Previous   |    Next

doi:10.2204/iodp.proc.334.104.2012

Paleomagnetism

We made pass-through magnetometer measurements on all split-core archive sections with variable measuring intervals (2–10 cm). Discrete samples were also collected from the working halves of Hole U1379C at a spacing of one sample per section (1.5 m). In order to isolate the characteristic remanent magnetization (ChRM), we subjected the cores to alternating-field (AF) demagnetization. The split cores were typically demagnetized up to 30 mT. In order to test the split-core data, we demagnetized 85 discrete samples using progressive AF demagnetization techniques and measured them in both the superconducting rock magnetometer and JR6 magnetometer (see “Paleomagnetism” in the “Methods” chapter [Expedition 334 Scientists, 2012]). Cores 334-U1379C-1H through 17H were cored with the APC system using a nonmagnetic cutting shoe. These cores were oriented with the Flexit orientation tool. Unfortunately, crucial orientation data were lost during the coring process, which hampers the magnetostratigraphy investigation for Site U1379. Cores 334-U1379C-18X through 118X were cored with the XCB system using a standard cutting shoe.

Natural remanent magnetization

Downhole variations of paleomagnetic data obtained at Site U1379 are shown in Figure F37. The natural remanent magnetization (NRM) intensity is on the order of 10^–2 to 10^–1 A/m. Variations in NRM intensity of archive-half cores are correlated with lithology. Paleomagnetic measurements indicate that the olive-green sands in lithostratigraphic Unit III (~651–880 mbsf) have the lowest NRM intensity. A few discrete peaks of higher NRM values appear in some depth intervals in Units II and III (e.g., at ~100, ~490, and ~890 mbsf; Fig. F37), which can be tied directly to the presence of volcanic tephra in these regions (see “Lithostratigraphy and petrology”). Another peak occurs at ~590 mbsf, which is above the fault zone (see “Structural geology”). Below the peak at ~590 mbsf in the fault zone, NRM intensity slightly decreases. Magnetic susceptibility data also show positive peaks at these intervals (see “Physical properties”).

As with cores recovered from nearly all ocean drilling programs, remagnetization imparted by the coring process is commonly encountered at Site U1379. NRM inclinations are strongly biased toward the vertical (mostly toward +90°) in a majority of cores. Upon AF demagnetization to 30 mT, a significant decrease in intensity (about one order of magnitude; Fig. F37) and a shift of inclination toward shallower values were observed. The inclination from the APC core (shallower than ~100 mbsf) became close to the expected time-averaged geomagnetic field inclination at this site (~±18°), whereas the inclinations from the XCB cores remain much steeper (~50°–60°).

The patterns of the magnetic declination are also different between APC- and XCB-cored sections. NRM declinations of APC cores are different from each other, which is expected. However, declinations of XCB cores show strong concentration along the +x-direction in the IODP coordinate scheme, with values highly clustered at 0°. Upon AF demagnetization, the declinations of XCB cores start to become randomly oriented. It appears that the core barrel assembly has a very strong effect on the declinations. The paleomagnetism of APC and XCB cores are discussed separately below.

Demagnetization behavior of APC cores

Although the APC core NRM obtained from the pass-through measurements were strongly affected by drilling overprint, as evidenced by the steep inclinations, AF demagnetization was effective in removing this overprint (Fig. F37). Moreover, declination is uniform within each core and different among cores (Fig. F38). This behavior is expected for the remanence of natural origin recovered by APC coring. APC coring minimizes the internal rotation of the core and therefore preserves the uniformity of remanence direction within the core. On the other hand, the core liner is arbitrarily oriented with respect to the direction of Earth’s magnetic field and so is the IODP coordinate for each core. This produces random declination among different cores, as shown in Figure F38.

The magnetic properties observed from the split cores were also confirmed by discrete sample measurements. The nearly vertical overprint was removed by AF demagnetization of 5–10 mT (e.g., Fig. F39A), and the stable component has a similar direction to those in the corresponding core sections (Fig. F37). AF demagnetization was extremely successful: 31 out of 32 tested samples revealed ChRM with maximum angular dispersion <15° (Kirschvink, 1980). Generally, NRM was completely demagnetized by ~40 mT.

Several spot readings in the pass-through measurements revealed negative inclinations (e.g., ~100 mbsf), but this behavior was not found in the ChRM of discrete samples except for a nearly horizontal inclination (–0.1°) in interval 334-U1379C-8H-4, 38 cm (Fig. F37).

Demagnetization behavior of XCB cores

NRM of XCB cores obtained by pass-through measurements also show a magnetic overprint with steep inclinations. Unlike APC cores, the inclinations in XCB cores are extremely stable and display only minor changes upon AF demagnetization at 30 mT (Fig. F39B). This stable overprint magnetization hindered retrieval of reliable paleomagnetic directions from the pass-through measurements of XCB cores.

