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doi:10.2204/iodp.proc.344.105.2013

Paleomagnetism

Four holes were drilled at Site U1412. Cores 344-U1412A-1H through 15H were cored with the APC using nonmagnetic core barrels and oriented with the FlexIT orientation tool. Cores 344-U1412A-16X through 25X were cored with the XCB system. Core 344-U1412B-1H was also cored with the APC, and Cores 344-U1412B-3X through 20X were cored with the XCB. Core 344-U1412B-21 was a drilled interval. Cores 344-U1412C-2R through 7R and 344-U1412D-1R through 3R were cored with the RCB. We measured the remanent magnetization of archive section halves from Cores 344-U1412A-1H through 23X, Core 344-U1412B-1H through Section 11X-1, Sections 344-U1412C-5R-2 through 9R-3, and Sections 344-U1412D-2R-1 through 3R-5. In order to isolate the characteristic remanent magnetization (ChRM), these archive section halves were demagnetized in an alternating field (AF) up to 40 mT and measured with the pass-through superconducting rock magnetometer (SRM) at 2.5 cm intervals. The remaining core sections were not measured because of the poor conditions of the recovered material. In order to verify the section data, we demagnetized 61 discrete samples using progressive AF and thermal demagnetization techniques and measured them in the SRM.

Natural remanent magnetization of cores

Downhole variations of paleomagnetic data obtained at Site U1412 are shown in Figures F32, F33, F34, and F35. The natural remanent magnetization (NRM) intensity for the carbonaceous silty clay to clayey silt of Unit I (204.74–338.50 mbsf; see “Lithostratigraphy and petrology”) is on the order of 10^–2 A/m (Figs. F32, F33). The clayey siltstone of Unit III (338.50–387.00 mbsf) has somewhat lower NRM intensity (on the order of 10^–3 A/m; Fig. F34). The calcareous ooze with nannofossils and diatoms of Unit II (204.74–338.50 mbsf) has the lowest NRM intensity (on the order of 10^–4 A/m; Fig. F34). Sediments recovered from Hole U1412D have a mean NRM intensity of 2.5 × 10^–3 A/m (Fig. F35). There are more significant variations in NRM intensity in lithostratigraphic Unit I than in Units II and III. We observed a strong correlation between the variations in magnetic susceptibility and the variations in NRM intensity (see “Physical properties”).

A few troughs of lower NRM values appear around ~12 and ~146 mbsf in Unit I (Fig. F32); these troughs can be linked directly to the presence of watery sediments in these intervals and are likely due to drilling disturbances. NRM measurements of discrete samples taken from the nonwatery sediments above and below these intervals (block crosses in Fig. F32) show intensity values that are similar to the section data, confirming that the watery sediments indeed caused the lower NRM troughs in the pass-through records.

Magnetic noise in the SRM and y-axis flux jumps

During initial drilling operations at Site U1412, we observed a large increase in the frequency and number of magnetic flux jumps in the y-axis superconducting quantum interference device (SQUID). These jumps produced large magnetic noise anomalies and adversely affected the measurements of some sections (see the uniform inclination values at the top of Fig. F33). Discussions with IODP staff helped us find the solution for this anomaly. This magnetic noise is mainly due to the so-called “antenna” effect described originally by Gary Acton and others in the “Assessment of Readiness of the Refurbished D/V JOIDES Resolution” (scientific.oceandrilling.org/​xmlui/​handle/​10914/​20521/) and most recently by the Expedition 342 scientists (see “Paleomagnetism” in Norris et al., in press). These reports led us to discover that there was a tiny interval (~0.2 cm) between the aluminum foil wrapped on the cable attached to the in-line degausser and the magnetic shielding assembly. The aluminum foil was used to prevent an ambient electromagnetic field (EMF) from interfering with the SRM, but the tiny gap allowed the EMF to be transmitted inside the SRM. Core sections (especially full 1.5 m length sections) acted like an antenna or wire that conducts the EMF along its length to the SQUID sensors, in a process termed radio-frequency magnetic induction. We rewrapped the power cable attached to the in-line degausser with new aluminum foil and taped the foil to the shielding assembly to make sure the foil was electrically grounded. The magnetic noise decreased immediately, and no major flux jumps were noticed after this grounding. Nevertheless, small flux jumps still exist and are believed to be caused by random disturbances from wiring in the laboratory and fluctuations in ship’s power during heavy operations on the rig floor. At present, we cannot prevent this kind of magnetic noise except by repeating the measurement where the small flux jumps occurred. In most cases, the repeated measurements were improved and free of the flux jumps.

