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doi:10.2204/iodp.proc.314315316.215.2011

Results

Paleomagnetism

One of the major experimental requirements in paleomagnetic research is to isolate the ChRM by selective removal of secondary magnetization. AF and thermal demagnetization of discrete samples revealed that samples have two magnetic components, a low-coercivity drilling overprint, which is removed after 10 to 40 mT or 400°C demagnetization, and the ChRM. Several representative examples are shown in Figure F7. It appears that AF demagnetization is more effective than the thermal procedure in isolating the ChRM. Although the amount and coercivity of overprinting varied, most of it seems to be removed with 20–35 mT AF demagnetization for the majority of samples, allowing us to isolate the ChRM direction using principal component analysis on higher field demagnetization steps (Table T1). Inclination values of ChRM are moderate downward or upward, roughly consistent with the expected inclination for the drill sites (expected inclination at drill sites = ±52°). Thus, sedimentary cores from Nankai Trough have recorded a stable component of magnetization with both normal and reversed inclinations (Table T1). A number of samples, however, have inclinations that do not resemble the time-averaged geomagnetic field. At several depth intervals in Site C0004, for example, remanent inclinations are consistently shallower (±20°–30°). Nevertheless, we note that the negative ChRM inclinations are not randomly distributed throughout the sequence but are confined to certain zones. Thus, the negative inclinations in these core sections lend support to the notion that the ChRM in these core sections may indicate a reversed polarity.

When compared to shipboard data, our results show that there is a discrepancy between mean inclinations calculated using the discrete samples and long-core measurements. Compared with corresponding discrete samples, the steeper inclination values observed in the long-core measurements may be due to an overprint that has not been completely removed.

Rock magnetism

Curie temperature determination of samples

Curie temperature determinations of samples from Expedition 316 sites are presented in Table T2. According to Curie temperatures, three different groups can be recognized. Group 1 is characterized by a single ferromagnetic phase with Curie temperatures between 570° and 590°C, compatible with that of Ti-poor titanomagnetites (five samples in Table T2). The cooling and heating curves are reasonably reversible (Fig. F8A). Group 2 has multiple magnetic phases. The thermomagnetic curves display one magnetic phase with Curie temperatures around 280°–390°C on heating, most likely titanomaghemite. The second high–Curie temperature phase is observed around 580°–600°C (Fig. F8B). The large difference between heating and cooling of the sample suggests a low-temperature oxidized titanomagnetite as the main magnetic mineral. Group 3 also has multiple magnetic phases. The irreversible thermomagnetic curve of this type displays one magnetic phase with Curie temperature <200°C on heating (Fig. F8C). The second high Curie temperature phase is observed around 570°C (five samples in Table T2).

Comparison of the two Curie temperatures (obtained by low-field continuous susceptibility and high-field magnetic moment versus temperature runs) on Sample 316-C0004D-30R-1, 124–126 cm (Table T2), reveals that Curie temperatures determined by susceptibility as a function of temperature are less than those determined by high-field magnetic moment runs (Table T2). It is not clear whether this is due to nonhomogeneous subsamples used in the two instruments.

Hysteresis loop parameters

Samples analyzed in this study indicate that rock samples from Expedition 316 sites show a predominance of multidomain domain size (Table T2). Only a few samples exhibit pseudosingle domain size, probably indicating a mixture of mutidomain and a significant amount of single-domain grains. The low values of J_r/J_s in Table T2 probably indicate the presence of ultra-small superparamagnetic grains as well as large multidomain grains (Dunlop and Özdemir (1997). We plotted examples of a room-temperature hysteresis loop for representative samples that exhibit pseudosingle domain and multidomain behavior (Fig. F9). Hysteresis experiments also indicate both diamagnetic and paramagnetic influence.

Low-temperature properties

The study of low–temperature phase transition in magnetic minerals has been increasingly utilized in rock magnetism (Banerjee, 1992; Moskowitz et al., 1998; Özdemir et al., 1993, 2002; Cui et al., 1994; Kosterov, 2001a, 2001b, 2002; Smirnov and Tarduno, 2002). Low-temperature measurements were made on representative samples to help characterize magnetic minerals and understand their rock magnetic properties. As shown in Figure F10, the low-temperature curves of SIRM both in zero-field warming and cooling display a variety of features. These include an unblocking temperature in the vicinity of 40–50 K, most likely caused by superparamagnetic magnetite particles (Moskowitz et al., 1993), and a decrease in remanence in the 100–120 K range (Fig. F10), most likely caused by the Verwey transition (Verwey et al., 1947). Figure F10B shows cooling and warming curves for Sample 316-C0006E-44X-6W, 24–26 cm, which has multidomain grain size. Remanence is lost at ~120–130 K as the sample both cools and warms through the Verwey transition. On the other hand, no obvious Verwey transition is observed for Sample 316-C0004D-28R-3W, 7–9 cm, during cooling to 10 K. Upon warming from 10 K, however, a tiny bend in remanence occurs near 120 K (Fig. F10D). According to Moskowitz et al. (1998), titanomagnetites with a high Ti replacement level (x > 0.04) could not exhibit the Verwey temperature.

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