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Results and discussion

Downcore variations of hysteresis parameters are shown in Figure F1. Hysteresis parameters for Site U1378 are shown in Table T1 and for Site U1379 in Table T2. The variations of Ms closely match that of magnetic susceptibility measured on ship (see the “Site U1378” and “Site U1379” chapters [Expedition 334 Scientists, 2012b, 2012c]), showing sporadic occurrence of layers with high values. Mrs/Ms slightly increases with depth with considerable scatter. Samples 334-U1379C-49X-3W, 132–134 cm, and 334-U1479C-50X-1W, 1341–136 cm, show exceptionally high Mrs/Ms. These samples also show elevated coercivity, indicating the presence of fine-grained magnetic minerals. Hcr/Hc of Site U1379 slightly decreases with depth, except for the high value of Sample 334-U1379C-6H-4W, 100–102 cm, and low values of the samples mentioned above. Hcr/Hc of Site U1378 does not show a noticeable trend with depth.

The interrelationships among hysteresis parameters were analyzed using Day plots (Day et al., 1977; Dunlop, 2002), as shown in Figure F2. Most samples plot in a pseudosingle-domain (PSD) region, except for Samples 334-U1379C-49X-3W, 132–134 cm, and 334-U1479C-50X-1W, 134–136 cm, which plot in a single-domain (SD) region. Samples exhibit departure from the theoretical mixing line between SD and coarse-grained, multidomain (MD) titanomagnetite, indicating the presence of superparamagnetic (SP) grains and/or multiple magnetic minerals with distinct coercivities (Tauxe et al., 1996; Dunlop, 2002).

For Site U1378, the departure from the SD-MD mixing line of titanomagnetite observed in Day plots shows some correlation with depth (Fig. F2A), where samples from shallow depths tend to lie close to the SD-MD mixing line. There are exceptions; samples from above 100 meters below seafloor (mbsf) depart from the mixing line, and some samples from 300–400 mbsf plot near to the mixing line. Ms values more closely correlate with the departure from the SD-MD mixing line, where high Ms samples plot close to the mixing line (Fig. F2C). The same is true for Site U1379 (Figs. F2B, F2C). For Site U1379, samples from the uppermost 100 m plot toward the coarse end of the PSD region. The dominant control of Ms on the Day plot parameters (Mrs/Ms and Hcr/Hc) is clearly seen by plotting these parameters against Ms (Fig. F3). For both sites, Mrs/Ms is systematically low for samples with Ms > 0.05 Am2/kg, whereas Hcr/Hc is not clearly related to Ms, resulting in a departure from the SD-MD mixing line in low Ms samples.

FORC measurements were conducted on samples from Site U1379 only and are shown in FORC diagrams (Roberts et al., 2000) (Fig. F4). The horizontal axis of a FORC diagram (Hc) represents coercivity, and the vertical axis (Hu) reflects magnetostatic interactions (e.g., Muxworthy and Williams, 2005). Some samples show FORC distributions with low Hc peak and spread along Hu, whereas others show broad Hc distribution extended to high coercivity with smaller spread in Hu. Although sample numbers are limited, the latter seems to correspond to samples that plot toward the finer (SD) side in Day plots.

Thermomagnetic experiments were conducted for two samples, 334-U1379C-13H-4W, 34–36 cm, and 334-U1379C-57X-5W, 111–113 cm (Fig. F5). These samples show Ms values of 8.67 mAm2/kg and 151 mAm2/kg, respectively. These samples were chosen to represent low Ms and high Ms samples, respectively. Sample 334-U1379C-13H-4W, 34–36 cm (low Ms), exhibits an increase in χ upon heating from ~375°C followed by a decrease to zero at ~580°C. The cooling curve is more elevated than the heating curve. Sample 334-U1379C-57X-5W, 111–113 cm (high Ms), shows an increase in χ upon heating at around –160°C to –150°C, indicating the presence of the Verwey transition of low-Ti titanomagnetite (Verwey, 1939). The heating curve of the sample suggests the presence of the magnetic phase with a Curie temperature of ~580°C, also indicating the presence of magnetite. The cooling curve is more elevated than the heating curve, but the change is smaller than that in the other sample.

The thermomagnetic curves of the high Ms sample and the hysteresis parameters lying on the SD-MD mixing line suggest that the main magnetic phase in samples with high Ms is low-Ti titanomagnetite. The magnetic phases in samples with low Ms are not clearly resolved, but the departure of hysteresis parameters from the SD-MD mixing line and the absence of the Verwey transition indicate that they contain magnetic minerals other than low-Ti titanomagnetite. Identification of magnetic minerals in the low Ms samples will be crucial for future paleomagnetic studies.