Previous   |    Next

doi:10.2204/iodp.proc.314315316.135.2009

Paleomagnetism

Pass-through magnetometer measurements on all split-core archive sections were made at 5 cm intervals. Archive half cores were demagnetized to 40 mT using alternating-field (AF) demagnetization. Before the measurement data were uploaded to the J-CORES database, we excluded data from voids and from the top and bottom ~15 cm to avoid end-core edge effects. We demagnetized a total of 88 discrete samples using both AF and thermal demagnetization techniques in order to test the half-core data and to identify magnetic carriers.

Natural remanent magnetization and magnetic susceptibility

Downhole variations of paleomagnetic data obtained at Site C0007 are shown in Figures F35 and F36. As in previous sites, drilling-induced remagnetization exists in the recovered cores (Figs. F35, F36, F37). However, we noticed that in cores drilled using a nonrotary core barrel, especially in those drilled using the ESCS (i.e., Cores 316-C0007B-5X though 15X, 43.00–138.18 m CSF), drilling has very little effect on magnetic declination but significantly affects magnetic inclination and natural remanent magnetization (NRM) intensity (Fig. F36). In almost all cases, we were able to remove drilling-induced remagnetization with 20 mT AF demagnetization and to isolate the characteristic remanent magnetization (ChRM) direction using higher fields.

Variations of magnetic properties among various lithologies are similar to those observed at Site C0006. In the nannofossil-bearing mud and sand cores in lithologic Unit I, both NRM intensity and magnetic susceptibility show a steady downhole increase with an average intensity of 5 mA/m and susceptibility value of 60 × 10^–3 SI units. Pleistocene aged sediments in Unit II have relatively high NRM intensity (average = 15 mA/m) and magnetic susceptibility (~190 × 10^–3 SI units). These are caused by the presence of numerous volcanic ash and sand layers throughout this unit, which have relatively high concentrations of magnetic minerals. Recovered cores in Unit III are hemipelagic mudstone and have the lowest NRM intensity (mean = ~0.25 mA/m) and magnetic susceptibility (mean = ~9 × 10^–3 SI units). Poor recovery of the underthrust trench wedge type sands and rocks in Unit IV limits paleomagnetic work, although a few pass-through whole-core magnetic susceptibility measurements give an indication for relatively high susceptibility values (>300 × 10^–3 SI units) (Fig. F36).

There are more significant variations in susceptibility within Unit II than Unit III. In particular, magnetic susceptibility records at both drilled locations in Site C0007 (i.e., Hole C0007C at ~159 m CSF and Hole C0007D at ~190 m CSF) show a similar discrete trough of lower susceptibility values, confirming the accuracy of the subunit boundaries defined by shipboard sedimentologists and also suggesting that this magnetic susceptibility trough can be used for stratigraphic correlation.

Discrete samples and core orientation

Remanent magnetization of discrete samples were investigated using stepwise AF or thermal demagnetization (Fig. F37). The steep downward component of magnetization imparted by the coring process can be removed by both demagnetization techniques, but thermal demagnetization appears to be more effective in removing this drilling-induced component (Fig. F37). 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 the 88 discrete samples is shown in Figure F38. Inclinations from these discrete samples are mostly concentrated at 52° (Fig. F38), suggesting these samples maintain an inclination close to the theoretically predicted value for the latitude of this site (52°) and indicating they may represent the primary ChRM. As with previous sites, results from discrete sample demagnetization in combination with the pass-through sample results were used to reorient core pieces to a common geographic framework. 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 measurement intervals with the same inclination sign from the archive half core data.

Magnetostratigraphy

We used ChRM inclinations from both pass-through and discrete measurements to define magnetic polarity sequences for Site C0007 (Fig. F39). Faulting and incomplete recovery affect and complicate the magnetostratigraphy at this site. Nevertheless, a few magnetic reversals identified on the basis of changes in sign of inclinations can be identified. Because biostratigraphic samples in the depth interval between 5 and 20 m CSF in Hole C0007C have been assigned ages ~0.43 Ma, the observed negative inclinations between 10.81 and 13.92 m CSF (Fig. F35) suggest that these sediments were deposited within the Emperor event (0.42 Ma), a short reversal within the upper part of the Brunhes normal chron according to the Vostok ice core data (Gradstein et al., 2004).

Shipboard biostratigraphic data suggest that sediments between 200 and 320 m CSF in Hole C0007D are 0.8–1.1 Ma. This information suggests that the relatively well defined record of change in polarity from normal to reversed at ~210 m CSF represents the Brunhes/​Matuyama Chron boundary (0.78 Ma), and the shift in polarity from reversed to normal at ~330 m CSF represents the end of the Jaramillo normal polarity subchron (1.07 Ma). Below 360 m CSF, a dominantly normal polarity sequence extends to at least 410 m CSF (Fig. F39). This normal polarity sequence appears to correspond to the Gauss normal chron as nannofossil biostratigraphic Zones NN15/NN14 and NN13 are also placed in this depth interval (see “Biostratigraphy”). At least three thrust faults are inferred in this depth interval (Fig. F39) (see “Structural geology”), preventing more precise reconstruction of the magnetostratigraphy.

Top of page   |    Previous   |    Next