Paleomagnetic demagnetization results

As with cores recovered from other Expedition 344 sites, remagnetization imparted by the coring process is common. NRM inclinations are strongly biased toward the vertical (mostly toward +90°) in a majority of cores. Upon AF demagnetization to 40 mT, a significant decrease in intensity (about an order of magnitude; Fig. F32) and a shift of inclination toward shallower values were observed. The inclinations from APC cores (at depths shallower than ~108 mbsf) come close to the expected time-averaged geomagnetic field inclination at this site (approximately ±18°), whereas the inclinations from XCB cores remain much steeper (~50°–60°). It is clear that the maximum level of AF demagnetization (40 mT) we applied was not enough to remove the overprints entirely. An example of good-quality AF demagnetization results is shown in Figure F36.

We demagnetized 61 discrete samples (43 from Hole U1412A, 9 from Hole U1412B, 7 from Hole U1412C, and 2 from Hole U1412D), using stepwise AF and thermal demagnetization (Figs. F32, F33, F34, F35). After removal of the drilling-induced component, the ChRMs of both normal and reversed polarities were observed for most of the samples (Fig. F34). A pair of sister samples was treated by AF and thermal demagnetization. The AF demagnetized sample displayed high resistance to AF demagnetization to 120 mT, and the thermally demagnetized sample showed unblocking temperature between 650° and 675°C, indicating that the main magnetic carrier (probably hematite) in this sample has a high Curie temperature and high coercivity. ChRM inclinations of discrete samples obtained from principal component analysis (PCA; Kirschvink, 1980) for the four holes are plotted in Figures F32, F33, F34, and F35.

Implications for magnetostratigraphy

We used ChRM inclinations and declinations from both discrete and pass-through measurements to define magnetic polarity sequences (Fig. F37). We recognize seven magnetozones in Hole U1412A. These magnetozones are defined as intervals with multiple consecutive samples with polarities that are distinctly different from neighboring intervals. The upper part of Hole U1412A is characterized by two reversed zones (R1 and R2) and three normal polarity zones (N1, N2, and N3). The lower part of the hole is characterized by one long normal interval (N4) and one reversed interval (R3). Shipboard micropaleontological studies indicate that the base of Hole U1412A (Core 344-U1412A-17X) is close to the Zone NN19/NN18 boundary (~1.9 Ma), the middle part of the hole (Core 6H) is younger than 1.3 Ma, and the uppermost two cores (Cores 1H and 2H) may be within Zone NN21 (younger than 0.29 Ma) (see “Paleontology and biostratigraphy”). Using these age points as a guide to correlate the observed magnetozones with those in the Gradstein et al. (2012) geomagnetic polarity timescale, we interpreted N1 as the upper part of the Brunhes Chron and N2 and N3 as the Jaramillo and Cobb Mountain Subchrons, respectively. Consequently, the long normally magnetized Zone N4 is likely the Olduvai Subchron (1.788–1.945 Ma) with its upper boundary placed at ~92 mbsf (Fig. F37). If true, this would suggest extremely high sediment accumulation rates for Hole U1412A, similar to those in Hole U1379C. Shore-based radiometric dating on tephra layers within Unit I (i.e., ~25 and 95 mbsf; Fig. F32) will provide an independent check on the age assignments based on magnetostratigraphic and biostratigraphic results.

Paleomagnetic measurements of nine discrete samples from Hole U1412B display a change from normal to reversed polarity at ~167 mbsf (Fig. F33). Changes in inclination sign are also observed for the seven samples studied from Hole U1412C (Fig. F34). Two discrete samples from Hole U1412D were measured for shipboard structural analyses (Fig. F35). Because of the small number of samples, we have no magnetostratigraphic interpretations at this time. Shore-based integrated work with micropaleontological data and paleomagnetic studies of discrete samples from each hole are required to estimate the timing and origin of the magnetization recorded by the sediments at Site U1412.

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