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doi:10.2204/iodp.proc.347.104.2015

Physical properties

This section summarizes the preliminary physical property results from Site M0060. Two holes were drilled at this site. Hole M0060A was drilled to 232.50 mbsf, and Hole M0060B was drilled to 85.7 mbsf. A more complete data set of physical properties was produced for Hole M0060A during the OSP (Fig. F14) because Hole M0060B was designated as a microbiology hole (see “Microbiology”) and was extensively subsampled onboard. Core recovery was generally good in Hole M0060A, except for a gap between ~101.5 and 117 mbsf and from ~198 mbsf to the base of the hole, where coring alternated between hammer sampling and open-hole intervals. Although all physical property measurements described in “Physical properties” in the Methods chapter (Andrén et al., 2015) were conducted for Site M0060, whole-round core shipboard P-wave velocity data appear to be highly affected by artifacts, similar to those seen at Site M0059 (Little Belt). Discrete P-wave and thermal conductivity data are too sparsely distributed to exhibit any discernable downcore trends, and noncontact electrical resistivity data show little variability. Color reflectance variations do not correspond to other physical properties and lithostratigraphic units.

Natural gamma ray

High-resolution natural gamma ray (NGR) values are <5 cps near the core top in lithostratigraphic Unit I (Fig. F14). These low values reflect high coarse-grained content within lithostratigraphic Unit I (see “Lithostratigraphy”). NGR exhibits generally higher (>6 cps) and variable values in lithostratigraphic Unit II. Negative excursions potentially reflect an increase in silt or sand content, whereas positive excursions may result from an increase in clay content (see “Lithostratigraphy”). NGR values increase to ~15 cps at ~25 mbsf, near the lithostratigraphic Unit II/Subunit IIIa boundary, continuing at approximately this level downcore to the lithostratigraphic Unit IV/V boundary. High NGR values are interpreted as increased clay-sized sediment in lithostratigraphic Units III and IV (see “Lithostratigraphy”). NGR values drop to near zero at ~95 mbsf at the boundary of lithostratigraphic Units IV and V. NGR values are low (<5 cps) in lithostratigraphic Unit VI from ~115 to ~145 mbsf, which is interpreted to result from increased silt and sand contents in this unit (see “Lithostratigraphy”). At ~145 mbsf, near the boundary of lithostratigraphic Units VI and VII, NGR values increase to ~15 cps. NGR is highly variable deeper than this throughout lithostratigraphic Unit VII.

Shipboard magnetic susceptibility

Magnetic susceptibility exhibits slightly increased values in lithostratigraphic Units I and II but near-zero values within all of lithostratigraphic Unit III (Fig. F14). lithostratigraphic Unit IV is characterized by higher and more variable values; this is also evident in the discrete values measured on cubes during the OSP (see “Paleomagnetism”). The synchronous increase in dry density suggests a lithologic change is reflected in the physical property measurements (see “Lithostratigraphy”). Below the break in recovery, from ~117 mbsf in lithostratigraphic Unit VI, excursions in magnetic susceptibility exhibit the highest magnitude in Hole M0060A. In lithostratigraphic Unit VII, values return to near zero.

Density

Discrete dry density values generally increase with depth and do not exhibit changes that correspond to lithologic boundaries (Fig. F14).

Gamma density was measured at 2 cm intervals during the offshore phase of Expedition 347. Values are not well correlated with the discrete bulk density measurements performed during the OSP (Fig. F15), either above the break in recovery (0–100 mbsf; r² = 0.28) or below (~117–200 mbsf; r² = 0.19), but exhibit an overall similar trend.

