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Anisotropy of magnetic susceptibility and paleomagnetism

The AMS analysis was conducted using a Kappabridge KLY 4 magnetic anisotropy susceptometer (AGICO Co. Ltd., Czech Rep.) in sediments with intercalated turbiditic sand and detrital carbonate layers.

AMS is demonstrated by the orientation of magnetic particles (Tarling and Hrouda, 1993). It can be represented by an ellipsoid defined by three principal axes: maximum (Kmax), intermediate (Kint), and minimum (Kmin) susceptibility. Hereafter, the AMS ellipsoid will be used throughout the paper. The AMS parameters magnetic lineation (L), magnetic foliation (F), anisotropy degree (P), corrected anisotropy degree (P′), and shape parameters T and q calculated from the three principal axes are reported (Tarling and Hrouda, 1993).

We measured AMS and paleomagnetism using the working and archive halves, respectively. A total of 68 samples from the siliciclastic sand layers and 12 from the detrital carbonate layers were selected for AMS measurements. The northern magnetized direction in the samples, which deviated during core sampling, was determined by paleomagnetic declinations and was used to correct the AMS orientations. To do this we used a 2G-Enterprise superconducting magnetometer (2G-Enterprises, CA, USA) available on the JOIDES Resolution. Declination data were obtained successively by applying 10 mT alternating-field demagnetizations (see the “Expedition 303 summary” and “Expedition 306 summary” chapters [Expedition 303 Scientists, 2006a; Expedition 306 Scientists, 2006]). Because the declination data are rotated gradually downward because of a twist during coring within one core, magnetic north was calculated using the least-squares method (Fig. F3). Magnetic north of the archive half cores was adjusted for working halves by adding 180°. A correction to magnetic north is applied by the following equation:

Kmax declination (corrected) =
Kmax declination – declination of paleomagnetism.

Magnetic hysteresis

Because all minerals in marine sediments contribute to AMS to various degrees, it is important to define the kind of mineral most responsible for the measured magnetic fabric. This is done by following a magnetic hysteresis analysis using a MicroMag AGM2900 Model (Princeton Co. Ltd., UK). The MicroMag measures susceptibility in high magnetic fields (Khf) of 500–900 G. Khf is generally a result of mainly paramagnetic minerals rather than ferrimagnetic minerals (Housen and Sato, 1995; Housen, 1997). Low-field susceptibility (Klf) in sediments measured at 0.04 mT using the KLY 4 can be subdivided into components that are contributed by both ferrimagnetic and paramagnetic minerals (Housen and Sato, 1995; Housen, 1997). The ratio of Khf/Klf is inversely proportional to the relative contribution of ferrimagnetic minerals to Klf (Housen and Sato, 1995; Housen, 1997).