Previous   |    Next

doi:10.2204/iodp.proc.343343T.103.2013

Paleomagnetism

Because of the unavailability of the Chikyu’s cryogenic magnetometer (see “Paleomagnetism” in the “Methods” chapter [Expedition 343/343T Scientists, 2013]), regular whole-core pass-through magnetic measurements of core sections could not be carried out during Expedition 343. Thus, shipboard paleomagnetic studies for Site C0019 consisted of measurements for natural remanent magnetization (NRM) and progressive demagnetization of discrete samples only with the ship’s spinner magnetometer and alternating field (AF) demagnetizer. We stepwise AF demagnetized 56 paleomagnetic cubes or minicores to evaluate the directional stability and coercivity spectrum of each sample. We analyzed the results in Zijderveld diagrams (Zijderveld, 1967) and calculated the characteristic remanent magnetization (ChRM) direction using principal component analysis (Kirschvink, 1980). Anisotropy of magnetic susceptibility (AMS) of these discrete samples was measured with the Kappabridge KLY 3S. The Königsberger ratio, which is defined as the ratio of remanent magnetization to the induced magnetization in Earth’s magnetic field, was also determined for the measured samples.

NRM, magnetic susceptibility, and Königsberger ratio

The most significant variations in NRM intensity and susceptibility for discrete samples from Site C0019 are well correlated with lithology (Fig. F43). The brown clayey mudstone in lithologic Units 5 and 6 have a mean NRM intensity of 4.33 × 10^–2 A/m, which is the highest mean value among all the samples. Within lithologic Unit 3, NRM intensities average ~3.94 × 10^–2 A/m except for a few discrete peaks of higher NRM intensities in some depth intervals (e.g., around 697.85, 770.44, and 801.04 mbsf). The siliceous/ash mud(stone) in lithologic Unit 1 has the lowest mean NRM intensity of 2.03 × 10^–2 A/m.

Variations in magnetic susceptibility generally parallel the variations in NRM intensity in lithologic Unit 1 (Fig. F43), where the magnetic susceptibility values are generally ~1.0 × 10^–3 SI. The high magnetic susceptibility in lithologic Unit 2 could be related to the presence of ashy mudstone (see “Lithology”). Magnetic susceptibility values tend to increase from the upper portion of lithologic Unit 3 and peak in the lower portion of Unit 3, and in Units 5 and 6, where magnetic susceptibility reaches ~4.5 × 10^–3 SI. The variation in susceptibility is most likely caused by variations in the magnetic mineral type or magnetic mineral content and needs more detailed shore-based rock magnetic measurements.

The Königsberger (Q) ratio is defined as the ratio of remanent magnetization to the induced magnetization in Earth’s magnetic field. In general, the Q ratio is used as a measure of stability to indicate a rock’s capability of maintaining a stable remanence relative to the induced magnetization. The current total field value of the International Geomagnetic Reference Field in Hole C0019E (46,498 nT = 37.0 A/m), was used to calculate the Königsberger ratio for rock samples at Site C0019:

where

• H = local geomagnetic field (A/m),
• K_m = bulk susceptibility (SI), and
• J_nrm = NRM intensity (A/m).

In general, Q ratios in a majority of samples in lithologic Units 1, 2, 5, and 6 are less than unity, suggesting that the total magnetization of samples contains dominantly induced magnetization. In contrast, Q ratios in a majority of samples in lithologic Unit 3 are higher than unity, with a mean value of 1.48, except for two Q ratio peaks at 697.85 and 787.08 mbsf. This may suggest that the magnetic minerals in the mudstone are insusceptible to an external magnetic field.

Anisotropy of magnetic susceptibility

Figure F44 shows the AMS parameters measured for discrete samples in Hole C0019E. In lithologic Unit 1, minimum susceptibility (K_min) inclinations ranges from 41° to 80°. Steeper K_min inclinations possibly represent the preservation of the primary fabric of the sediments. The low anisotropy degree parameter (P′) reveals that the three principal axes of magnetic ellipsoids have small variations. Negative shape parameters (T) indicate that magnetic ellipsoids have a prolate component. In general, P′ values increase gradually with depth in lithologic Unit 3, indicating an increasing degree of deformation or progressive compaction with depth. In lithologic Units 5 and 6, the anisotropy degree seems to gradually decrease, the positive shape parameter represents an oblate shape of the ellipsoid, and some K_min inclinations are steeper than 60°, possibly preserving the sedimentary or compaction fabric. Further shore-based study is required to better clarify these observations.

