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doi:10.2204/iodp.proc.308.209.2009

Material and methods

Magnetic techniques

A total of 1721 oriented cubic samples (6.2 cm³ volume) of wet bulk sediment were collected onboard the R/V JOIDES Resolution from the working core halves of all four recovered drill sites at an average spacing of 0.5 m. Holes U1319A, U1320A, and U1322B were sampled throughout the entire recovered depth ranges. The deepest Hole U1324B (612 meters below seafloor [mbsf]) was only sampled to 386 mbsf because consolidation prevented hand sampling at greater depths.

All samples were subject to systematic paleomagnetic analysis comprising stepwise AF demagnetization up to 100 mT peak field using the automated 2G super-conducting quantum interference device magnetometer at the University of Bremen, Germany. AF steps were increased in 5 mT intervals between 0 and 50 mT and 10 mT intervals between 60 and 100 mT. Orthogonal AF demagnetization in x-, y-, and z-directions was followed by measurement of the remaining remanent magnetization.

Samples from Hole U1324B were measured using the method described by Dankers and Zijderveld (1981) where magnetizations are determined after each orthogonal AF demagnetization in order to correct for the acquisition of GRM. This laboratory magnetization is known to form during AF demagnetization in an orientation perpendicular to the direction of the last applied AF (e.g., Dunlop and Özdemir, 1997). Although this measurement technique is three times more time-consuming than the standard routine, it allows the GRM and NRM signals to be separated.

Please note that the sample x-, y-, z-coordinate system of the University of Bremen magnetometer differs from the x_s-, y_s-, z_s-orientation of the JOIDES Resolution shipboard cryogenic magnetometer. This is important for the comparison of shipboard and laboratory data and can be easily reconciled taking the following definitions into account:

x = –y_s, y = x_s, and z = z_s.

In the IODP standard drilling procedure, one complete drill hole is composed of a certain number of individual drill cores (~10 m long). Because each core has a different azimuth orientation relative to the others, the observed NRM declinations were corrected by rotation of the entire cores (e.g., Channell et al., 2008) so that the mean declination is oriented to 0° as theoretically assumed based on a geocentric axial dipole (GAD) hypothesis for sediments with ages of the Brunhes Chron. Calculated total inclination and declination values for each site are given in Table T1. The mean inclination and declination of all individual drill cores are given in Table T2. In particular, the core-wise declination values may serve as a reorientation tool for other studies because they give the angle for which data from that core has to be rotated with respect to 0° to obtain the correct geographic orientation.

Scanning electron microscopy techniques

Samples for electron microscopic analyses of the magnetic carriers were obtained from extracts using a wet magnetic separation technique developed by von Dobeneck et al. (1987). Approximately 10 mL of sediment was dispersed in demineralized water by ultrasonic agitation using sodium polyphosphate [Na₄P₂O₇ · 10H₂O] as a peptizing agent. Magnetic extracts obtained after 48 h cycling in the extraction apparatus were washed three times with demineralized water to purify the magnetic grains from any remaining clay mineral coatings. Microscopic analyses were performed using a FEI XL30 SFEG scanning electron microscope (SEM) at 15 kV acceleration voltage and 1.5 nm beam size at the Center of Electron Microscopy Utrecht (EMU) at Utrecht University. A drop of the washed magnetic extract was applied onto a carbon sticker previously stuck on a standard Al stub. After evaporation of the extraction fluid, a thin carbon coating of a few nanometers was applied on the dispersed magnetic particles. Secondary electron and backscattered electron (BSE) imaging techniques were used for visualizations and energy dispersive X-ray spectroscopy (EDS) was performed to examine the elemental composition (Goldstein et al., 1981). The EDAX PhiRhoZ processing software was utilized to (semi)quantify the obtained elemental spectra.

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