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doi:10.2204/iodp.proc.346.108.2015

Paleomagnetism

Paleomagnetic samples and measurements

Paleomagnetic investigations at Site U1427 included measurement of magnetic susceptibility of whole-core and archive-half split-core sections, and natural remanent magnetization (NRM) of archive-half sections. NRM was measured before and after alternating field (AF) demagnetization with a 20 mT peak field for all archive-half sections from Hole U1427A at 5 cm intervals. Because of increased core flow and limited measurement time available at the paleomagnetism station, NRM of archive-half sections from Hole U1427B was measured only after 20 mT AF demagnetization at 5 cm intervals. The FlexIT core orientation tool (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) was used to orient Cores 346-U1427A-2H through 25H. The APC-collected core orientation data for Hole U1427A are reported in Table T16.

We collected one paleomagnetic discrete cube sample (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) from the first section of Cores 346-U1427A-1H through 81H, 85X, and 87X (triangles in Fig. F43A, F43B). Stepwise AF demagnetization on all discrete samples collected from Hole U1427A was performed at successive peak fields of 0, 5, 10, 15, 20, 25, 30, 40, 50, and 60 mT to verify the reliability of the split-core measurements and to determine the demagnetization behavior of the recovered sediment. Following each demagnetization step, NRM of the discrete samples was measured with the samples placed in the “top-toward” or “+z-axis toward magnetometer” orientation (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) on the discrete sample tray.

We processed data extracted from the shipboard Laboratory Information Management System (LIMS) database by removing all measurements collected from disturbed and void intervals and all measurements that were made within 10 cm of the section ends, which are slightly biased by measurement edge effects. The XCB-cored archive-half sections of Hole U1427A frequently contain drilling “biscuits” surrounded by as much disturbed material as intact material, strongly compromising the quality of the resulting paleomagnetic data. We removed data collected during the disturbed intervals in core sections of the four measured XCB cores. For declination data from cores in Hole U1427A where FlexIT tool data are available, we corrected the declination values for each core using the estimated orientation angles. A modified version of the UPmag software (Xuan and Channell, 2009) was used to analyze the NRM data of both the split-core section and the discrete cube samples. The disturbed and void intervals used in this process are reported in Table T17. The processed NRM inclination, declination, and intensity data after 20 mT AF demagnetization are reported in Table T18 and shown in Figure F43.

Natural remanent magnetization and magnetic susceptibility

NRM intensity after 20 mT AF demagnetization in the two measured holes at Site U1427 is similar in magnitude for overlapping intervals, mostly ranging between ~10^–4 and 10^–3 A/m. For the uppermost ~280 m of the recovered sediment, NRM intensity of the measured core sections after 20 mT AF demagnetization is mostly on the order of 10^–3 A/m. Deeper than ~280 m CSF-A until the bottom of the holes, NRM intensity drops to the order of 10^–4 A/m and appears to be noisier than that of sediment from shallower than ~280 m CSF-A. The noisier NRM intensity data deeper than ~280 m CSF-A until the bottom of the holes is accompanied by large scatter in paleomagnetic direction data.

The upper ~120 m of the holes contain intervals of heavily bioturbated deep dark gray and deep dark greenish gray sediment with thicknesses between ~0.5 and 4.0 m. These dark intervals are divided into two types: Type I intervals (Sections 346-U1427A-5H-1, 7H-1~7H-2, and 9H-1~9H-2) are composed of mainly siliciclastics with black to dark greenish gray color, and Type II intervals (Sections 346-U1427A-12H-3~12H-5 and 13H-2~13H-4) are composed of nannofossil-rich clayey silt and nannofossil ooze with dark greenish gray to grayish brown (see “Lithostratigraphy”).

The color of sediment from the two interval types changed quickly after the cores were split. We monitored NRM after 20 mT AF demagnetization for selected core sections from the two interval types through time. NRM after 20 mT AF demagnetization of archive-half core Sections 346-U1427A-7H-1 through 7H-4 (~49–53.5 m CSF-A, Type I interval) and 12H-1 through 12H-5 (~97–104 m CSF-A, Type II interval) were measured as soon as the cores were split and then remeasured after ~2–6 h, after 1 day, and finally after 3 days. The results indicate that core sections from both interval types show some decay in NRM intensity through time, and NRM decay in Type II sediment appears larger than that in Type I sediment (Fig. F44). The decay in NRM intensity could be related to magnetic mineral alternation because of oxidation after the core was split.