The declinations of XCB cores further testify to the existence of this stable overprint. As shown in Figure F37A, NRM of XCB cores is almost invariably oriented parallel to the +x-direction, with declinations highly clustered at 0°. Because XCB coring produces internal rotation of the core, we expect remanence of natural origin to be randomly oriented throughout. The ~0° declinations of NRM of XCB cores strongly suggest an artificial overprint. Such orientation could derive from a radially inward magnetization induced by the coring process (e.g., Stokking et al., 1993). Interestingly, upon AF demagnetization the highly clustered declination pattern became random (Fig. F37B), suggesting that at least part of the radial-inward overprint was removed. Inclinations after AF demagnetization still maintained moderate to steep downward directions, suggesting that magnetization is still contaminated by the vertical overprint.

In an attempt to extract reliable paleomagnetic information from XCB cores, we performed progressive AF demagnetization experiments on discrete samples taken from the working halves. We observed steep inclinations and ~0° declinations in NRM measurements of the discrete samples (Fig. F39), indicating the existence of the radial-inward overprint. Seventeen samples out of 54 tested samples revealed ChRM with maximum angular dispersion <15°. Some other samples are either completely dominated by the artificial overprint or revealed scattered demagnetization behavior (Fig. F40). All samples showed rapid removal of near-vertical overprint at 5–15 mT demagnetization steps. The 17 ChRMs revealed a relatively uniform distribution for the declination. This may imply the success of removal of the radial overprint; however, the sample number is small and care must be taken. The inclination shows a broad distribution. Some inclination values are clearly higher than the theoretically expected value for the latitude of this site (17.3°), pointing to the incomplete removal of the vertical overprint or being affected by the steeper titling of bedding as observed in depth intervals below 600 mbsf (see “Structural geology”).

Implications for core orientation

Results from discrete sample demagnetization also provide an opportunity to evaluate the accuracy of the archive-half core remanence data that are used in combination with the discrete sample results to reorient core pieces to a common geographic framework. Paleomagnetic core reorientation method has been successfully used for both continental and oceanic outcrops (e.g., Fuller, 1969; Kodama, 1984; Shibuya et al., 1991). This method is to assume 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, represent the expected magnetic direction at site. The orientation of the paleomagnetic ChRM, which specifies the rotation of the core relative to the geographic coordinates, is then used to restore the azimuth of the core. For intervals of particular interest for structural geology at Site U1379, we used the stable ChRM isolated from progressive demagnetization of discrete samples.

For APC cores, both the pass-through and the discrete sample measurements indicate that after AF demagnetization, we can recover remanence of natural origin. XCB cores are more severely affected by drilling overprints. AF demagnetization appears successful in removing the radial overprint, implying the declination of ChRM may be reliable even when the vertical overprint was not completely removed. However, we suspect that some ChRM with steep (>45°) inclination either still exhibits a steep drilling overprint or they are affected by bed titling, and caution should be used with these core reorientation data.

Implication for magnetostratigraphy

We used ChRM inclinations from discrete measurements to define magnetic polarity sequences for Site U1379, although at low-latitude areas such as Site U1379, a near-180° shift in declination in the cores would be a more reliable sign of the polarity transition. For the uppermost part of lithostratigraphic Unit II, both pass-through and discrete sample measurements do not indicate reversed polarity of ChRM. Because the orientation data from the Flexit tool were lost, we cannot compare the declinations across APC cores to check whether there is a near-180° shift in declinations. However, we tentatively conclude that the APC-cored interval in Unit II is within the Brunhes Chron (<0.78 Ma).

In the lowermost part of lithostratigraphic Unit II, only one relatively well defined polarity interval was identified in the downhole magnetostratigraphic records at ~701–704 mbsf (Fig. F37B). Sections 334-U1379C-83X-3 and 83X-4 show dominantly reversed polarity after AF demagnetization. Discrete samples taken from these two sections also show negative inclinations, consistent with magnetization of these cores in a reversed field. Lower Pleistocene biostratigraphic Zone NN19 is also placed at this interval (see “Paleontology and biostratigraphy”). Using calcareous nannoplankton zonal schemes for the eastern equatorial Pacific for the lower boundary of Zone NN19 (2.3 Ma), this observed reversed polarity may correlate with Chron C1r.2r (1.185–1.778 Ma). If true, this would suggest extremely fast sedimentation accumulation rates (>388 m/m.y.) for Hole U1379C.

Paleomagnetic characterization of basement rocks

At the time of this writing, we have not obtained the shipboard paleomagnetic samples from the basement. Pass-through magnetic measurements indicate that NRM intensity of the basement rocks is on the same order as that of the overlying sediments.

Preliminary pass-through paleomagnetic data have revealed important magnetic signatures that await further verification in terms of age and origin. A strong radial overprint is observed in most cores below ~90 mbsf. The relative intensity of the two magnetic overprint components varies along the cores, suggesting that some physical properties of the sediment itself may play a role in the acquisition of the overprint. The core barrel assembly also has a very strong effect on the overprint. The average inclination of the cores recovered using the standard assembly is clearly different from that of the cores where the nonmagnetic assembly was used. Further integrated work with shipboard micropaleontological data and structural measurements and paleomagnetic study on discrete samples from each section are required to constrain the timing and origin of the magnetization recorded by the Hole U1379C sediments.

Top of page   |    Previous   |    Next