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doi:10.2204/iodp.proc.344.206.2015

Methods

Magnetic hysteresis

Magnetic hysteresis properties of samples were measured with an alternating gradient magnetometer (Princeton Measurements Corporation MicroMag 2900) at the Paleomagnetism Laboratory of the University of California at Davis (UC Davis; USA). We selected a range of lithologies that correspond to the major lithostratigraphic units identified during the expedition (Expedition 334 Scientists, 2012b; see the “Input Site U1381,” “Input Site U1414,” “Mid-slope Site U1380,” and “Upper slope Site U1413” chapters [Harris et al., 2013b, 2013c, 2013e, 2013f]). All hysteresis measurements presented in this study were made on basalt chips or sediment specimens taken from the working halves of core sections (see the “Methods” chapter [Harris et al., 2013d] for curatorial procedures used on board the JOIDES Resolution).

Magnetic hysteresis properties are presented using Day plots (Day et al., 1977) and FORC diagrams (Pike et al., 1999; Roberts et al., 2000). Day plots display the ratio of saturation remanent magnetization to saturation magnetization (M_rs/M_s) versus the ratio of coercivity of remanence to coercive force (H_cr/H_c) and can be used to characterize the bulk magnetic assemblage of particles as being single-domain (SD), pseudosingle-domain (PSD), or multidomain (MD), as defined by Dunlop (2002). However, the combination of different mineral compositions and grain sizes and the interactions among magnetic grains can lead to results that are difficult to interpret.

FORC diagrams can be used to discriminate the coercivities of magnetic grains and their interactions. A FORC diagram is a contour plot of H_a versus H_c, where H_a represents the applied magnetic field interacting with the sample and H_c represents the coercivity of the magnetic particles in the sample (Pike et al., 1999). The shapes of the coercivity and interaction distributions are indicative of magnetic properties and can aid in determining the types of magnetic minerals present in the samples. For example, a population of noninteracting SD grains with a single coercivity (H1) would be represented by contours with small vertical and horizontal range around H_a = 0, H_c = H1. Contours that are elongated narrowly along the H_a = 0 axis indicate the presence of populations of noninteracting SD grains with varying coercivities, whereas contours with a larger vertical span indicate interactions between particles or within particles, such as within PSD or MD grains. Interactions between multiple populations of SD, PSD, and/or MD grains can produce much larger vertical spread and sometimes lobes and valleys on the FORC diagrams. For the diagrams presented here, we used a minimum of 120 curves to construct each FORC diagram.

Natural remanent magnetization

The NRMs of a few basalt samples were measured to investigate their demagnetization behavior. Cubes with volumes of 1.0–1.9 cm³ and masses of 2.6–5.3 g were cut out of the working halves of core sections and come from the same intervals as the basalt chips used for the hysteresis measurements. The cube samples were step-wise, progressively demagnetized in an alternating field (AF) up to peak fields of 60 mT, with one sample demagnetized up to 100 mT. The magnetization direction and intensity was measured after each demagnetization step with a 2G Enterprises 755R cryogenic magnetometer. The magnetic susceptibility of the cubes was measured before the samples were demagnetized.

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