Paleomagnetic direction of discrete samples

Remanent magnetization of discrete samples was investigated using stepwise AF demagnetization techniques to extract the primary component of magnetizations acquired at the time of deposition. Figure F45 illustrates the magnetic stability behavior of representative samples. Typically, two remanent magnetization components were isolated during AF demagnetization. The first- and second-removed components are here referred to as a low-coercivity component and a high-coercivity component, respectively. The low-coercivity component has generally steep inclination and demagnetized below 10–20 mT. Low-coercivity components with steep inclinations are found in numerous DSDP, ODP, and IODP paleomagnetic studies and are likely to be induced during drilling (Richter et al., 2007). The high-coercivity component is demagnetized above 10–20 mT and univectorially decays toward the origin of the vector plots. Some samples show curved trajectories rather than acute angles between two components because of overlapping coercivity, but most samples have a linear segment in an interval of high demagnetization levels. Three samples show no linear segments in the demagnetization diagram, so we omitted these samples from further paleomagnetic analyses. We regarded the high-coercivity component as the ChRM and calculated its direction using principal component analysis (Kirschvink, 1980).

Figure F46 shows the ChRM direction for the downhole profile. Except for the lower part of lithologic Unit 3 (770–820.01 mbsf), ChRM inclination of most samples ranges between ±20° and 60°, which is somewhat shallower than the expected inclination (±57.3°) for the latitude of Hole C0019E (37°56′N). Samples from the lower part of Unit 3 (770–820.01 mbsf) show ChRM inclination shallower than ±57.3°. Steep tilt of beds (see “Structural geology”) after the acquisition of remanent magnetization is a possible explanation for the shallow inclination observed in the lower part of Unit 3. It is notable that all discrete samples within the upper part of Unit 3 (688.5–725.165 mbsf) except for the uppermost sample have positive ChRM inclination.

ChRM declination of Hole C0019E is generally not consistent even within each section of the cores, possibly because of severe biscuiting of the core (see “Lithology”). ChRM declination can be utilized for reorientation of beddings and deformation structures observed within coherent pieces with paleomagnetic samples (see “Structural geology”).

Paleomagnetic direction of archive halves

Remanent magnetization of archive halves of the core samples collected during Expedition 343 were measured at 2 cm intervals using the 2G long core superconducting rock magnetometer during Expedition 343T. Routine paleomagnetic measurements are typically performed with AF demagnetization at 5, 10, 15, and 20 mT. However, paleomagnetic results of the discrete samples showed that AF demagnetization below 15 mT is insufficient for removing the low-coercivity component of the core samples obtained during this expedition, which is likely to be induced during drilling. Therefore, we adopted AF demagnetization levels of 15 and 20 mT. We adopted AF demagnetization at 5, 10, 15, and 20 mT for Core 343-C0019E-17R, where no paleomagnetic discrete samples were measured. We omitted paleomagnetic measurements of Cores 2R and 3R because these cores are made up of drilling breccia (see “Structural geology”).

We adopted a sampling interval of 2 cm, which is less than the typical interval for pass-through paleomagnetic measurement of 5 cm. The dense sampling interval was adopted in order to determine paleomagnetic directions of as many coherent biscuits of the core samples as possible.

The NRM directions and remanent magnetization directions after AF demagnetization at 20 mT are shown in Figure F46 with the ChRM directions of discrete samples. The ChRM directions of discrete samples are fairly consistent with remanent magnetization directions after demagnetization at 20 mT for the corresponding depths of the archive halves.

Within lithologic Unit 1 and the upper part of lithologic Unit 3 (178.50–185.23 and 688.50–725.115 mbsf), distribution of inclinations after demagnetization at 20 mT centers at ±40°, which is shallower than the expected inclination (±57.3°). Positive inclination dominates at intervals between 184.2 and 184.8, 689.7 and 690.4, and 696.9 and 722.7 mbsf, whereas negative inclination dominates at intervals between 180.8 and 184.1, 689.0 and 689.4, and 696.1 and 696.6 mbsf. It is likely that these intervals belong to certain normal and reversed chronozones, respectively. However, identification of the specific chronozone for the intervals is quite difficult because of lack of shipboard biostratigraphy data. Shallow inclinations dominate in the lower part of the lithologic Unit 3 (770.00–820.01 mbsf), which is possibly due to steep tilt of beds (see “Structural geology”) within the interval. Positive inclinations were obtained from the sheared clay of lithologic Unit 4 (822.10–822.36 mbsf). Inclination fluctuates between shallow and steep values within lithologic Units 5 and 6 (824.00–833.47 mbsf) and polarity zones are difficult to identify within these lithologic units.

Top of page   |    Previous   |    Next