Previous   |    Next

doi:10.2204/iodp.proc.334.203.2015

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 M_s 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. M_rs/M_s 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 M_rs/M_s. These samples also show elevated coercivity, indicating the presence of fine-grained magnetic minerals. H_cr/H_c 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. H_cr/H_c 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. M_s values more closely correlate with the departure from the SD-MD mixing line, where high M_s 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 M_s on the Day plot parameters (M_rs/M_s and H_cr/H_c) is clearly seen by plotting these parameters against M_s (Fig. F3). For both sites, M_rs/M_s is systematically low for samples with M_s > 0.05 Am²/kg, whereas H_cr/H_c is not clearly related to M_s, resulting in a departure from the SD-MD mixing line in low M_s 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 (H_c) represents coercivity, and the vertical axis (H_u) reflects magnetostatic interactions (e.g., Muxworthy and Williams, 2005). Some samples show FORC distributions with low H_c peak and spread along H_u, whereas others show broad H_c distribution extended to high coercivity with smaller spread in H_u. 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 M_s values of 8.67 mAm²/kg and 151 mAm²/kg, respectively. These samples were chosen to represent low M_s and high M_s samples, respectively. Sample 334-U1379C-13H-4W, 34–36 cm (low M_s), 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 M_s), 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 M_s sample and the hysteresis parameters lying on the SD-MD mixing line suggest that the main magnetic phase in samples with high M_s is low-Ti titanomagnetite. The magnetic phases in samples with low M_s 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 M_s samples will be crucial for future paleomagnetic studies.

Top of page   |    Previous   |    Next