DATA RESULTS

Susceptibility

Magnetic susceptibility measured in discrete samples from Hole 1006A indicated a combination of weak positive susceptibility and diamagnetism (Fig. 1; Table 1). The core susceptibility profile clearly shows several cycles of variation. Mixed positive and diamagnetic responses characterize the upper ~100 m, from mostly diamagnetic between ~100 and ~300 meters below seafloor (mbsf) to two positive intervals and one diamagnetic interval in the bottom 300 m (~300-600 mbsf) of the core. The interval between ~540 and ~560 mbsf is marked by the highest positive susceptibilities (up to +2), besides the upper few meters of the core (Fig. 1).

Magnetic Remanence and Sample Reliability

Shore-based discrete sample analyses indicate that the carbonate sediments in Hole 1006A exhibit weak to moderate magnetic moments (Fig. 2; Table 2). Mean intensity for the 778 samples is 9.47 × 10-8 Am2/kg (median = 6.04 × 10-8 Am2/kg). Similar to the susceptibility profile, magnetic intensity at the NRM level exhibits distinct intervals of variation, including considerable difference between adjacent samples (Fig. 2). These variations in magnetic intensity are evident in both the scatter plot and the three-point smoothing (Fig. 2). The weakest remanent magnetization (~5 × 10-8 Am2/kg) is found in the interval between ~50 to ~200 mbsf. Below ~200 mbsf, the intensity is generally around 1 × 10-7 Am2/kg with two slightly stronger intervals (Fig. 2). A histogram of intensity distribution within the range of 1 × 10-8 and 1 × 10-7 Am2/kg shows a widely scattered distribution (Fig. 3A). The distribution of samples between 1 × 10-7 and 1 × 10-6 Am2/kg shows a rapid decline in the number of samples in the stronger intensity classes (Fig. 3B). Samples with NRM intensity of <5 × 10-8 Am2/kg were usually deemed either Class B or Class C samples. Class A (N = 106, 13.6%) and Class B (N = 102, 13.1%) samples comprised 27.7% of the 778 total samples and are tentatively judged as reliable.

AF demagnetization was responsible for the almost complete decay of magnetic remanence. Usually only 10% of the original remanence remained after AF fields of 25 to 30 mT (Fig. 4). Median destructive field for most samples was in the range of 5-15 mT. This relatively "soft" remanence is often characteristic of magnetite with comparatively low magnetostatic interaction and/or magnetite that has undergone partial oxidation to maghemite (McNeill, 1997).

A significantly steep (>70°) inclination angle was measured at the NRM level in many samples below ~200 mbsf. This steep inclination greatly exceeds the axial dipole value (43°) expected for the site latitude of ~25°N. We interpret this steep inclination as some type of drilling/coring overprint or remagnetization(?), as the drill pipe can produce a magnetic field with very steep inclination. AF demagnetization was not able to effectively remove the steep angles in many of the samples; as a result, they were classified as the Class C type because of the uncertainty in reliability. Although not quantified, many of the samples with higher intensities did not exhibit the steep inclination found in the more moderate to weak samples.

Rock Magnetics

Isothermal remanent magnetization acquisition was performed on selected samples from Hole 1006A (Fig. 5). Most of the samples show acquisition patterns and characteristics (Fig. 5A) similar to those of single-domain magnetite. Of the seven IRM samples, most reached saturation before or near 100 mT (Fig. 5A) and have ratios from IRM acquisition of near 1 (within the single-domain magnetite field on Fig. 5B).

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