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doi:10.2204/iodp.proc.314315316.134.2009

Paleomagnetism

We made pass-through magnetometer measurements on all split-core archive sections at 5 cm intervals. In order to isolate characteristic remanent magnetization (ChRM), we subjected the cores to alternating-field (AF) demagnetization. The archive-half cores were typically demagnetized to 40 mT. Before measurement data were uploaded to the J-CORES database, we excluded the top and bottom ~15 cm to avoid end-core edge effects (response of the magnetometer’s pickup coils is ~20 cm) as well as those for voids caused by whole-round sampling. In order to test the archive-half core data and identify magnetic carriers, we demagnetized a total of 102 discrete samples using both AF and thermal techniques.

Natural remanent magnetization and magnetic susceptibility

Paleomagnetic data obtained at Site C0006 during Expedition 316 exhibit significant variations in demagnetization behavior among various recovered lithologies. Drilling-induced remagnetization exists in the recovered cores (Figs. F32, F33, F34), but it had less effect compared with those in Hole C0004C sediments. Interestingly, drilling-induced remagnetization in a few cores is of opposite polarity (see Figs. F32, F33B); perhaps a different type of core barrel was used to core these sediments. In almost all cases, we were able to remove drilling-induced remagnetization with 20–30 mT AF demagnetization and to isolate ChRM direction using higher fields.

The most significant variations in natural remanent magnetization (NRM) intensity and susceptibility for whole cores from Site C0006 are well correlated with lithology. Paleomagnetic measurements indicate that mudstones in Unit III have the lowest NRM intensity (averaging ~0.1 mA/m) compared to those in turbidite Unit II (~12 mA/m) and nannofossil-bearing mud and sand in Unit I (~2 mA/m). Variations in magnetic susceptibility generally correlate with variations in NRM intensity (Fig. F32). Magnetic susceptibility values are generally ~19 × 10^–3 SI for sediments in Unit I, >200 × 10^–3 SI for Unit II, and 12 × 10^–3 SI for Unit III. A few discrete peaks of higher NRM and susceptibility values appear in some depth intervals in Units I and II (e.g., around 18, 27, 161, 231, and 326 m CSF; see Fig. F32) and visibly correlate with the presence of volcanic ash in these regions (see “Lithology”).

Paleomagnetic stability tests and core orientation

Remanent magnetization of discrete samples was investigated using stepwise AF or thermal demagnetization (Fig. F33). In most cases, the steep downward component of magnetization imparted by the coring process is easily removed by AF demagnetization. Thermal demagnetization also successfully removed this drilling-induced component (Fig. F34). Most samples show unblocking temperatures between 200° and 400°C, indicating that titanomagnetites are likely the main magnetic carriers in these samples. A histogram of inclinations isolated from discrete samples is shown in Figure F35. Inclinations from archive-half core pieces at <100 m CSF are concentrated at approximately ±45° (Fig. F32), similar to the expected dipole inclination at the site (52°). A pronounced shallowing of inclinations is evident in the interval from ~120 to 380 m CSF, where inclination values are more typically shallower than 30°. This interval corresponds to the observed zones of faulting and fracturing (see “Structural geology”), suggesting that inclination values may have been affected by faulting and tilting. Results from discrete sample demagnetization also provide an opportunity to evaluate the accuracy of archive-half core remanence data that are used (in combination with discrete sample results) to reorient core pieces to a common geographic framework. This paleomagnetic core reorientation method has been successfully used for both continental and oceanic outcrops (e.g., Fuller, 1969; Kodama, 1984; Shibuya et al., 1991). Assuming the direction of stable remanent magnetization (either viscous remanent magnetization or primary magnetization) with respect to a common reference line that is scribed the length of the core represents the expected magnetic direction at the site, the orientation of paleomagnetic ChRM, which specifies the rotation of the core relative to the geographic coordinates, is then used to restore the core azimuth. For intervals of particular interest for structural geology at Site C0006, we used either the stable ChRM isolated from progressive demagnetization of discrete samples and small homogeneous segments (that contain structural features) or an average declination from 3–4 consecutive measuring intervals with the same inclination sign from archive-half core data. The combined data sets suggest that deformation bands in cored units generally strike southwest and dip northwest or southeast (See “Structural geology”).

Magnetostratigraphy

We used ChRM inclinations from both pass-through and discrete measurements to define magnetic polarity sequences for Site C0006. As at Site C0004, magnetic polarity sequences at Site C0006 could not be completed because of incomplete recovery and whole-round sampling of archive-half sections. Faulting also affects this site and complicates magnetostratigraphy. Nevertheless, a few magnetic reversals discerned on the basis of changes in sign of inclinations can be identified, which constitute partial magnetostratigraphic records at Site C0006. Biostratigraphic samples between 5 and 10 m CSF in Hole C0006D have been assigned ages of ~0.43 Ma. Thus, the observed predominantly negative inclination in this interval suggests that these sediments were deposited within the Emperor event (0.42 Ma), a short reversed polarity event within the upper part of the Brunhes normal chron (Gradstein et al., 2004). This event is likely recorded in Holes C0006C and C0006E, each ~2.5 m from Hole C0006D, and at Site C0004 (~20 km northwest of Site C0006). If confirmed by further shore-based study, this record could serve as a chronostratigraphic marker to correlate sedimentary layers in the Nankai Trough region.

On the basis of shipboard micropaleontological data, the uppermost 80 m of sediment in Hole C0006E has an age in the range of 0.8–1.1 Ma. The Brunhes/​Matuyama Chron boundary (0.78 Ma) is placed at 32.75 m CSF as magnetic inclination at this depth changes polarity from normal to reversed (Fig. F36). This match is definite because the Matuyama is the only reversed chron of this particular age. Between 145 and 192 m CSF, cores have been assigned to ages ranging from 1.6 to 1.67 Ma (see “Biostratigraphy”). Therefore, the shift in polarity from reversed to normal at 160 m CSF would correspond to the onset of the Gilsa Subchron (1.68 Ma), and the upper part of Hole C0006E contains only a partial record of the Matuyama Chron and no record of the Jaramillo Subchron (0.99–1.07 Ma).

Biostratigraphic data suggest that sediments between 200 and 410 m CSF may be 0.8–1.1 Ma in age. This information suggests that the relatively well defined record of a reversed interval between 250 and 345 m CSF and the shift in polarity from reversed to normal at ~345 m CSF represent the upper part of the Matuyama Chron and the onset of the Jaramillo normal polarity subchron and infer a relatively rapid sediment accumulation rate (~180 m/m.y.) for the early Pleistocene in Holes C0006E and C0006F. Alternatively, the thickness of the strata could also be due to thrust faulting in the region.

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