Previous   |    Next

doi:10.2204/iodp.proc.314315316.136.2009

Paleomagnetism

By the end of Expedition 316, coring penetrated to ~329 m CSF in Hole C0008A and ~176 m CSF in Hole C0008C and recovered various sediments and rocks (see “Lithology”). At Site C0008, we continued to perform measurements of archive half core sections and progressive demagnetization measurements of discrete samples. Pass-through magnetometer measurements on split-core archive sections were made at 5 cm intervals. Archive-half cores were demagnetized to 40 mT using alternating-field (AF) demagnetization. A number of core sections that have distinctive flow-in structures were not measured or sampled. 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. In order to test accuracy of the half-core data and provide additional insight into remanence magnetization direction and carriers, we demagnetized a total of 80 discrete samples using AF (77) and thermal (3) demagnetization techniques. Because of the time limit at the end of the expedition, pass-through measurements were carried out with only two demagnetization steps of natural remanent magnetization (NRM) and 40 mT for the cores from Hole C0008C and their data were processed postcruise. For this reason, discrete sample measurements were only performed with samples from Hole C0008A.

Natural remanent magnetization and magnetic susceptibility

Downhole variations of paleomagnetic data obtained from Holes C0008A and C0008C are shown in Figures F18 and F19, respectively. In Hole C0008A, sedimentary cores recovered from 0 to 45 m CSF continue to be nannofossil-rich mud and sand layers and volcanic ashes. These sediments have low NRM intensity (mean = ~10 mA/m) and low magnetic susceptibility (mean = ~12 × 10^–3 SI units). Sediments between ~40 and 150 m CSF have a higher NRM intensity (mean = ~39 mA/m) and magnetic susceptibility (mean = ~21 × 10^–3 SI units). Cores between 160 and 270 m CSF have the strongest NRM intensity (mean = ~207 mA/m) and magnetic susceptibility (mean = ~119 × 10^–3 SI units). As shown in Figure F18, magnetic susceptibility (Fig. F18A) and NRM intensity variations (Fig. F18B, green) through sedimentary units are closely correlated. Two significant increases in magnetic susceptibility are present at ~45 and 165 m CSF. Interestingly, the stable inclinations (Fig. F18C, red crosses) also switch polarity from normal to reversed at ~165 m CSF, suggesting that characteristic susceptibility and NRM intensity response might be useful for identifying geomagnetic event boundaries. High and low susceptibility peaks throughout Hole C0008A range from 824 × 10^–3 to 5 × 10^–3 SI units. These variations in susceptibility are probably caused by variations in the magnetic mineral type or variations in the content of magnetic minerals in the observed volcanic ashes and silty clay with depth. Below 270 m CSF, poor recovery of the sand layers in lithologic Unit II limits magnetic characterization of this unit. A few pass-through whole-core magnetic susceptibility measurements give an indication of relatively higher susceptibility values (>350 × 10^–3 SI units) (Fig. F18A).

Two significant magnetic susceptibility increases can also be found in Hole C0008C at ~48 and 108 m CSF (Fig. F19A). Mean values are 13 × 10^–3 SI units (0–48 m CSF), 27 × 10^–3 SI units (48–108 m CSF), and 84 × 10^–3 SI units (108–176 m CSF), which are consistent with those values at corresponding depths in Hole C0008A. NRM intensity varies from 14 mA/m (0–48 m CSF) to 150 mA/m (48–108 m CSF) and 156 mA/m (108–176 m CSF) in Hole C0008C, which shows that the middle part does not agree with values in Hole C0008A. Although the second magnetic susceptibility increase in Hole C0008C appeared 40 m shallower than that in Hole C0008A, there are ~10 m thick characteristically low susceptibility zones (~10 × 10^–3 SI units) just above the increase. A gradual downward increase of inclination values starts from ~108 m CSF in harmony with the second increase of magnetic susceptibility and NRM intensity in Hole C0008A. This resemblance suggests that the boundary at 150 m CSF in Hole C0008A can correspond to the one at 108 m CSF in Hole C0008C.

Discrete samples and core orientation

Remanent magnetization of discrete samples was investigated using stepwise AF or thermal demagnetization (Figs. F20, F21). The steep downward component of magnetization imparted by the coring process can be removed by both demagnetization techniques, but AF demagnetization appears to be more effective in removing this drilling-induced component (Fig. F20). Thus, AF demagnetization was preferred over the thermal technique for sediments and rocks in Hole C0008A. We also noted that several samples from 100 m CSF display a higher stability of remanent magnetization (Fig. F20C), suggesting that the magnetization of these samples are carried by grains with high coercive force. The characteristic inclinations from 80 discrete samples are mostly concentrated at 52°, suggesting these samples maintain an inclination close to the expected dipole inclination for the latitude of this site (52°) and indicating they may represent the primary magnetization when the sediments were deposited.

At Site C0008, we also obtained representative paleomagnetic directions from several intervals for structural analysis and core orientation. The preliminary results suggest that overall paleomagnetic data are reasonably robust to provide information about the geographic north and serve as reference directions for structural studies (see “Structural geology”).

Magnetostratigraphy

We used stable inclinations from both pass-through and discrete measurements to define magnetic polarity sequences for Holes C0008A and C0008C (Fig. F22, F23). For Hole C0008A, both calcareous nannofossils and radiolarians allowed us to tentatively correlate certain parts of the magnetic polarity interval recorded in the sediments with the geomagnetic reversal timescale. Biostratigraphic samples at a mean depth of 17.0 m CSF have been assigned ages between 1.04 and 1.08 Ma; hence, the observed positive inclinations at this depth range (Fig. F22) suggest that these sediments were likely deposited within the Jaramillo Subchron (0.99–1.07 Ma; see Gradstein et al., 2004) instead of the Brunhes Chron. The polarity shift from normal to reversed at 24.8 m CSF (in Section 316-C0008A-3H-10) may thus represent the beginning of the Jaramillo normal polarity subchron. The Emperor event (0.42 Ma), a short reversal within the upper part of the Brunhes normal chron, appears not to have been recorded by the recovered cores in Hole C0008A, although two discrete samples at 5–8 m CSF revealed negative inclinations (Fig. F22).

Below 220 m CSF, a dominantly normal polarity sequence appears to correspond to the Olduvai normal subchron, as the available coeval biostratigraphic indicators are also placed in this depth interval. Shipboard biostratigraphic data suggest that sediments below 186 m CSF are older than 1.6 Ma and nannofossil biostratigraphic Zone NN18 (~2.06 Ma) is placed at 259 m CSF (see “Biostratigraphy”). This information suggests that the dominantly normal polarity sequence between 220 and 270 m CSF may represent part of the Olduvai Subchron (1.77–1.95 Ma).

For Hole C0008C, the polarity shift from normal to reversed occurred at 30.5 m CSF, which might correspond to the beginning of the Jaramillo Subchron (Fig. F23). A short spike of polarity change at 7.3 m CSF within biostratigraphic Zone NN20 appears to be related to the Emperor event (0.42 Ma).

The polarity changes to normal at 160 m CSF, which is located between biostratigraphic data of 1.67 Ma at ~143 m CSF and younger than 1.98 Ma at ~167 m CSF. This normal polarity is considered to be the end of Olduvai Subchron (1.77 Ma) and appears 60 m shallower than in Hole C0008A. The shallowing of corresponding depth between Holes C0008A and C0008C was found in two independent data sets such as magnetic susceptibility and inclination.

In summary, preliminary shipboard paleomagnetic studies revealed some important magnetostratigraphic signatures at Site C0008 that await verification by shore-based studies.

Top of page   |    Previous   |    Next