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doi:10.2204/iodp.proc.314315316.133.2009

Paleomagnetism

Shipboard paleomagnetic studies for Site C0004 consisted of continuous measurements of archive half core sections and progressive demagnetization measurements of discrete samples in a similar fashion to that performed during Expedition 315 (see “Paleomagnetism” in the “Expedition 315 Site C0001” chapter). A total of 16 discrete samples were stepwise thermally or alternating-field (AF) demagnetized to evaluate the directional stability and coercivity/​unblocking temperature spectra of each sample. The anisotropy of magnetic susceptibility (AMS) was measured on seven discrete samples with the Kappabridge KLY 3. Volume magnetic susceptibility of these discrete samples was measured before the AMS measurement. The Königsberger ratio was also determined for these samples.

Natural remanent magnetization and magnetic susceptibility

Within the recovered sediments, there are considerable variations in magnetic properties and demagnetization behavior among the various lithologies. The most important observations at Site C0004 are summarized below.

Natural remanent magnetization (NRM) intensities of the archive halves from Holes C0004C and C0004D span more than two orders of magnitude (ranging from 0.02 to >80 mA/m; Fig. F32). NRM intensity peaks at 352.7 m CSF (Sample 316-C0004D-46R-2, 80 cm), corresponding to the silty clay with volcanic sand layers (see “Lithology”). Variations in magnetic susceptibility are consistent with variations in NRM intensity (see Fig. F32 for an example). Magnetic susceptibility values are generally ~10 × 10^–3 SI for the upper slope sediment Unit I, prism Subunit IIB, and fault-bounded Unit III but significantly higher (>50 × 10^–3 SI) for the underthrust slope sediment deposits in Unit IV, which contain volcanic and sand layers (see “Lithology”). Magnetic susceptibility rapidly decreases and then increases within Sections 316-C0004C-9H-5 (~78 m CSF) and 15X-1 (~118 m CSF), marking the beginning and end of the breccia observed in Subunit IIA. These susceptibility variations were verified by further measurements of corresponding discrete samples (see “Anisotropy of magnetic susceptibility”).

Pervasive remagnetization imparted by the coring process is commonly encountered, as noted during previous Deep Sea Drilling Project/​ODP/​IODP legs (e.g., Gee et al., 1989; Zhao et al., 1994). This remagnetization is characterized by NRM inclinations that are strongly biased toward vertical (mostly toward +90°) in a majority of cores. As shown in Figure F32, with AF demagnetization to 40 mT a significant decrease in intensity and a shift of inclination toward shallower or negative values were observed for intervals with normal or reversed polarity, respectively, suggesting the presence of drilling-induced remagnetization. In two intervals (~350 and 370 m CSF) where recovered sediments are dominated by hemipelagic silty clay with sand layers, however, remagnetization appears to have only affected inclination and to have little effect on NRM intensity (Fig. F32).

The most diagnostic feature in the paleomagnetic data obtained at Site C0004 is that changes in magnetic polarity can be correlated with changes in biostratigraphic zonations. For example, the uppermost 15 m of sediments at Hole C0004C is known to be of Pleistocene age, based on biostratigraphic age markers (Zones NN20–NN19, >0.291 and <0.9 Ma). Therefore, the normal polarity of Cores 316-C0004C-1H through 2H suggests that these sediments were deposited during the Brunhes Chron (i.e., <0.781 Ma). In the lower part of Hole C0004D, sediments between 314.6 m CSF (Sample 316-C0004D-38R-1, 12 cm) and 398.8 m CSF (Sample 56R-CC, 14 cm) show reversed polarity. Biostratigraphic Zone NN19 with well-defined FO events is also placed at the beginning of this interval, suggesting the reversed polarity should correlate with the Matuyama Chron. In addition, biostratigraphic data suggest that sediments below 398 m CSF (Sample 316-C0004D-56R-CC, 14 cm) may be <2 Ma. This information is in good agreement with paleomagnetic observations and suggests that the shift of polarity from reversed to normal at ~390 m CSF should correspond to the beginning of the Olduvai Subchron (C2n; 1.778–1.945 Ma).

