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doi:10.2204/iodp.proc.314315316.215.2011

Methods and materials

The paleomagnetic and rock magnetic data presented in this paper are from measurements performed at the paleomagnetism laboratories at University of California at Santa Cruz (UCSC; USA) and at the Institute for Rock Magnetism of the University of Minnesota (USA). Progressive AF and thermal demagnetization experiments were carried out to identify the magnetic components and investigate the nature of the remanent magnetization. Thermal demagnetization was carried out with a UCSC-built oven with a residual field of ~7 nT. AF demagnetization was performed using the automatic degaussing system built into the 2G Enterprises cryogenic magnetometer. Changes in the intensity and direction of remanent magnetization vectors during demagnetization experiments were analyzed using orthogonal vector end-point projections (Zijderveld, 1967). Magnetic component directions were identified using principal component analysis (Kirschvink, 1980). Data were processed using software developed by Enkin (1994; gsc.nrcan.gc.ca/​sw/​paleo_e.php).

For rock magnetic characterization, samples were subjected to several magnetic measurements. These included

  1. Curie temperature determinations using both low and high applied fields (0.05 and 1 T, respectively);

  2. Hysteresis loop parameter measurement: saturation magnetization (Js), saturation remanence (Jr), coercivity (Hc), and remanent coercivity (Hcr); and

  3. Saturation isothermal remanent magnetization as a function of temperature (10–300 K).

Curie temperatures were determined by measurement of low-field magnetic susceptibility or induced moment versus temperature (using both the Kappabridge susceptometer at UCSC and the Princeton MicroMag vibrating sample magnetometer at the University of Minnesota). To avoid oxidation that could lead to chemical alteration, we conducted thermomagnetic analyses in an inert helium or argon atmosphere. We used a graphic method (Grommé et al., 1969) to determine the Curie temperature that uses the intersection of two tangents to the thermomagnetic curve that bounds the Curie temperature. This method is most straightforward when done by hand, even though it tends to underestimate Curie temperatures with the two other methods presented by Moskowitz (1981) and Tauxe (1998). Hysteresis parameters were determined on a Princeton Measurements corporation Alternating Gradient Force Magneotmeter (MicroMag) at UCSC using 50–100 mg rock chips. Low-temperature measurements were made from 10 K to room temperature on 100–300 mg subsamples in a Quantum Design magnetic property measurement system (MPMS) at the University of Minnesota. Samples were given a saturation isothermal remanent magnetization (SIRM) in a steady magnetic field of 2.5 T at room temperature (300 K) and then cooled in a zero field to 10 K; the remanence was measured at 5 K intervals. The sample was then given a SIRM in a field of 2.5 T before warming it to 300 K in zero field while measuring the remanence value every 5 K in sweep measurement fashion. Unlike in high-temperature measurements, there is no risk of oxidation of a sample since it is at low temperature and is not heated.

A total of 606 discrete paleomagnetic samples were used for shore-based magnetic studies. These 2.5 cm cylindrical samples were drilled from the core sections that contained long pieces, generally taken from the least deformed parts. In all cases, the uphole direction was recorded on the sample with an orientation arrow before removal from the core section. All samples were kept in a low-field environment (field-free room) to prevent viscous remanence acquisition.