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doi:10.2204/iodp.proc.347.105.2015

Paleomagnetism

To fulfill the main objectives of the OSP paleomagnetic work, we carried out basic analyses of the natural remanent magnetization (NRM) and magnetic susceptibility of discrete specimens of known volume and mass magnetic susceptibility (see “Paleomagnetism” in the “Methods” chapter [Andrén et al., 2015a]). A total of 309 discrete samples were taken from Holes M0061A (128 samples), M0061B (53 samples), and M0061C (128 samples) according to the site splice, with a higher density of sampling in the upper 11.5 m. Magnetic susceptibility (χ) ranges between 0.1 × 10–6 and 1.4 × 10–6 m3/kg through the sequence, with the highest value found at the boundary between Subunits IIb and IIa, which is characterized by black (iron sulfide) staining.

Most of the paleomagnetic pilot samples recovered from Units IV and III carried a relatively intense NRM that reached 380 × 10–3 A/m. This NRM was easily demagnetized and has low magnetic stability. In addition, this relatively coarse sand interval has very scattered inclinations, with an average close to 15°. At 11.5 meters composite depth (mcd) (in Unit III), the NRM intensity decreases and the inclination increases to approach a geocentric axial dipole (GAD) prediction. An interval of high NRM intensity at the boundary between Subunits IIb and IIa is associated with observations of iron sulfide precipitation. The majority of samples in the top part of Unit III and Units II and I carried normal polarity NRMs, with secular variations around the GAD prediction. Pilot samples with relatively high χ taken from Unit IV and up to the middle of Unit III (11.5 mcd) acquired gyroremanent magnetization (GRM) during alternating field (AF) demagnetization above 50 mT, and these samples are associated with inclinations that approach the GAD prediction of 74°. The acquisition of a GRM indicates the presence of authigenic greigite (Fe3S4), which is responsible for secondary chemical remanent magnetizations of strictly unknown age (Snowball, 1997). The upper 6 m (part of Subunit IIa and Unit I) contains inclination data that can be correlated to features in a regional master curve (Snowball et al., 2007), but these require independent corroboration.

Discrete sample measurements

A total of 309 discrete samples were obtained from Holes M0061A, M0061B, and M0061C. Samples were recovered at intervals of ~50 cm from within the site splice.

Magnetic susceptibility

The results of the magnetic analyses are shown in Figure F18. Magnetic susceptibility (χ), which was normalized to sample mass, predominantly ranges between 0.1 × 10–6 and 0.6 × 10–6 m3/kg, with one outlier at 1.4 × 10–6 m3/kg. Samples taken within Unit IV have χ values between 0.2 × 10–6 and 0.4 × 10–6 m3/kg. Overlying Unit III has relatively lower χ values that generally do not exceed 0.3 × 10–6 m3/kg, with the exception of some high values (0.6 × 10–6 m3/kg) around 9 and 11 mbsf. Unit II has variable χ, and Subunit IIb is characterized by values of <0.2 × 10–6 m3/kg. Unit I, which is organic-rich laminated clay, has χ values close to 0.2 × 10–6 m3/kg.

Sediment wet density and χ are positively related, although the majority of the data are from the upper 11.5 m because of the higher sampling density in the upper part of the sequence. One significant observation is that shallow positive (<40°) and negative inclinations are associated with a wide range of NRM intensity.

Natural remanent magnetization and its stability

Results of the pilot sample demagnetization (Fig. F19) indicate that a low AF of 5 mT is sufficient to remove a weak viscous remanent magnetization (VRM). Three different responses to the sequential AF demagnetization are displayed by samples from Site M0061. Category 1 includes the samples from the relatively coarse grained Units IV and III, which lose 50% of their NRM intensity at alternating fields less than 15 mT, with a small residual component left at 40 mT. These samples subsequently acquired a gyroremanent magnetization (GRM) at field levels >60 mT, with the vector moving into a plane perpendicular to the last demagnetization axis. Category 2, which includes all other pilot samples except those in Unit I, 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. Category 3 has a relatively high magnetic stability, with >50% of the NRM removed between 15 and 30 mT, which can indicate a narrow magnetic grain size distribution.

After removal of the viscous overprint, the NRM intensity of the samples recovered from Site M0061 lies between 0.07 × 10–3 and 380 × 10–3 A/m and there is a general positive relationship with χ (Fig. F18). It is notable that the NRM intensity of Unit I and the interval of distinct iron sulfide precipitation between Subunits IIb and IIa is particularly high relative to χ, which suggests a more efficient recording of the geomagnetic field than in the other units.

Paleomagnetic directions

The directions of the subsample paleomagnetic vectors are illustrated by the inclination data in Figure F18. The inclination data from Unit IV and the lower part of Unit III are scattered, with the majority of the inclination values on the positive side of the diagram. Only a few samples from these two units approach the GAD prediction for this site location. In contrast, the inclination data from Units I and II and the uppermost part of Unit III group closer to the GAD prediction, but there is a bias toward shallow inclinations, particularly between 10 and 9 mbsf. It is notable that the samples taken from the upper part of Unit IV and Unit III, which have high χ values, plot relatively far away from the GAD prediction. The variable magnetic properties and different categories of response to AF demagnetization, which include samples that acquire GRM, probably preclude using the paleomagnetic data for relative dating purposes in Units IV and III. In particular, pilot samples that acquire GRM are restricted to intervals with inclinations that are close to the GAD prediction. These samples probably contain a secondary chemical remanent magnetization (CRM) carried by authigenic greigite (Fe3S4), which is known to acquire GRM (Snowball, 1997). The time lag between sediment deposition and greigite precipitation is unknown and, therefore, the ability to use the paleomagnetic data at Site M0061 for relative dating purposes is most likely restricted to Unit I and the upper half of Unit II, shallower than 5 mbsf. The interval of high inclinations grouped around a depth of ~2 mcd may correspond to one of the late Holocene inclination features identified in the FENNOSTACK regional master curve (Snowball et al. 2007), such as ε1 (~2650 cal y BP) or γ (1290 cal y BP).

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