Paleomagnetic stability tests and general polarity sequences

Because of the rotary technique used for drilling in Hole C0004D, relative rotation frequently occurred between different segments of sediment within the core. This may cause apparent changes in the declination of stable remanent magnetization. Consequently, magnetic polarity has been assigned on the basis of the inclination of the stable remanent magnetization. As Site C0004 is situated at moderate latitude in the Northern Hemisphere, positive (downward directed) inclinations are taken to signify normal polarity and negative (upward directed) inclinations signify reversed polarity. NRM was measured at 5 cm intervals for each core section, followed by AF demagnetization at 5, 10, 20, and 40 mT peak fields. In most cases, an unambiguous polarity determination of the stable component of magnetization was achieved after the 40 mT treatment. Figure F33 illustrates the stable behavior of several samples; this behavior demonstrates the removal of a normal component of magnetization and the isolation of a more stable reversed component that univectorially decays toward the origin of the vector plots (Zijderveld, 1967). Because the maximum level of AF demagnetization on the ship’s cryogenic magnetometer was not always able to remove remagnetization, several discrete samples were stepwise thermally or AF demagnetized (Fig. F33). An example of demagnetization behavior of a sample during thermal demagnetization is illustrated in Figure F33C. A secondary component of magnetization was removed at low temperatures (200°C), and a characteristic remanent magnetization (ChRM) component having higher unblocking temperatures could be identified. The response of ChRM to AF and thermal demagnetization suggests that ChRM in most samples is carried by fine (i.e., single-domain to pseudosingle-domain) low-Ti titanomagnetite grains.

Demagnetization of core sections sometimes showed antipodal relative declinations within the same piece of core section (Fig. F34). After AF demagnetization at 40 mT, the reversal indicated by the difference in polarity of inclination was confirmed by the near 180° change in declination. This positive “antipodal test” is perhaps the most compelling argument for isolating primary ChRM, although this test is not sufficient by itself. We also noticed that the inclination values of ChRM are moderately downward or upward consistent with the expected inclination for the site (52.6° for normal polarity or –52.6° for reversed polarity).

A number of pass-through measurements, however, have inclinations that do not resemble the time-averaged geomagnetic field (expected inclination = ±52°). In particular, Cores 316-C0004C-2H through 18H (~15–134 m CSF) and Cores 316-C0004D-1R through 13R (~105–180 m CSF) have remanent inclinations that are consistently shallower (±20°–30°) than predicted. Shipboard studies revealed that the sedimentary layers may have tilted after formation (see Fig. F35). Assuming the regional strike for the sedimentary sequence is northeast–southwest with dips ranging from 40° to 60°, a moderate tilt correction (~45°) along a northeast fold axis would restore these shallow inclinations to the expected dipole inclination at Site C0004. Another possible explanation is that drilling-induced remagnetization was not completely removed from the recovered sediments, and therefore these sediments do not necessarily have an inclination corresponding to that expected from a geocentric axial dipole.

The magnetostratigraphy at Site C0004 indicates several magnetic reversals that may be discerned on the basis of changes in sign of inclinations on cores from Holes C0004C and C0004D (Fig. F36; Table T10). Each of the major polarity zones in Figure F36 is defined by several measurements of the same polarity. On the basis of shipboard micropaleontological data, the uppermost 15 m of sediment in Hole C0004C is Pleistocene in age. Therefore, the normal polarity of Core 316-C0004C-1H suggests that these sediments were deposited during the Brunhes Chron. The first evidence for reversed magnetization occurs at 6.73 m CSF in Section 316-C0004-2H-1, 55 cm, which may correspond to the Emperor event (0.42 Ma; see Gradstein et al., 2004). The Brunhes/​Matuyama Chron boundary (0.78 Ma) is placed at 15.87 m CSF, as the magnetic inclination at this depth changes polarity from normal to reversed. Sediments between 17.92 and 40.18 m CSF show normal polarity, indicating that the Jaramillo Subchron (0.99–1.07 Ma) may be recorded in these sediments and implying a relatively rapid sediment accumulation rate. Biostratigraphic marker R. asanoi (0.9–1.078 Ma) is also placed in this depth interval (see Fig. F29). Below 40.99 m CSF, the dominantly reversed polarity correlates well with the Matuyama reversed polarity chron. This dominantly reversed polarity sequence extends to 102.20 m CSF. Biostratigraphic data suggest that the sediments below 98.51 m CSF (Sample 316-C0004C-12X-CC, 5 cm) are older than 2.52 Ma but younger than 3.65 Ma. This information suggests that the shift of polarity from reversed to normal at ~102 m CSF should correspond to the Matuyama/​Gauss boundary (2.58 Ma). Within the sequence assigned to the Matuyama reversed chron, one apparently thin normal polarity zone is present in the depth range of 50.78–54.20 m CSF in Hole C0004C and at 356.68 m CSF in Hole C0004D. This short normal polarity zone appears to correspond to the Olduvai Subchron (1.77–1.95 Ma). Key nannofossil markers around this depth in both Holes C0004C and C0004D are in good agreement with the paleomagnetic data, suggesting that these sediments span the Pliocene/​Pleistocene boundary (Zone NN19a). Below the Matuyama/​Gauss boundary, a dominantly normal polarity sequence extends to at least the bottom of Hole C0004C at 135.00 m CSF (Fig. F36). This dominantly normal polarity sequence should correspond to the Gauss normal chron. Unfortunately, sedimentary sequences in Hole C0004D are structurally disrupted with several age-reversed intervals. Consequently, these age-reversed intervals prevent us from constructing a magnetostratigraphy older than the Gauss Chron.