The AF demagnetization behavior of the 12 measured discrete samples is illustrated in Figure F45. Declination and inclination values acquired from the discrete sample measurement generally agree well with the split-core measurement after 20 mT AF demagnetization. All samples exhibit a steep, normal overprint that was generally removed after AF demagnetization at peak fields of ~15–20 mT, demonstrating that the 20 mT AF demagnetization is, in general, sufficient to eliminate the drilling overprint. Compared with the two discrete samples from 174.73 and 221.55 m CSF-A, measured discrete samples from deeper depth levels appear to have lower coercivity. NRM measurement of discrete samples from the deep depth with weak intensity often appears to be affected by acquisition of an anhysteretic remanent magnetization, possibly due to bias caused by ambient magnetic field during demagnetization.

Magnetic susceptibility measurements were taken on whole cores from all holes as part of the Whole-Round Multisensor Logger (WRMSL) analysis and on archive-half sections using the Section Half Multisensor Logger (SHMSL) (see “Physical properties”). The WRMSL acquired susceptibility was stored in the database in raw meter units. These were multiplied by a factor of 0.68 × 10^–5 to convert to the dimensionless volume SI unit (Blum, 1997). A factor of (67/80) × 10^–5 was multiplied by the SHMSL acquired susceptibility stored in the database. Magnetic susceptibility measurement is consistent between the two instruments and across the different holes for overlapping intervals, and varies mostly between 5 × 10^–5 and 20 × 10^–5 SI (Fig. F43, fourth panel). Magnetic susceptibility of sediment in both holes, in general, mimics NRM intensity, suggesting that the magnetic minerals that carry NRM are the same or at least coexist with those that dominate magnetic susceptibility.

Magnetostratigraphy

Paleomagnetic inclination and declination data of the holes show patterns that allow for the determination of magnetic polarity for the uppermost ~280 m of recovered sediment. Both magnetic declination and inclination after 20 mT AF demagnetization were used when possible for the magnetostratigraphic interpretation at Site U1427. The geomagnetic field at the latitude of Site U1427 (35.965°N) has an expected inclination of 55.43°, assuming a geocentric axial dipole field model, which is sufficiently steep to determine magnetic polarity in APC cores that lack horizontal orientation. We identified the Brunhes/Matuyama boundary (0.781 Ma) at ~295.3 m CSF-A in Hole U1427A and at ~293.7 m CSF-A in Hole U1427B (Table T19).

Above the identified Brunhes/Matuyama boundary, inclinations after 20 mT AF demagnetization from both holes vary closely around the expected normal polarity inclination at Site U1427. In Hole U1427A, the FlexIT-corrected declinations (green dots in Fig. F43A) are mostly stable and vary around 0°. Right below the Brunhes/Matuyama boundary, inclinations from both holes are apparently dominated by shallow and negative values. The interpreted Brunhes/Matuyama boundary is consistent with the stepwise demagnetization data from the measured discrete samples. Discrete samples from above the boundary (Fig. F45A–F45D) show well-defined characteristic remanence with positive inclinations, whereas discrete samples from right below the Brunhes/Matuyama boundary (Fig. F45E, F45F) apparently have negative inclinations at high demagnetization steps. The depth level of the Brunhes/Matuyama boundary in Hole U1427A agrees well with the LO of planktonic foraminifer N. kagaensis (~0.70 Ma) at 242.86 m CSF-A (Section 346-U1427A-28H-CC) as well as the LO of calcareous nannofossil R. asanoi (~0.91 Ma) at 346.95 m CSF-A (Section 346-U1427A-51H-1W, 75 cm) (see “Biostratigraphy”).

Below the Brunhes/Matuyama boundary, NRM inclinations after 20 mT AF demagnetization show mostly positive values that are apparently steeper than the expected normal polarity dipole inclination at the site location and scattered intervals with shallow and negative inclinations. Increased coring disturbance, strong drill string overprint, the lack of core orientation, and the large scatter in paleomagnetic declinations makes magnetostratigraphic interpretations difficult for the deep part of sediment recovered at Site U1427.

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