Paleomagnetism

To achieve the main objectives of the OSP paleomagnetic work, magnetic susceptibility measurements and rudimentary 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]). The discrete samples were taken from Hole M0060A, with a gap in sampling between 116 and 101 mbsf in the mid–lower sections of Unit V because of poor core recovery. Magnetic susceptibility (χ) ranged between 0.1 × 10^–6 and 7 × 10^–6 m³/kg through the sequence, with values >2 × 10^–6 m³/kg confined to Unit VI, an interval of fine to medium massive well-sorted sand. Elevated χ values were also observed in an interval between 87 and 83 mbsf in Unit IV, most of Subunit IIIb, and the upper 22.23 mbsf of the hole (Units II and I). The stability of paleomagnetic pilot samples recovered from silt- and sand-rich intervals between the bottom of the hole (232.5 mbsf) and the top of Unit VI (116.7 mbsf) was poor, and this group of samples has very scattered inclinations with an average close to 0°. The majority of samples in Units III, II, and I carried normal polarity NRMs, with inclinations up to 30° shallower than a geocentric axial dipole (GAD) prediction. Samples with relatively high χ taken from Unit III and in particular Subunit IIIa acquired gyroremanent magnetization (GRM) during alternating field (AF) demagnetization above 60 mT, and these samples are associated with initial inclinations that approach the GAD prediction of 71°.

Discrete sample measurements

A total of 297 discrete samples were obtained from Hole M0060A. 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 F16. Magnetic susceptibility (χ), which was normalized to sample mass, ranges between 0.1 × 10^–6 and 7 × 10^–6 m³/kg. Samples taken from Unit VII have χ values <0.5 × 10^–6 m³/kg. Overlying Unit VI has high χ values that exceed 2 × 10^–6 m³/kg in distinct intervals close to the bottom and top. The χ of Units IV and III is variable and includes one interval in Unit IV (87–83 mbsf) and one in Subunit IIIb (66–57.5 meters composite depth [mcd]) in which the values exceed 1 × 10^–6 m³/kg. One sample (347-M0060A-18H-1, 61–63 cm) in Subunit IIIa has an anomalously high χ value; otherwise the values are <0.2 × 10^–6 m³/kg and constant in the interval between 57.6 and 22.3 mcd. Unit II displays χ values that reach 0.7 × 10^–6 m³/kg, which trends upcore toward lower values that are characteristic of Unit I.

Sediment wet density and χ are not related to each other, although χ ranges over half an order of magnitude, which suggests that changes of 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 (Fig. F16). The samples from Subunit IIIb have high χ/NRM ratios.

Natural remanent magnetization and its stability

Results of the pilot sample demagnetization (Fig. F17) indicate that an AF of 5 mT is sufficient to remove a weak viscous remanent magnetization. Three different responses to the sequential AF demagnetization are displayed by samples from Site M0060. Category 1 includes the samples from the relatively coarse grained Units VII and VI, and they lose 50% of their NRM intensity at alternating fields <20 mT, with a small residual component left at 30 mT that is unaffected by more intense demagnetization levels. Category 2, which includes all other samples except those in Subunit IIIb and the interval 87–83 mbsf, 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 more complex behavior, which is characterized by the removal of a significant viscous remanence at the 5 mT demagnetization level. These samples subsequently acquired a GRM at field levels above 60 mT, with the vector moving into a plane perpendicular to the last demagnetization axis.

After removal of the viscous overprint, the NRM intensity of the samples recovered from Site M0060 lies in the range between 0.07 × 10^–3 and ~250 × 10^–3 A/m, with one outlier showing >500 × 10^–3 A/m, and displays a general positive relationship with χ (Fig. F16). The relationship between NRM intensity and χ is, however, less obvious in Subunit IIIb, in which large peaks in NRM intensity are not reflected in the χ data.

Paleomagnetic directions

The directions of the paleomagnetic vectors are illustrated by the inclination data in Figure F16. The inclination data from Units VII, VI, and I are scattered, with an approximately equal number of positive and negative values. Only a few samples from these three units approach the GAD prediction for this site location. In contrast, the inclination data from Units IV and III group closer to the GAD prediction, but there is a bias toward lower values and some data points are negative or close to zero. It is notable that the samples taken from Unit IV and Subunit IIIb, which have high χ values and high NRM/χ, plot within a few degrees of the GAD prediction. 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. In particular, pilot samples that have high NRM/χ and 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 timing between sediment deposition and greigite precipitation is unknown and, therefore, the ability to use the paleomagnetic data at Site M0060 for relative dating purposes is very restricted.

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