Paleomagnetic and biostratigraphic age determinations are compatible with each other in Hole C0004C (Fig. F29). This compatibility allows a determination of sedimentation rates and a better definition of the times at which significant changes in sedimentation rate occurred.

Paleomagnetically determined sedimentation rates for Hole C0004C

Pleistocene and Pliocene sediments cored in Hole C0004C yielded a preliminary magnetic polarity stratigraphy (Fig. F36; Table T10) and age-depth profile (Fig. F29) from which preliminary sedimentation rates can be calculated. The Brunhes/​Matuyama (0.78 Ma), Jaramillo (0.99–1.07 Ma), Olduvai (1.77–1.95 Ma), and Matuyama/​Gauss (2.58 Ma) chrons are tentatively determined at depths of 15.87, 17.92–40.18, 50.78–54.20, and 102.20 m CSF, respectively. If correct, these calibration points allow the determination of sedimentation accumulation rate values and the assignment of “absolute” ages to the biostratigraphic zonal boundaries identified in Hole C0004C. The lower Pliocene–Holocene sediment accumulation rates at Site C0004C are 20.4 m/m.y. for the middle Pleistocene and 39.6 m/m.y. for the late Pliocene. Furthermore, if the identification of the Olduvai Subchron is correct, a decrease in accumulation rate or a short hiatus is inferred within the interval of 1.77–1.95 Ma.

Anisotropy of magnetic susceptibility

AMS measurements were carried out on discrete samples from the area of special scientific interest to inspect magnetic properties and fabrics. Seven samples were collected across the unconformity in Section 316-C0004C-9H-5, which forms a boundary between Unit I and Subunit IIA, upper slope sediments and upper prism. Although the equipment (AGICO Kappabridge KLY 3) is so sensitive that it is barely stable onboard because of the movement of the vessel, we accomplished the analyses with low error values. Magnetic susceptibility of the samples was measured in various orientations and resulted in a magnetic ellipsoid with three principal axes from maximum (K1) to minimum (K3). Parameters of anisotropy degree and shape parameter are expressed as

L = K1/K2

and

F = K2/K3,

respectively.

Bulk magnetic susceptibility showed a significant decrease below the unconformity, which is consistent with the results of continuous whole-core logging using the multisensor core logger (MSCL) (Table T11). Although L and F values vary in the hanging wall in ranges of 1.002 < L < 1.024 and 1.010 < F < 1.030, measurements in the footwall are relatively low and stable (1.003 < L < 1.007 and 1.003 < F < 1.012). With such low L and F values, three principal axes of magnetic ellipsoids have only little variation so that the shapes of the ellipsoids are close to spherical. Even the results do not show significant preferences; a small variance in the hanging wall possibly indicates some structural disturbance or mineralogical difference from the foot wall. Further onshore study is required to make this question clear.

As shown in Table T11, the decrease in magnetic susceptibility and intensity at the unconformity might indicate some mineralogical change of magnetic carrier. The hanging wall consists of silty clay, whereas the footwall consists of green brecciated silty clay. Shipboard XRD measurements revealed that breccia below the unconformity contains a slightly higher iron component (see “Lithology” for details), which is not consistent with magnetic data. Resolution of this discrepancy will require further detailed mineralogical investigations.

The Königsberger ratio, Q, is defined as the ratio of remanent magnetization to the induced magnetization in Earth’s magnetic field. In general, the Königsberger ratio is used as a measure of stability to indicate a rock’s capability of maintaining a stable remanence. The International Geomagnetic Reference Field (IGRF) value at Site C0004 (45,706 nT = 36.39 mA/m) was used for calculating the Königsberg ratio (Q):

Q = J_nrm [mA/m]/[K_m (SI) × H (mA/m)],

where

• H = local geomagnetic field,
  
• K_m = bulk susceptibility, and
  
• J_nrm = NRM intensity.

Results show that Q ratios in a majority of samples are <1, suggesting that the total magnetization of the sediments contains dominantly induced magnetization. The relatively low Q ratios may also explain the pervasive drilling-induced remagnetization imparted to these cores.

In sum, preliminary shipboard paleomagnetic data revealed important magnetic signatures that await further verification in terms of age and origin. Further integrated work with biostratigraphic and structural studies is required to constrain the timing and origin of magnetization recorded by the Site C0004 sediments.

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