^–6 and 0.2 × 10^–6 m³/kg.

Natural remanent magnetization and its stability

The intensity of the NRM displays a wide range over almost 4 orders of magnitude, suggesting a variable magnetic mineralogy and/or efficiency of recording of the geomagnetic field. The NRM intensity spans between 0.3 × 10^–3 and 2.6 A/m. The NRM intensity displays a consistent positive relationship with χ in Units VII and VI. In Units III, II, and I, this relationship is disturbed by the occurrences of samples with relatively high NRM intensity compared to χ. Four categories of response to AF demagnetization were observed in the pilot samples (Fig. F22). The majority of pilot samples taken from Units VII, VI, and V, which contain laminated (varved) clays, display a linear orthogonal vector that trends toward the origin during AF demagnetization, and the carrier(s) of remanence have medium to high coercivity, with a residual NRM intensity of ~15% after the treatment at 80 mT. This category (Category 1) has relatively low inclinations between 30° and 40°. Category 2 samples also contain a medium- to high-coercivity carrier of the NRM, but the inclinations are close to the geocentric axial dipole (GAD) prediction for the site location (73°). Category 3 includes pilot samples with relatively low NRM intensity and low χ and they acquire a relatively strong GRM during AF demagnetization that causes the sample intensity to increase at AF > 50 mT and the vector to lie in the plane perpendicular to the last demagnetization axis (Snowball, 1997). The majority of the pilot samples obtained from Units II and I belong to Category 4, which is defined by a low- to medium-coercivity carrier of the NRM. The NRM is most effectively removed between a narrow range of AF steps (10–40 mT), which indicates a narrow magnetic grain size distribution.

Paleomagnetic directions

Core sections recovered from Hole M0063D were often found to be disturbed by drilling and degassing in the upper 0.75 m of cores, which resulted in scattered inclination data. With the exceptions of Units IIIb and V, which contain disturbed and convoluted structures, the identification and removal of these data points improve the fidelity of the inclination record. However, the variable magnetic properties downhole and different categories of response to AF demagnetization, which includes samples that acquire GRM, probably preclude using the paleomagnetic data for relative dating purposes. The most notable observation is that the relatively strong NRM intensity in relation to χ is associated with Category 4 pilot samples, which are generally confined to the laminated Subunits Id and Ib. Similar magnetic enhancement of Baltic Sea sediments was reported by Reinholdsson et al. (2013) and was assigned to the preservation of greigite (Fe₃S₄) magnetofossils in laminated sapropels that formed during periods of basin-wide hypoxia due to natural and anthropogenically induced increases in Baltic Sea primary production (e.g., Zillen et al., 2008).

Our main conclusion is that the variable magnetic properties, acquisition of GRM, and presence of disturbed intervals in Hole M0063D preclude comparison between the directional data and regional paleomagnetic secular variation (PSV) master curves applicable to the Holocene.

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