Previous   |    Next

doi:10.2204/iodp.proc.342.103.2014

Paleomagnetism

We completed a preliminary paleomagnetism study of the two APC cores in Hole U1402B with the primary objective of establishing a magnetostratigraphy of the site to provide chronostratigraphic age control. The natural remanent magnetization (NRM) of each archive section half was measured at 2.5 cm intervals before and after demagnetization treatment in a peak alternating field (AF) of 20 mT. NRM measurement on the archive half of Section 342-U1402B-1H-1 was not conducted (not intentionally). Two sections, 1H-3 and 1H-4, were not measured because they were not split. We processed the data by removing measurements made within 7.5 cm of section ends and from disturbed intervals as described in the Laboratory Information Management System (LIMS) database.

Reversal patterns can be easily determined at the site latitude (~40°N) by changes in inclination polarity. When cores are azimuthally oriented, north- and south-directed declinations can clarify magnetic polarity. Cores at Site U1402, however, were not oriented because the FlexIT tool was not used during coring.

Eight discrete samples, taken in plastic Natsuhara-Giken sampling cubes, were collected from working section halves. Samples were typically taken from the least disturbed region closest to the center of each section. The samples were first subjected to measurements of the anisotropy of magnetic susceptibility (AMS), including bulk susceptibility. Subsequently, the samples were step-wise AF demagnetized at 10 and 20 mT. Three of the samples were further demagnetized at 30, 40, 60, and 80 mT. All discrete sample data were volume corrected to 7 cm³. Although Sample 342-U1402B-1H-1W, 75–77 cm, was only half filled (~3.5 cm³), it was volume corrected to 7 cm³.

Results

Downhole paleomagnetism data for Hole U1402B are presented in Figure F5. Cores from Hole U1402B have steep inclination values (70°–90°) measured prior to demagnetization that generally become shallower after AF demagnetization at 20 mT. We interpret this behavior as a substantial drilling overprint, primarily a viscous isothermal remanent magnetization (IRM), that is typical for cores from Deep Sea Drilling Project, ODP, and IODP operations.

AF demagnetization results for the eight discrete samples are summarized in Table T3. Stepwise AF demagnetization on the samples reveals a stable component of magnetization that is resolved for treatment levels above 10 mT (Fig. F6A, F6C). Sample 342-U1402B-1H-2W, 79–81 cm, did not record a stable component (Fig. F6B); it was collected from a horizon that probably consists of magnetically soft (very low coercivity) minerals (~2.3 mbsf; Fig. F5). Our results indicate that viscous overprinting is successfully removed by AF demagnetization up to 10 mT. Thus, NRM following 20 mT demagnetization is a reliable indicator of characteristic remanent magnetization (ChRM). NRM intensities and directions after 20 mT demagnetization are generally consistent between the archive halves and the discrete samples (Fig. F5). Three samples (8.13, 10.80, and 12.33 mbsf) record inclinations that are inconsistent with the pass-through magnetometer data. These samples were taken from Core 342-U1402B-2H, and the disturbance probably affected the declination results (see below).

For the uppermost ~3 m, inclination and declination show generally stable directions. Except for the horizon at ~2.3 mbsf, inclination varies between ~30° and ~60°. These values are consistent with the expected inclination of ~60° from the geocentric axial dipole (GAD) field at the latitude of Site U1402. In contrast, the 7–14 mbsf depth interval (Core 342-U1402B-2H) shows generally steep inclination and highly variable declination. We attribute this variability to core disturbance; the plastic liner for Core 2H shattered during recovery. See “Lithostratigraphy” for a full discussion of core disturbance.

Magnetostratigraphy

If we tentatively include the 7–14 mbsf depth interval (Core 342-U1402B-2H), then sediment from Hole U1402B is characterized by normal polarity except for a short interval at 6.15–6.20 mbsf (interval 342-U1402B-1H-5A, 15–20 cm). Site U1402 is a reoccupation of Site 1073, at which Chron C1n (Brunhes normal polarity chron) was identified downhole to 515 mbsf (Shipboard Scientific Party, 1998). We follow this prior work and also ascribe the normal polarity observed in Hole U1402B sediment to Chron C1n. This interpretation is consistent with the biostratigraphic results (see “Biostratigraphy”). The brief polarity reversal observed in both inclination and declination at 6.15–6.20 mbsf may record a short geomagnetic reversal event. A similarly shallow and short reversal event was reported at 14.8–15.7 mbsf at Site 1073 (Shipboard Scientific Party, 1998) and was provisionally identified as either the Laschamp or Blake excursion. Nannofossil biostratigraphy indicates, however, that the base of Hole U1402B cannot be older than 70 ka (see “Biostratigraphy”), so we interpret the reversal at 6.15–6.20 mbsf to be the ~38–40 ka Laschamp excursion (Plenier et al., 2007). Shore-based radiocarbon dating may provide additional age control to verify this conclusion.

Magnetic susceptibility and anisotropy of magnetic susceptibility

Bulk susceptibility measured on discrete samples is summarized in Table T4. Downhole variation for whole-round magnetic susceptibility (WRMS) and discrete sample magnetic susceptibility (DSMS) are shown in Figure F5. The WRMS data for Hole U1402B are shown in raw form; they have not been trimmed at section ends or filtered for obvious outliers, so noise in the data probably reflects edge effects or spurious measurements. We multiplied the WRMS data, which are in instrument units, by a factor of 0.577 × 10^–5 to convert to approximate SI volume susceptibilities (see “Paleomagnetism” in the “Methods” chapter [Norris et al., 2014b]). WRMS and DSMS data agree very well after this conversion, and we attribute small absolute differences to the fact that the conversion factor applied to the WRMS data is not constant downhole because of changes in core diameter and density; only discrete samples provide calibrated susceptibility values in SI units. Magnetic susceptibility of the discrete samples is invariant with depth and is between 250–450 µSI.

AMS results for the discrete samples are summarized in Table T4. The eigenvalues associated with the maximum (τ₁), intermediate (τ₂), and minimum (τ₃) susceptibilities indicate that the magnetic fabric becomes increasingly oblate with depth (Fig. F7). Moreover, both the inclination of V₃ (minimum principal eigenvector) and the degree of anisotropy (P; τ₁/τ₃) increase with depth. These changes in magnetic fabric are common in sediment (Schwehr et al., 2006) and can be readily attributed to compaction of clays with burial. The very low V₃ inclination at 8.13 mbsf may be due to core disturbance in this interval.

Top of page   |    Previous   |    Next